How to calculate a calorie deficit

Calculating a deficit takes three steps: work out what you burn at rest, multiply for how much you move, then subtract. Everything below is in kcal, which is the same unit your food label calls a Calorie. Here it is end to end.

If you would rather skip the arithmetic, the deficit calculator does all of this and adds a finish date.

Step 1 — Resting burn

Use the Mifflin–St Jeor equation, which is what most clinicians and apps use:

Men:   BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

In imperial, convert first: kg = lb ÷ 2.205, cm = inches × 2.54.

Example — a 35-year-old woman, 160 lb (72.6 kg), 5’5” (165 cm):

10 × 72.6  = 726
6.25 × 165 = 1,031
5 × 35     = 175
726 + 1,031 − 175 − 161 = 1,421 kcal/day

Step 2 — Total burn

Multiply the resting figure by an activity factor:

Lifestyle	Factor
Desk job, little movement	1.2
Light activity, 1–3 sessions a week	1.375
Moderate, 3–5 sessions	1.55
Heavy, 6–7 sessions	1.725
Physical job or two sessions a day	1.9

Continuing the example, lightly active:

1,421.6 × 1.375 ≈ 1,955 kcal/day maintenance

Pick the lower of two options when you are unsure. Overestimating activity is the most common error in this entire calculation, and it inflates maintenance by hundreds of calories.

Step 3 — Subtract

Choose a deficit of 15–25%, or work backwards from a target rate using 3,500 kcal per pound (7,700 per kg):

Target rate	Daily deficit
0.5 lb / week	250 kcal
1 lb / week	500 kcal
1.5 lb / week	750 kcal
2 lb / week	1,000 kcal

For our example at 1 lb a week:

1,955 − 500 = 1,455 kcal/day to eat

And the timeline for a 20 lb goal:

20 lb × 3,500 = 70,000 kcal
70,000 ÷ 500  = 140 days ≈ 20 weeks

The check that matters more than the formula

Everything above is an estimate stacked on an estimate — which is also why two calculators rarely agree. Mifflin–St Jeor carries roughly ±10%, and the activity factor is blunt. The calculated number is a starting hypothesis, not a measurement.

So test it against reality:

Hold the calculated intake for three weeks.
Weigh daily, compare weekly averages.
Lost about as predicted? The estimate was good.
Lost less? Your real maintenance is lower — subtract another 100–150 kcal.
Lost more? You can eat a little more, which is usually the wiser choice.

That feedback loop beats any formula, because it uses your actual body rather than a population average. If three weeks pass with nothing at all, the usual causes are worth working through before you cut further.

Keeping it current

A deficit calculated today decays. Lose 10 lb and your resting burn falls by roughly 50 kcal a day; incidental movement tends to drop too. Recalculate every 10 lb, or let something track the real average for you — which is what Caldown does, recomputing the finish date from your measured deficit rather than the one you assumed at setup.