800 Calories a Day Weight Loss Calculator
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Weight Loss Calculator (800 Calorie Diet)
This calculator helps estimate the potential weekly and total weight loss when following a very low-calorie diet of 800 calories per day. It also estimates the duration until a target weight is reached, assuming a consistent deficit.
Your Weight Loss Projection
—
Estimated Weekly Loss: — lbs
Estimated Time to Target: — weeks
Total Weight Loss: — lbs
Total Calories Burned (Net): — kcal
Formula: Daily Calorie Deficit = (BMR * Activity Factor) – Daily Calorie Intake. Weekly Loss = Daily Deficit * 7 / 3500. Time to Target = Total Weight Loss / Weekly Loss.
Calorie Deficit Breakdown
Weight Loss Progress Table
Weight Loss Milestones
| Week |
Starting Weight (lbs) |
Ending Weight (lbs) |
Weight Lost This Week (lbs) |
Total Weight Lost (lbs) |
What is an 800 Calories a Day Weight Loss Plan?
An 800 calories a day weight loss plan is an extremely low-calorie diet (VLCD). It involves drastically reducing daily food intake to only 800 calories, creating a significant calorie deficit. This type of diet is typically undertaken for rapid, short-term weight loss under strict medical supervision. It's crucial to understand that an 800 calories a day weight loss diet is not suitable for everyone and carries potential risks. It should only be considered after consulting with a healthcare professional to ensure it aligns with individual health needs and goals.
This approach is primarily designed for individuals with a high body mass index (BMI) or obesity who need to lose weight quickly for health reasons, such as before surgery or to manage conditions like type 2 diabetes. Common misconceptions include believing that consuming fewer calories always leads to linear, risk-free weight loss. However, an 800 calories a day weight loss plan requires careful management of nutrient intake to prevent deficiencies and should never be followed long-term without medical guidance.
800 Calories a Day Weight Loss Formula and Mathematical Explanation
The core of estimating weight loss on an 800-calorie diet relies on understanding the concept of calorie deficit. A deficit of approximately 3,500 calories is generally considered equivalent to losing one pound of body fat. Our 800 calories a day weight loss calculator uses the following steps:
- Calculate Total Daily Energy Expenditure (TDEE): This is the total number of calories your body burns in a day, accounting for your Basal Metabolic Rate (BMR) and your activity level.
- TDEE = BMR × Activity Factor
- Calculate Daily Calorie Deficit: This is the difference between the calories your body burns (TDEE) and the calories you consume.
- Daily Deficit = TDEE – Daily Calorie Intake (800 calories)
- Calculate Estimated Weekly Weight Loss: Using the 3,500-calorie rule for one pound of fat.
- Weekly Weight Loss (lbs) = (Daily Deficit × 7 days) / 3500 calories/lb
- Calculate Total Weight Loss: This is the difference between your current weight and your target weight.
- Total Weight Loss = Current Weight – Target Weight
- Calculate Estimated Time to Reach Target: This estimates how many weeks it will take to achieve the total weight loss.
- Time to Target (weeks) = Total Weight Loss / Weekly Weight Loss
Variables Table for 800 Calories a Day Weight Loss Calculation
Variables Used in Calculation
| Variable |
Meaning |
Unit |
Typical Range |
| Current Weight |
Your starting body weight. |
Pounds (lbs) |
100 – 1000+ |
| Target Weight |
Your desired body weight. |
Pounds (lbs) |
50 – 500+ |
| BMR |
Calories burned at rest. |
Kilocalories (kcal) |
800 – 2500+ |
| Activity Factor |
Multiplier for activity level. |
Unitless |
1.2 – 1.9 |
| Daily Calorie Intake |
Calories consumed per day. |
Kilocalories (kcal) |
Typically 800 for this plan |
| Daily Calorie Deficit |
Net calorie deficit per day. |
Kilocalories (kcal) |
Varies significantly |
| Weekly Weight Loss |
Estimated pounds lost per week. |
Pounds (lbs) |
Varies significantly |
| Time to Target |
Estimated weeks to reach goal. |
Weeks |
Varies significantly |
The 800 calories a day weight loss calculator provides an estimate. Actual results can vary due to individual metabolic rates, body composition, adherence to the diet, and other physiological factors.
