Calorie Deficit Calculator

Your total daily energy expenditure (TDEE) = BMR × activity factor. BMR is calories your body burns at complete rest (this calculator uses the Mifflin-St Jeor equation — the most accurate of the published formulas for the general population). Activity factor accounts for movement, exercise, and the thermic effect of food.

1 lb of body fat stores roughly 3,500 kcal of usable energy. To lose 1 lb of fat per week you need an average daily deficit of 500 kcal (3,500 ÷ 7). Half a pound per week = 250 kcal/day. The math is identical whether you create the deficit by eating less, moving more, or both.

Cutting more than 1,000 kcal/day below TDEE accelerates the loss but also accelerates the downsides: muscle loss, fatigue, hunger that breaks adherence, hormonal disruption, and metabolic adaptation (your TDEE drops, shrinking the deficit you thought you had). Most evidence-based protocols cap weekly loss at ~1% of body weight.

Hard floors: never eat below about 1,500 kcal/day for men or 1,200 kcal/day for women without medical supervision. Below those numbers it's hard to get enough protein and micronutrients, and the body shifts into a defensive metabolic state that makes long-term loss harder, not easier.

Prioritize protein. Aim for 0.7–1.0 g per pound of goal body weight. Protein is the most satiating macro, has the highest thermic effect (you burn ~25% of its calories digesting it), and preserves muscle while you lose fat.

Add steps before adding cardio. Walking 8–10k steps/day quietly adds 200–400 kcal of expenditure without the appetite spike that hard cardio causes. Strength training 2–4×/week preserves the muscle that keeps your BMR up as you lose weight.

Track the average, not the day. Bodyweight fluctuates 2–5 lb day-to-day from sodium and glycogen. Use a 7-day moving average to see real trend, and reassess your TDEE every 10–15 lb lost — the deficit shrinks as you get smaller.

The calculator opens with a 32-year-old woman, 170 lb, 5'6" (66 in), lightly active. Converting: 170 lb = 77.1 kg, 66 in = 167.6 cm. Mifflin-St Jeor for women is (10 × kg) + (6.25 × cm) − (5 × age) − 161, so (10 × 77.1) + (6.25 × 167.6) − (5 × 32) − 161 = 771 + 1,047.5 − 160 − 161 = 1,497 kcal BMR.

Multiplying by the light-activity factor of 1.375 gives a TDEE of about 2,059 kcal. That is maintenance — the intake at which weight holds steady. Choosing the recommended 0.5 lb/week pace sets a 250 kcal daily deficit, for an intake target of roughly 1,809 kcal. A 20 lb goal at that pace takes 40 weeks.

Switching to the 1 lb/week pace doubles the deficit to 500 kcal, drops intake to about 1,559 kcal, and cuts the timeline to 20 weeks. Both targets stay above the 1,200 kcal floor for women, so neither triggers the warning. A 1.5 lb/week pace would push intake to 1,309 kcal — still above the floor but close enough that hitting protein and micronutrient targets becomes genuinely difficult.

Note what happens as the weight comes off. At 150 lb the same woman's BMR drops to about 1,406 kcal and her TDEE to 1,933 — roughly 125 fewer calories per day than at the start. Her original 1,809 kcal target now represents a deficit of only 124 kcal, or a quarter pound a week. This is why weight loss stalls without recalculation, and it is not a metabolic defect: a smaller body simply costs less to run.

The '3,500 kcal equals one pound' rule comes from a 1958 estimate of the energy stored in a pound of adipose tissue, and as a rule of thumb for a few weeks it works well. Extended over months it systematically overpredicts, because it treats your energy expenditure as a fixed number when it is actually a moving target that falls as you lose weight.

Three things shrink the deficit you thought you had. Your BMR drops because there is less tissue to maintain — roughly 6–8 kcal per pound lost. The energy cost of movement drops for the same reason; carrying 20 fewer pounds up the same flight of stairs is cheaper work. And non-exercise activity thermogenesis tends to fall unconsciously during a deficit as you fidget less, take the elevator, and move with less urgency, which can account for a further 100–300 kcal a day.

Researchers at the NIH modeled this and produced a more realistic long-run rule: a sustained deficit of about 10 kcal per day produces roughly one pound of eventual loss, with half of it arriving in the first year and the plateau approached over about three years. A 500 kcal daily deficit therefore trends toward about 50 lb of eventual loss, not the 52 lb per year that naive 3,500-kcal arithmetic predicts indefinitely.

The practical takeaway is not that the calculator is wrong — the linear estimate is accurate over the 8–12 week horizon most people plan in — but that you should recalculate every 10–15 lb rather than assuming the original target carries you to the finish. Expect the timeline to run somewhat longer than the initial 'weeks to goal' figure, and treat a slowdown as arithmetic rather than failure.

The Mifflin-St Jeor equation was validated in generally healthy adults, and the deficit guidance assumes an uncomplicated weight-loss goal. Several groups need individualized advice rather than a formula: anyone who is pregnant or breastfeeding, where energy needs rise by roughly 340–450 kcal a day and a deficit is contraindicated; children and adolescents, who need growth-appropriate intake and pediatric supervision; and adults over about 70, where preserving muscle mass usually matters more than losing fat.

It is also unsuitable for anyone with a history of disordered eating. Calorie targets and daily tracking are known triggers for restrictive patterns, and for these individuals the risk of the tool outweighs its usefulness. If tracking numbers has previously escalated into restriction, work with a clinician rather than an equation.

Medical conditions that shift the math include untreated or newly treated thyroid disease, type 1 and insulin-dependent type 2 diabetes (where intake changes require medication adjustment), chronic kidney disease with protein restrictions, and recent bariatric surgery. Several common medications — corticosteroids, some antipsychotics, insulin, and beta blockers — alter appetite or expenditure enough that the formula's estimate drifts.

Finally, the equation estimates. Measured resting metabolic rate in a lab varies from the Mifflin-St Jeor prediction by around 10% in either direction for a meaningful minority of people, and the activity multiplier is the cruder half of the calculation. Treat the output as a starting hypothesis, track actual results for two to three weeks, and adjust the target based on what your body does rather than what the formula predicted.