Calculator Weight Loss on an 800 Calorie a Day Diet

Estimate your potential weight loss and understand the key factors of an extremely low-calorie diet.

Calculate Your Potential Weight Loss

Weight Loss Projection Over Time

Day	Calories Consumed	Estimated Calories Burned (TDEE)	Daily Calorie Deficit	Cumulative Weight Loss (kg)
0	800			0.0

What is an 800 Calorie Diet?

An 800 calorie diet is an extremely low-calorie diet (VLCD) that drastically restricts daily food intake. It's significantly below the generally recommended daily intake for adults, which typically ranges from 2000-2500 calories for women and 2500-3000 for men. Such diets are designed for rapid weight loss and are often medically supervised due to their restrictive nature and potential health implications. It's crucial to understand that an 800 calorie diet is not suitable for everyone and should generally only be undertaken under the guidance of a healthcare professional or registered dietitian. Common misconceptions suggest it's a quick fix for any weight problem, but its suitability is highly specific.

Who should consider it (with caution and medical supervision)? Individuals with significant obesity who need to lose weight rapidly for medical reasons, such as before surgery or to manage conditions like type 2 diabetes or severe sleep apnea, might be prescribed a VLCD. It is absolutely NOT recommended for general weight loss, individuals with a history of eating disorders, pregnant or breastfeeding women, children, or individuals with certain chronic health conditions without explicit medical approval. Always consult a doctor before starting any diet this restrictive.

800 Calorie Diet Weight Loss Formula and Mathematical Explanation

The core principle behind weight loss on any diet, including an 800 calorie plan, is achieving a calorie deficit. This means consuming fewer calories than your body burns. The calculator utilizes established physiological principles to estimate this deficit and subsequent weight loss.

The Calculation Steps:

1. Estimate Basal Metabolic Rate (BMR): This is the number of calories your body burns at rest. We use the Mifflin-St Jeor equation, which is widely considered accurate:
   • For men: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
   • For women: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
2. Estimate Total Daily Energy Expenditure (TDEE): This is your BMR multiplied by an activity factor to account for your daily movement and exercise.
3. Calculate Daily Calorie Deficit: This is your TDEE minus your daily calorie intake (800 kcal).
4. Calculate Total Calorie Deficit: Daily Calorie Deficit multiplied by the number of days on the diet.
5. Estimate Weight Loss: Divide the Total Calorie Deficit by the approximate number of calories in one kilogram of fat (around 7700 kcal).

Variables Used in the Calculator:

Variable	Meaning	Unit	Typical Range/Input
Current Weight	Your starting weight before the diet.	kg	User Input (e.g., 70-150 kg)
Target Weight	Your desired weight goal.	kg	User Input (e.g., 50-120 kg)
Height	Your current height. Needed for TDEE calculation.	cm	User Input (e.g., 150-200 cm)
Age	Your current age. Needed for TDEE calculation.	Years	User Input (e.g., 18-80 years)
Gender	Biological sex assigned at birth. Affects BMR calculation.	–	Male / Female
Activity Level	Average weekly physical activity. Adjusts TDEE.	Category	Sedentary, Light, Moderate, Very Active, Extra Active
Diet Duration	Number of days the 800 kcal diet is followed.	Days	User Input (e.g., 7-90 days)
Calorie Intake	Fixed daily calorie consumption.	kcal/day	800 (Fixed)
Calories per kg Fat	Estimated energy content of 1 kg of body fat.	kcal/kg	~7700 (Standard approximation)
TDEE	Total Daily Energy Expenditure – calories burned daily.	kcal/day	Calculated (e.g., 1500-3000 kcal/day)
Daily Calorie Deficit	Difference between TDEE and intake.	kcal/day	Calculated (e.g., 700-2200 kcal/day)
Estimated Weight Loss	Total weight lost over the diet duration.	kg	Calculated (e.g., 1-10+ kg)

Practical Examples (Real-World Use Cases)

Let's illustrate with two distinct scenarios:

Example 1: Moderately Active Woman

• Inputs:
• Current Weight: 85 kg
• Target Weight: 75 kg
• Height: 165 cm
• Age: 35 years
• Gender: Female
• Activity Level: Moderately Active
• Diet Duration: 45 days

Calculation Breakdown:

• Estimated BMR (Female): (10 * 85) + (6.25 * 165) – (5 * 35) – 161 = 850 + 1031.25 – 175 – 161 = 1545.25 kcal
• Estimated TDEE (Moderately Active Factor ~1.55): 1545.25 * 1.55 = ~2395 kcal/day
• Daily Calorie Deficit: 2395 – 800 = ~1595 kcal/day
• Total Calorie Deficit: 1595 * 45 days = ~71775 kcal
• Estimated Weight Loss: 71775 / 7700 = ~9.3 kg

Interpretation: Over 45 days, this individual could potentially lose around 9.3 kg. This highlights the significant, rapid weight loss achievable with such a strict deficit, underscoring the importance of medical monitoring during this period.

Example 2: Sedentary Man

• Inputs:
• Current Weight: 100 kg
• Target Weight: 90 kg
• Height: 180 cm
• Age: 50 years
• Gender: Male
• Activity Level: Sedentary
• Diet Duration: 30 days

Calculation Breakdown:

• Estimated BMR (Male): (10 * 100) + (6.25 * 180) – (5 * 50) + 5 = 1000 + 1125 – 250 + 5 = 1880 kcal
• Estimated TDEE (Sedentary Factor ~1.2): 1880 * 1.2 = ~2256 kcal/day
• Daily Calorie Deficit: 2256 – 800 = ~1456 kcal/day
• Total Calorie Deficit: 1456 * 30 days = ~43680 kcal
• Estimated Weight Loss: 43680 / 7700 = ~5.7 kg

Interpretation: For this individual, a 30-day stint on an 800 calorie diet could result in approximately 5.7 kg of weight loss. The lower activity level means a smaller portion of the deficit comes from TDEE, but the 800 kcal intake still creates a substantial deficit.

How to Use This 800 Calorie Diet Weight Loss Calculator

Using the calculator is straightforward and designed to provide a quick estimate:

1. Enter Current Weight: Input your current body weight in kilograms.
2. Enter Target Weight: Input the weight you aim to reach.
3. Input Height & Age: Provide your height in centimeters and your age in years. These are crucial for accurately estimating your TDEE.
4. Select Gender: Choose Male or Female to apply the correct BMR formula.
5. Choose Activity Level: Select the option that best describes your typical weekly physical activity.
6. Specify Diet Duration: Enter the number of consecutive days you intend to follow the 800-calorie diet.
7. Click 'Calculate': The calculator will process your inputs and display your estimated total weight loss in kilograms, along with key intermediate metrics like total calorie deficit, estimated weekly loss, and average daily deficit.
8. Interpret the Results: Understand that these are estimates. Actual results can vary. The primary result shows the total kilograms you might lose. Intermediate results provide context on the magnitude of the deficit.
9. Use the Projection Table & Chart: Examine the table and chart to visualize the day-by-day progression of your potential weight loss, calorie expenditure, and deficit.
10. Reset Functionality: If you wish to recalculate with different parameters, use the 'Reset' button to return the fields to sensible default values.
11. Copy Results: Use the 'Copy Results' button to easily save or share your calculated figures.

Decision-Making Guidance: The results from this calculator should serve as an informational tool, not a definitive plan. If the potential weight loss is significant, consult with a healthcare provider to discuss the safety and appropriateness of an 800 calorie diet for your specific health status and goals. If you are considering longer-term weight management strategies, explore resources on sustainable balanced eating plans.