Practical Examples (Real-World Use Cases)
Here are two practical examples demonstrating how the 800 calories a day weight loss calculator can be used:
Example 1: Pre-Surgical Weight Loss
Scenario: Sarah needs to lose a significant amount of weight before a scheduled surgery. She weighs 220 lbs and her target weight is 190 lbs. Her doctor estimated her BMR to be around 1700 kcal, and she is following a sedentary lifestyle (Activity Factor 1.2). She will be on a medically supervised 800-calorie diet for a limited period.
Inputs:
- Current Weight: 220 lbs
- Target Weight: 190 lbs
- BMR: 1700 kcal
- Activity Factor: 1.2 (Sedentary)
- Daily Calorie Intake: 800 kcal
Calculation using the calculator:
- TDEE = 1700 × 1.2 = 2040 kcal
- Daily Deficit = 2040 – 800 = 1240 kcal
- Weekly Weight Loss = (1240 × 7) / 3500 ≈ 2.48 lbs/week
- Total Weight Loss = 220 – 190 = 30 lbs
- Time to Target = 30 / 2.48 ≈ 12.1 weeks
Interpretation: On an 800 calories a day weight loss plan, Sarah could expect to lose approximately 2.5 lbs per week, reaching her target of 190 lbs in about 12 weeks. This projection helps her and her medical team plan the duration of the VLCD.
Example 2: Managing Type 2 Diabetes
Scenario: John is aiming to improve his metabolic health and manage his type 2 diabetes. He weighs 180 lbs and wants to reach 160 lbs. His BMR is estimated at 1500 kcal, and he engages in light exercise 2-3 times a week (Activity Factor 1.375). He is advised by his endocrinologist to follow a temporary 800-calorie diet.
Inputs:
- Current Weight: 180 lbs
- Target Weight: 160 lbs
- BMR: 1500 kcal
- Activity Factor: 1.375 (Lightly Active)
- Daily Calorie Intake: 800 kcal
Calculation using the calculator:
- TDEE = 1500 × 1.375 = 2062.5 kcal
- Daily Deficit = 2062.5 – 800 = 1262.5 kcal
- Weekly Weight Loss = (1262.5 × 7) / 3500 ≈ 2.53 lbs/week
- Total Weight Loss = 180 – 160 = 20 lbs
- Time to Target = 20 / 2.53 ≈ 7.9 weeks
Interpretation: John can anticipate losing around 2.5 lbs per week. To lose 20 lbs and reach his goal weight of 160 lbs, it would take approximately 8 weeks on this 800 calories a day weight loss regime. This provides a clear timeline for his health improvement plan.
How to Use This 800 Calories a Day Weight Loss Calculator
Using the 800 calories a day weight loss calculator is straightforward. Follow these steps to get your personalized weight loss projection:
- Enter Current Weight: Input your current body weight in pounds (lbs).
- Enter Target Weight: Input the weight you aim to achieve in pounds (lbs). Ensure your target weight is realistic and healthy for your height.
- Enter Basal Metabolic Rate (BMR): Provide your BMR in kilocalories (kcal). If you don't know your BMR, you can use an online BMR calculator or consult your doctor. It's the number of calories your body burns at rest.
- Select Activity Level: Choose the option that best describes your average daily physical activity from the dropdown menu. This influences your Total Daily Energy Expenditure (TDEE).
- Daily Calorie Intake: This field is pre-filled with 800 calories, as this calculator is specifically for that diet. You cannot change this value.
- Click 'Calculate': Once all fields are filled, press the 'Calculate' button.
Reading the Results:
- Primary Highlighted Result: This shows your estimated total weight loss in pounds.
- Estimated Weekly Loss: The projected amount of weight (in lbs) you are likely to lose each week.
- Estimated Time to Target: The approximate number of weeks needed to reach your target weight.
- Total Weight Lost: The total difference between your current and target weight.
- Total Calories Burned (Net): The cumulative net calorie deficit over the estimated period to reach your goal.
- Explanation: A brief summary of the formulas used.
