Achieving a healthy body composition involves more than just reducing weight; it's about reducing body fat while preserving lean muscle mass. This calculator helps you estimate the amount of fat you need to lose and the time it might take to reach your target body fat percentage, based on your current stats and desired rate of loss.
Body fat percentage is the proportion of your total body weight that is fat. Lowering this percentage is a key indicator of improved health and fitness.
How the Calculator Works
The calculation involves several steps:
Calculate Current Fat Mass: We first determine how many kilograms of fat you currently have. This is done by multiplying your current weight by your current body fat percentage.
Calculate Target Fat Mass: Next, we estimate your lean body mass (everything that isn't fat). Then, we calculate the total target weight that would correspond to your desired body fat percentage, assuming your lean body mass remains constant. The difference between your current weight and this target weight represents the total weight you need to lose.
Calculate Total Fat to Lose: The total amount of fat to lose is the difference between your current fat mass and your target fat mass.
Estimate Time to Reach Goal: Finally, we divide the total amount of fat to lose by your desired weekly weight loss rate to estimate the number of weeks required to reach your target.
The Math Behind It:
Let:
CW = Current Weight (kg)
CBFP = Current Body Fat Percentage (%)
TBFP = Target Body Fat Percentage (%)
WWLR = Desired Weekly Weight Loss Rate (kg/week)
1. Current Fat Mass (CFM) = CW * (CBFP / 100)
2. Lean Body Mass (LBM) = CW – CFM
3. Target Weight (TW) = LBM / ((100 – TBFP) / 100)
4. Total Weight to Lose (TWL) = CW – TW
5. Total Fat to Lose (TFL) = CFM – (TW * (TBFP / 100))
6. Weeks to Target (WTT) = TFL / WWLR
Important Considerations:
This calculator provides an estimation. Individual results may vary.
Sustainable fat loss is typically around 0.5kg to 1kg per week. Losing weight too quickly can lead to muscle loss and other health issues.
It's crucial to combine a healthy, calorie-controlled diet with regular physical activity for effective and healthy fat loss.
Consulting with a healthcare professional or a registered dietitian is recommended before starting any significant weight loss program.
function calculateFatLoss() {
var currentWeight = parseFloat(document.getElementById("currentWeight").value);
var currentBodyFatPercentage = parseFloat(document.getElementById("currentBodyFatPercentage").value);
var targetBodyFatPercentage = parseFloat(document.getElementById("targetBodyFatPercentage").value);
var weeklyWeightLossRate = parseFloat(document.getElementById("weeklyWeightLossRate").value);
var resultElement = document.getElementById("result");
resultElement.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(currentWeight) || currentWeight <= 0) {
resultElement.innerHTML = "Please enter a valid current weight (kg).";
return;
}
if (isNaN(currentBodyFatPercentage) || currentBodyFatPercentage 100) {
resultElement.innerHTML = "Please enter a valid current body fat percentage (0-100%).";
return;
}
if (isNaN(targetBodyFatPercentage) || targetBodyFatPercentage = currentBodyFatPercentage) {
resultElement.innerHTML = "Please enter a target body fat percentage lower than your current one and greater than 0%.";
return;
}
if (isNaN(weeklyWeightLossRate) || weeklyWeightLossRate <= 0) {
resultElement.innerHTML = "Please enter a valid desired weekly weight loss rate (kg/week).";
return;
}
// Calculations
var currentFatMass = currentWeight * (currentBodyFatPercentage / 100);
var leanBodyMass = currentWeight – currentFatMass;
var targetWeight = leanBodyMass / ((100 – targetBodyFatPercentage) / 100);
var totalWeightToLose = currentWeight – targetWeight;
var totalFatToLose = currentFatMass – (targetWeight * (targetBodyFatPercentage / 100));
var weeksToTarget = totalFatToLose / weeklyWeightLossRate;
// Display results
var resultHtml = "