Key Factors That Affect 800 Calorie Diet Weight Loss Results

While the calculator provides an estimate, numerous factors can influence actual weight loss outcomes on an 800 calorie diet:

• Metabolic Adaptation: As you drastically cut calories, your metabolism may slow down to conserve energy, reducing your TDEE and potentially slowing weight loss over time.
• Muscle Mass Preservation: Very low-calorie diets can lead to muscle loss, not just fat loss. Muscle tissue burns more calories at rest than fat, so losing muscle can further lower your TDEE. Adequate protein intake (within the 800 kcal limit, which is challenging) and strength training are crucial but difficult to manage effectively on such a restricted intake.
• Hydration Levels: Water weight fluctuations can significantly impact the scale daily. Initial rapid drops may include water loss, not just fat. Staying well-hydrated is key for overall health and can aid metabolism.
• Individual Metabolism Variations: People's bodies burn calories at different rates due to genetics, hormones, and other physiological differences. The Mifflin-St Jeor equation provides an estimate, but individual results will vary.
• Diet Adherence and Accuracy: Consistently sticking to exactly 800 calories is difficult. Small deviations, unmeasured calories (e.g., from cooking oils, sauces), or inaccurate tracking can alter the total deficit.
• Nutrient Deficiencies: An 800 calorie diet makes it extremely challenging to meet all essential vitamin and mineral requirements. This can lead to fatigue, hair loss, mood changes, and other health issues. Supplementation is often necessary but requires medical guidance.
• Medical Conditions & Medications: Underlying health issues like thyroid disorders or medications can affect metabolism and weight loss efficacy.
• Sleep Quality: Poor sleep can disrupt hormones that regulate appetite (ghrelin and leptin) and increase cortisol levels, potentially hindering weight loss and increasing cravings.

Frequently Asked Questions (FAQ)

Is an 800 calorie diet safe?

For most adults, an 800 calorie diet is considered too low and potentially unsafe without strict medical supervision. It can lead to nutrient deficiencies, fatigue, gallstones, heart problems, and other serious health issues. It should only be considered under the guidance of a healthcare professional for specific medical indications.

How much weight can I realistically lose on an 800 calorie diet?

As shown by the calculator, significant rapid weight loss is possible, potentially 1-2 kg per week or more initially. However, this includes water and potentially muscle mass, not just fat. Long-term sustainable loss is often better achieved through less restrictive methods.

What are the risks of an 800 calorie diet?

Risks include fatigue, dizziness, nutrient deficiencies, muscle loss, electrolyte imbalances, gallstones, hair loss, irritability, and potential heart rhythm disturbances. Extended periods on such a diet can also lead to binge eating or disordered eating patterns.

Can I do an 800 calorie diet long-term?

No, an 800 calorie diet is generally intended for short-term, medically supervised use only. Long-term adherence is unsustainable and poses significant health risks. Gradual reintroduction of calories is necessary post-diet.

What should I eat on an 800 calorie diet?

Focus should be on nutrient-dense, low-calorie foods like lean proteins (fish, chicken breast), non-starchy vegetables (leafy greens, broccoli, peppers), and small portions of fruits. Meal replacements (shakes) are often used in medically supervised programs to ensure nutritional completeness. Consult a dietitian for a specific meal plan.

Will I regain the weight after stopping the 800 calorie diet?

Weight regain is a common risk if the transition back to normal eating is not managed carefully. Metabolic adaptation and a return to previous eating habits without lifestyle changes often lead to weight rebound. A gradual increase in calories and focusing on sustainable habits are key.

Are there alternatives to an 800 calorie diet for weight loss?

Yes, absolutely. More sustainable and healthier alternatives include creating a moderate calorie deficit (e.g., 500-750 kcal/day) through a balanced diet, increasing physical activity, incorporating strength training to build muscle, improving sleep, and managing stress. Exploring healthy weight management strategies is recommended.

Does the calculator account for water weight?

The calculator primarily estimates fat loss based on calorie deficit. Initial rapid weight loss often includes significant water loss, especially when reducing carbohydrate intake. The calculator does not explicitly model water weight fluctuations but provides an overall estimate based on calorie balance.

Related Tools and Internal Resources

• Calorie Deficit CalculatorUnderstand the fundamental principle of weight loss.
• TDEE CalculatorEstimate your daily calorie needs based on activity.
• Healthy Diet Plans OverviewExplore options for sustainable nutrition.
• Macronutrient Ratio CalculatorLearn how to balance protein, carbs, and fats.
• Fitness Goal Setting GuideSet realistic and achievable fitness objectives.