Decision-Making Guidance:
The results provide a valuable projection, but remember they are estimates. Discuss these projections with your healthcare provider. If the calculated time to reach your goal seems too long or too short, consider if your target weight is appropriate or if adjustments to activity levels (if safe and recommended) are feasible. Always prioritize safety and consult a professional before starting or modifying any extreme diet like an 800 calories a day weight loss plan.
Key Factors That Affect 800 Calories a Day Weight Loss Results
While the 800 calories a day weight loss calculator provides a good estimate, several real-world factors can influence your actual results:
- Metabolic Adaptation: As you lose weight, your metabolism can slow down. Your BMR may decrease, meaning your body burns fewer calories at rest. This can reduce your calorie deficit and slow down weight loss over time.
- Muscle Mass Preservation: Severe calorie restriction, especially without adequate protein and resistance training, can lead to loss of muscle mass. Muscle burns more calories than fat, so losing it can further decrease your metabolism.
- Hydration Levels: Water weight fluctuations can significantly impact the scale in the short term, masking true fat loss. Proper hydration is crucial for metabolic function.
- Nutrient Deficiencies: An 800-calorie diet makes it challenging to obtain all essential vitamins and minerals. Deficiencies can negatively affect overall health, energy levels, and potentially metabolic processes. This is why medical supervision is critical.
- Hormonal Changes: Extreme dieting can disrupt hormones related to hunger (ghrelin, leptin), stress (cortisol), and thyroid function, all of which can affect metabolism and weight loss.
- Adherence and Consistency: The calculator assumes perfect adherence. In reality, occasional slip-ups or underestimating calorie intake from beverages or sauces can alter the actual calorie deficit. Long-term consistency is key.
- Digestive Health: Reduced food intake can affect gut motility and the gut microbiome, potentially impacting nutrient absorption and overall well-being.
- Medical Conditions & Medications: Underlying health issues or medications can influence metabolism, appetite, and weight management, making individual results vary.
Understanding these factors highlights why an 800 calories a day weight loss approach requires careful monitoring and professional guidance to mitigate risks and optimize outcomes.
Frequently Asked Questions (FAQ)
Is 800 calories a day safe for weight loss?
For most individuals, an 800-calorie diet is considered very low and should only be undertaken for short periods under strict medical supervision due to the risk of nutrient deficiencies, muscle loss, and other health complications. Consult your doctor first.
How much weight can I realistically lose on 800 calories a day?
The amount varies significantly based on your starting weight, BMR, and activity level. A deficit of 1000-1500 calories per day could lead to 2-3 lbs of loss per week, but this is an estimate and may include water weight initially.
What are the side effects of an 800 calorie diet?
Common side effects include fatigue, dizziness, headaches, irritability, constipation, gallstones, and nutrient deficiencies. Medical monitoring is essential to manage these risks.
Can I follow an 800 calorie diet long-term?
No, an 800-calorie diet is not sustainable or safe for long-term use. It's intended for short-term, rapid weight loss under medical guidance. A balanced, sustainable diet is recommended for long-term health and weight management.
What is the difference between BMR and TDEE?
BMR (Basal Metabolic Rate) is the calories your body burns at complete rest. TDEE (Total Daily Energy Expenditure) includes your BMR plus the calories burned through physical activity and the thermic effect of food.
Should I use supplements on an 800 calorie diet?
If medically supervised, a doctor may recommend specific vitamin and mineral supplements to prevent deficiencies common with such low calorie intake. Never self-prescribe supplements.
How can I ensure I'm getting enough protein on 800 calories?
Focus on lean protein sources like chicken breast, fish, tofu, or protein powders (if approved by your doctor). Distributing protein intake throughout the day can help with satiety.
What happens after I stop the 800 calorie diet?
It's crucial to transition gradually to a more balanced, higher-calorie intake to avoid rapid weight regain. Working with a dietitian or nutritionist is highly recommended to create a sustainable eating plan post-VLCD.
