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Diet Weight Estimator

This calculator projects body weight from a starting weight, an average daily calorie surplus or deficit, and a time period. It uses a simplified energy conversion to show the direction and approximate scale of change.

The model is most useful for short planning comparisons. Real weight change is nonlinear because energy expenditure, water, glycogen, appetite, and adherence change over time.

Weight projection

kg
kcal/day
weeks
Result
—
estimated body weight after the period
Estimated weight change—
Total calorie balance—
Average weekly change—

1. Enter current weight

Use your starting body weight in kilograms.

2. Enter calorie balance

Use a negative number for a deficit and a positive number for a surplus.

3. Choose the period

Enter the number of weeks for the projection.

4. Review projected weight

The main result adds the modeled weight change to starting weight.

5. Check the weekly rate

Use the weekly estimate to compare scenarios.

6. Treat the result as a scenario

Update assumptions from observed trends rather than relying on a long fixed projection.

Total calorie balance = daily balance × 7 × weeks
Estimated weight change (kg) = total calorie balance ÷ 7,700
Projected weight = current weight + estimated change

The model assumes 7,700 kcal per kilogram, a common simplification rather than a fixed biological rule.

What the result means

The main result is the projected body weight if the entered average calorie balance produced a linear change.

Longer projections are increasingly uncertain. Rapid planned losses or gains and eating-disorder concerns should be discussed with a qualified professional.

Given: 80 kg, a 500 kcal daily deficit, and 8 weeks.

Calculation: Total balance = −500 × 7 × 8 = −28,000 kcal. Estimated change = −28,000 ÷ 7,700 = −3.64 kg. Projected weight = 80 − 3.64 = 76.36 kg.

Result: Approximately 76.4 kg after 8 weeks.

Why is 7,700 kcal used per kilogram?

It is a convenient energy-equivalent approximation. Actual tissue change includes varying proportions of fat, lean tissue, glycogen, and water.

Should a calorie deficit be entered as negative?

Yes. Enter deficits as negative values and surpluses as positive values.

Why might scale weight change faster in the first week?

Water, glycogen, sodium, and digestive contents can shift quickly. Those changes are not the same as a stable change in body tissue.

Does metabolism stay constant during the projection?

No. The simple model does not adjust for changing body weight, activity, or metabolic adaptation.

How can I improve the estimate?

Use a rolling average of measured weight and update the calorie-balance assumption periodically. Shorter recalibrated projections are generally more informative than one long forecast.