Weekly Body Fat Calculator

1. Use comparable measurements
Choose readings taken with the same body-fat method and similar timing conditions.

2. Enter starting data
Add body weight and body-fat percentage from the beginning of the week.

3. Enter ending data
Add the corresponding measurements from the end of the week.

4. Choose the weight unit
Select pounds or kilograms and use that unit for both weights.

5. Interpret cautiously
Review percentage-point and estimated fat-mass changes, recognizing that short-term readings can fluctuate.

Body-fat change (percentage points) = Ending body fat % − Starting body fat %
Fat mass = Body weight × Body fat % ÷ 100
Fat-mass change = Ending fat mass − Starting fat mass

Scale-weight change is ending weight minus starting weight. The calculation assumes each body-fat percentage applies to the weight entered for the same date.

What the result means

A negative percentage-point result means the ending body-fat reading was lower than the starting reading; a positive result means it was higher.

Short-term body-fat estimates can be strongly affected by hydration and method error, so trends across multiple measurements are usually more informative.

Given: Starting weight 180 lb at 24% body fat and ending weight 178.5 lb at 23.5%.

Calculation: Starting fat mass = 180 × 0.24 = 43.2 lb. Ending fat mass = 178.5 × 0.235 = 41.9475 lb. Body-fat change = 23.5 − 24 = −0.5 percentage points. Fat-mass change = 41.9475 − 43.2 = −1.2525 lb.

Result: −0.5 percentage points, about −1.25 lb estimated fat mass, and −1.5 lb scale weight.