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var targetWeightInput = document.getElementById("targetWeight");
var bmrInput = document.getElementById("bmr");
var activityFactorSelect = document.getElementById("activityFactor");
var dietCaloriesInput = document.getElementById("dietCalories");
var currentWeightError = document.getElementById("currentWeightError");
var targetWeightError = document.getElementById("targetWeightError");
var bmrError = document.getElementById("bmrError");
var dietCaloriesError = document.getElementById("dietCaloriesError");
var resultsContainer = document.getElementById("results-container");
var primaryResultDiv = document.getElementById("primaryResult");
var weeklyLossSpan = document.getElementById("weeklyLoss");
var timeToTargetSpan = document.getElementById("timeToTarget");
var totalLossSpan = document.getElementById("totalLoss");
var netCaloriesBurnedSpan = document.getElementById("netCaloriesBurned");
var weightLossChartCtx = document.getElementById("weightLossChart").getContext("2d");
var weightLossChartInstance = null;
var calorieDeficitChartCtx = document.getElementById("calorieDeficitChart").getContext("2d");
var calorieDeficitChartInstance = null;
var chartKeyDiv = document.getElementById("chartKey");
var calorieChartKeyDiv = document.getElementById("calorieChartKey");
var tableBody = document.querySelector("#progressTable tbody");
var defaultValues = {
currentWeight: 180,
targetWeight: 160,
bmr: 1600,
activityFactor: "1.375",
dietCalories: 800
};
function validateInput(inputElement, errorElement, minValue, maxValue) {
var value = parseFloat(inputElement.value);
var isValid = true;
if (isNaN(value)) {
errorElement.textContent = "Please enter a valid number.";
errorElement.style.display = "block";
isValid = false;
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return isValid;
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function updateChartData(currentWeight, targetWeight, weeklyLoss, timeToTargetWeeks) {
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var weightData = [];
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for (var i = 0; i <= timeToTargetWeeks && i < 52; i++) { // Limit to 52 weeks for practicality
labels.push("Week " + i);
weightData.push(currentChartWeight);
currentChartWeight -= weeklyLoss;
if (currentChartWeight 0 && parseFloat(labels[labels.length – 1].split(' ')[1]) === timeToTargetWeeks) {
weightData[weightData.length – 1] = targetWeight;
} else if (labels.length > 0 && parseFloat(labels[labels.length – 1].split(' ')[1]) < timeToTargetWeeks) {
labels.push("Week " + Math.ceil(timeToTargetWeeks));
weightData.push(targetWeight);
}
if (weightLossChartInstance) {
weightLossChartInstance.destroy();
}
weightLossChartInstance = new Chart(weightLossChartCtx, {
type: 'line',
data: {
labels: labels,
datasets: [{
label: 'Projected Weight (lbs)',
data: weightData,
borderColor: 'var(–primary-color)',
backgroundColor: 'rgba(0, 74, 153, 0.2)',
fill: true,
tension: 0.1
}, {
label: 'Target Weight (lbs)',
data: Array(labels.length).fill(targetWeight),
borderColor: 'var(–success-color)',
borderDash: [5, 5],
backgroundColor: 'rgba(40, 167, 69, 0.1)',
fill: false,
tension: 0
}]
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options: {
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maintainAspectRatio: false,
scales: {
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title: {
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text: 'Weight (lbs)'
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chartKeyDiv.innerHTML = '
● Projected Weight |
● Target Weight';
}
function updateCalorieDeficitChart(bmr, activityFactor, dietCalories) {
var tdee = bmr * activityFactor;
var deficit = tdee – dietCalories;
var maintenance = dietCalories; // Calories consumed
if (weightLossChartInstance) {
var maxWeeks = weightLossChartInstance.data.labels.length > 0 ? weightLossChartInstance.data.labels.length : 10; // Use chart weeks or default
} else {
var maxWeeks = 10;
}
var totalDeficit = deficit * maxWeeks;
if (calorieDeficitChartInstance) {
calorieDeficitChartInstance.destroy();
