Calorie Deficit to Weight Loss Timeline

Estimate how long it takes to lose a target amount of weight from a given daily calorie deficit, using the 3,500-calories-per-pound rule.

Inputs

How many calories below maintenance you eat each day.

Amount of weight you want to lose.

Unit for the target weight.

Result

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How to use this calculator

  • Enter your daily calorie deficit (calories eaten below maintenance).
  • Enter the amount of weight you want to lose and its unit.
  • Read the estimated weeks and days to reach your goal.
  • Check the pace indicator to keep within a safe weekly rate.

About this calculator

Weight loss comes down to an energy deficit: eating fewer calories than your body burns. A long-standing rule of thumb, attributed to Max Wishnofsky, holds that a pound of body fat stores roughly 3,500 calories (about 7,700 calories per kilogram), so a sustained daily deficit predicts how fast you will lose. This calculator multiplies your target weight loss by that energy density to find the total deficit needed, then divides by your daily deficit to estimate the days and weeks to your goal. It also checks your implied weekly rate against the widely recommended safe range of one to two pounds (about half a kilogram to one kilogram) per week. Remember the rule is a simplification — real loss slows as your body adapts, and the figure is best treated as a planning estimate rather than a precise schedule.

How it works — the formula

Total deficit = Weight to lose × (3,500 kcal/lb or 7,700 kcal/kg) Days = Total deficit ÷ Daily deficit Weekly rate = Daily deficit × 7 ÷ kcal per unit

The energy stored in the target weight divided by the daily energy gap gives the time; the rule treats fat as a fixed energy store.

Source: CDC — Losing Weight

Worked examples

Example 1
500 kcal/day deficit, lose 20 lb
Inputs:
deficit=500, target=20, unit=lb
Output:
70,000 kcal ÷ 500 = 140 days (20 weeks)
Example 2
750 kcal/day, lose 10 kg
Inputs:
deficit=750, target=10, unit=kg
Output:
77,000 ÷ 750 ≈ 103 days
Example 3
1,000 kcal/day, lose 15 lb
Inputs:
deficit=1000, target=15, unit=lb
Output:
52,500 ÷ 1,000 ≈ 52.5 days

Limitations

  • Linear model; ignores metabolic adaptation that slows real loss.
  • Does not distinguish fat, muscle, or water weight.
  • Not individualized — true maintenance calories vary by person.

Educational estimate, not medical advice; consult a clinician for a weight-loss plan.

Frequently asked

How many calories are in a pound of fat?+
About 3,500 calories, per the classic Wishnofsky rule (roughly 7,700 calories per kilogram). So a cumulative 3,500-calorie deficit corresponds to losing about one pound of body fat in this simplified model.
How big should my daily deficit be?+
A deficit of about 500 calories a day yields roughly one pound of loss per week, a commonly recommended safe pace. Larger deficits lose faster but are harder to sustain and risk muscle loss and nutrient shortfalls.
Why does real weight loss slow down?+
As you lose weight, your body needs fewer calories, so the same intake produces a smaller deficit (metabolic adaptation). Water-weight shifts, hormonal changes, and reduced non-exercise movement also blunt the simple linear prediction.
Is the 3,500-calorie rule accurate?+
It is a useful approximation for short-term planning but overstates long-term loss because it ignores adaptation. Modern models (like the NIH Body Weight Planner) predict a slower, curving trajectory. Treat this as a rough estimate.
What is a safe rate of weight loss?+
Most health authorities suggest one to two pounds (about 0.5–1 kg) per week. Faster loss can be appropriate under medical supervision but raises the risk of muscle loss, gallstones, and rebound weight gain.
Does exercise count toward the deficit?+
Yes. Your total daily deficit is the gap between calories burned (including exercise) and calories eaten. Burning more through activity or eating less both widen the deficit and shorten the timeline — though appetite often rises to partly offset exercise.

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