}
calorieDeficitChartInstance = new Chart(calorieDeficitChartCtx, {
type: 'bar',
data: {
labels: ['Calories Consumed', 'Net Deficit'],
datasets: [{
label: 'Daily Calories',
data: [maintenance, deficit],
backgroundColor: [
'rgba(40, 167, 69, 0.6)', // Diet Calories (Green)
'rgba(220, 53, 69, 0.6)' // Net Deficit (Red)
],
borderColor: [
'rgba(40, 167, 69, 1)',
'rgba(220, 53, 69, 1)'
],
borderWidth: 1
}]
},
options: {
responsive: true,
maintainAspectRatio: false,
plugins: {
title: {
display: true,
text: 'Daily Calorie Balance (Approximate)'
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tooltip: {
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var label = context.dataset.label || ";
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label += ': ';
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if (context.parsed.y !== null) {
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title: {
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text: 'Calories (kcal)'
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calorieChartKeyDiv.innerHTML = '
● Daily Intake |
● Daily Net Deficit';
}
function populateProgressTable(currentWeight, targetWeight, weeklyLoss) {
tableBody.innerHTML = "; // Clear previous rows
var currentRowWeight = currentWeight;
var totalLost = 0;
var maxWeeks = 52; // Limit table entries
for (var week = 0; week 0) {
weightLostThisWeek = Math.max(0, weeklyLoss); // Ensure non-negative
if (currentRowWeight – weeklyLoss < targetWeight) {
weightLostThisWeek = currentRowWeight – targetWeight;
currentRowWeight = targetWeight;
} else {
currentRowWeight -= weeklyLoss;
}
totalLost = currentWeight – currentRowWeight;
} else {
// Week 0
weightLostThisWeek = 0;
currentRowWeight = currentWeight;
totalLost = 0;
}
if (startingWeightForRow 0) break; // Stop if target reached
var row = tableBody.insertRow();
row.insertCell(0).textContent = week === 0 ? "Start" : week;
row.insertCell(1).textContent = startingWeightForRow.toFixed(1);
row.insertCell(2).textContent = currentRowWeight.toFixed(1);
row.insertCell(3).textContent = weightLostThisWeek.toFixed(1);
row.insertCell(4).textContent = totalLost.toFixed(1);
if (currentRowWeight 0) {
break; // Exit loop once target is met or surpassed
}
}
}
function calculateWeightLoss() {
var currentWeight = parseFloat(currentWeightInput.value);
var targetWeight = parseFloat(targetWeightInput.value);
var bmr = parseFloat(bmrInput.value);
var activityFactor = parseFloat(activityFactorSelect.value);
var dietCalories = parseFloat(dietCaloriesInput.value);
var isValid = true;
isValid = validateInput(currentWeightInput, currentWeightError) && isValid;
isValid = validateInput(targetWeightInput, targetWeightError, 0, currentWeightInput.value ? parseFloat(currentWeightInput.value) : undefined) && isValid; // Target cannot be > current
isValid = validateInput(bmrInput, bmrError, 500) && isValid; // Min BMR reasonable
// dietCalories is fixed, no validation needed here other than it's a number
if (currentWeight <= targetWeight) {
targetWeightError.textContent = "Target weight must be less than current weight.";
targetWeightError.style.display = "block";
isValid = false;
} else {
targetWeightError.style.display = "none";
}
if (!isValid) {
resultsContainer.style.display = "none";
return;
}
var tdee = bmr * activityFactor;
var dailyDeficit = tdee – dietCalories;
var totalWeightLoss = currentWeight – targetWeight;
var weeklyLoss = (dailyDeficit * 7) / 3500;
var timeToTargetWeeks = totalWeightLoss / weeklyLoss;
var netCaloriesBurned = dailyDeficit * timeToTargetWeeks * 7;
if (dailyDeficit = 0 ? timeToTargetWeeks.toFixed(1) : "N/A";
totalLossSpan.textContent = totalWeightLoss.toFixed(1);
netCaloriesBurnedSpan.textContent = netCaloriesBurned.toFixed(0);
resultsContainer.style.display = "block";
updateChartData(currentWeight, targetWeight, weeklyLoss, timeToTargetWeeks);
updateCalorieDeficitChart(bmr, activityFactor, dietCalories);
populateProgressTable(currentWeight, targetWeight, weeklyLoss);
}
function resetCalculator() {
currentWeightInput.value = defaultValues.currentWeight;
targetWeightInput.value = defaultValues.targetWeight;
bmrInput.value = defaultValues.bmr;
activityFactorSelect.value = defaultValues.activityFactor;
dietCaloriesInput.value = defaultValues.dietCalories;
currentWeightError.style.display = "none";
targetWeightError.style.display = "none";
bmrError.style.display = "none";
dietCaloriesError.style.display = "none";
resultsContainer.style.display = "none";
if (weightLossChartInstance) weightLossChartInstance.destroy();
if (calorieDeficitChartInstance) calorieDeficitChartInstance.destroy();
chartKeyDiv.innerHTML = "";
calorieChartKeyDiv.innerHTML = "";
tableBody.innerHTML = "";
// Reset chart canvas content visually if needed
weightLossChartCtx.clearRect(0, 0, weightLossChartCtx.canvas.width, weightLossChartCtx.canvas.height);
calorieDeficitChartCtx.clearRect(0, 0, calorieDeficitChartCtx.canvas.width, calorieDeficitChartCtx.canvas.height);
}
function copyResults() {
var currentWeight = currentWeightInput.value;
var targetWeight = targetWeightInput.value;
var bmr = bmrInput.value;
var activityLevelText = activityFactorSelect.options[activityFactorSelect.selectedIndex].text;
var dietCalories = dietCaloriesInput.value;
var totalLoss = totalLossSpan.textContent;
var weeklyLoss = weeklyLossSpan.textContent;
var timeToTarget = timeToTargetSpan.textContent;
var netCaloriesBurned = netCaloriesBurnedSpan.textContent;
var resultsText = "800 Calories a Day Weight Loss Projection:\n\n";
resultsText += "— Inputs —\n";
resultsText += "Current Weight: " + currentWeight + " lbs\n";
resultsText += "Target Weight: " + targetWeight + " lbs\n";
resultsText += "BMR: " + bmr + " kcal\n";
resultsText += "Activity Level: " + activityLevelText + "\n";
resultsText += "Daily Calorie Intake: " + dietCalories + " kcal\n\n";
resultsText += "— Results —\n";
resultsText += "Total Weight Loss: " + totalLoss + "\n";
resultsText += "Estimated Weekly Loss: " + weeklyLoss + "\n";
resultsText += "Estimated Time to Target: " + timeToTarget + "\n";
resultsText += "Total Net Calories Burned: " + netCaloriesBurned + " kcal\n\n";
resultsText += "Key Assumptions:\n";
resultsText += "- Consistent daily calorie deficit.\n";
resultsText += "- 3500 calorie deficit equals 1 lb of fat loss.\n";
resultsText += "- Stable metabolic rate and no significant muscle loss.\n";
try {
navigator.clipboard.writeText(resultsText).then(function() {
alert("Results copied to clipboard!");
}).catch(function(err) {
console.error("Failed to copy: ", err);
alert("Failed to copy results. Please copy manually.");
});
} catch (err) {
console.error("Clipboard API not available: ", err);
alert("Your browser does not support automatic copying. Please copy the results manually.");
}
}
// Initialize calculator on load
window.onload = function() {
resetCalculator(); // Set default values and clear results
// Optionally call calculateWeightLoss() here if you want initial calculation
};
// Add event listeners for real-time validation and calculation
currentWeightInput.addEventListener('input', calculateWeightLoss);
targetWeightInput.addEventListener('input', calculateWeightLoss);
bmrInput.addEventListener('input', calculateWeightLoss);
activityFactorSelect.addEventListener('change', calculateWeightLoss);
dietCaloriesInput.addEventListener('input', calculateWeightLoss); // Though fixed, good practice
// Initial call to set default values visually and prepare canvas
document.addEventListener("DOMContentLoaded", function() {
resetCalculator(); // Sets default values from defaultValues object
var canvas = document.getElementById("weightLossChart");
if(canvas) {
// Initialize empty chart or placeholder
weightLossChartInstance = new Chart(weightLossChartCtx, {
type: 'line',
data: { labels: [], datasets: [] },
options: { scales: { y: {beginAtZero: false} }, plugins: { legend: {display: true} } }
});
}
var canvas2 = document.getElementById("calorieDeficitChart");
if(canvas2) {
calorieDeficitChartInstance = new Chart(calorieDeficitChartCtx, {
type: 'bar',
data: { labels: [], datasets: [] },
options: { scales: { y: {beginAtZero: true} }, plugins: { legend: {display: true} } }
});
}
});