Calorie Deficit Calculator

A calorie deficit is the gap between the energy your body burns in a day and the energy you eat. Burn 2,400 kcal and eat 2,000 kcal, and you have a 400 kcal deficit. Your body covers that gap by drawing on stored fat, which is why a sustained calorie deficit is the one mechanism every effective weight-loss diet has in common — keto, intermittent fasting, low-carb, and portion control all work by creating one.

The size of the gap decides your pace. As a working rule, one pound of body fat stores about 3,500 kcal (roughly 7,700 kcal per kg), so a daily deficit of 500 kcal translates to about half a kilogram or one pound of fat loss per week. Double the deficit and the model says you lose weight twice as fast — though your body pushes back in ways the simple math does not capture.

That is exactly what the calculator above does: it estimates your maintenance calories, applies the deficit you choose (or works backward from a goal date), and converts the result into a daily calorie target, a weekly pace, and a realistic goal date.

A calorie deficit is calculated in three steps: estimate your Basal Metabolic Rate (BMR), scale it up to your total daily burn (TDEE), then subtract your chosen deficit.

Step 1: Calculate BMR (Mifflin-St Jeor Equation)

The Mifflin-St Jeor equation (1990) is the most accurate predictive formula for modern populations:

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

Step 2: Apply Your Activity Multiplier

Multiply BMR by the factor that matches your typical week to get your TDEE — the maintenance calories you burn per day:

Step 3: Subtract Your Deficit

Final Formula: Daily Calorie Target = TDEE − Calorie Deficit

Worked Example

A 35-year-old woman who is 5'4" (163 cm) and 165 lb (75 kg): BMR = (10 × 75) + (6.25 × 163) − (5 × 35) − 161 ≈ 1,430 kcal. She exercises about four days per week, so she uses the 1.55 multiplier: 1,430 × 1.55 ≈ 2,215 kcal to maintain.

She picks a moderate 500 kcal deficit: 2,215 − 500 = 1,715 kcal per day, a pace of roughly 0.45 kg (1 lb) per week. The calculator runs all three steps automatically — and you can cross-check the TDEE estimate with our TDEE Calculator or your resting baseline with the BMR Calculator.

The right calorie deficit to lose weight depends on how much you want to lose and how much hunger you are willing to manage. Most results fall into three lanes:

Two safety rails matter more than ambition. First, keep your daily intake above about 1,200 kcal for women and 1,500 kcal for men — below that it becomes very hard to get enough protein, vitamins, and minerals. Second, aim to lose no more than roughly 1% of your body weight per week; faster than that, a growing share of the loss comes from muscle rather than fat. The calculator flags any plan that crosses either line.

If you are not sure where to start, pick Moderate. You can always step up after two weeks; starting aggressive and failing is far more costly than starting moderate and adjusting.

As a planning average, yes — as a weekly guarantee, no. The rule comes from the fact that a pound of body fat stores about 3,500 kcal, so eating 500 kcal under maintenance for seven days "owes" 3,500 kcal, which the body covers from fat. Over a month or two, most people on a consistent 500 kcal deficit do land near a pound a week.

Three real-world effects bend the curve. Metabolic adaptation: as you get lighter, your BMR and daily burn fall, so yesterday's deficit quietly shrinks — the same 1,700 kcal intake produces slower loss at 175 lb than it did at 200 lb. Water swings: the first two weeks of a new deficit drop glycogen and its attached water, which makes the scale race ahead of the fat math. Adherence noise: a heavy dinner or a weekend away can erase a week's deficit on the scale while barely touching the trend.

Use the 3,500 rule to plan, then let your weekly average weight — not any single day — referee the estimate. When your loss stalls for two to three weeks, recalculate: our Maintenance Calorie Calculator shows your new, lower maintenance level.

Expect visible scale movement inside two weeks, but read it in two phases. Phase one (days 1-10): the deficit drains glycogen and releases its stored water, so the scale can drop 1-2 kg (2-4 lb) quickly — encouraging, but mostly water. Phase two (from week two onward): loss settles into the pace your deficit actually supports, about 0.45 kg (1 lb) per week on a moderate deficit.

From there, the timeline math is straightforward: 5 kg (10 lb) at a moderate pace takes about 10-12 weeks; 10 kg (20 lb) takes around 5 months. That is why the goal-date mode exists — if a date needs a pace you would not choose for yourself, pick a milder deficit and a later date instead. Judge progress by your weekly average, and expect individual weeks to swing plus or minus a kilogram around the trend.

A calorie deficit and intermittent fasting solve two different problems, which is exactly why they pair so well. The deficit decides how much you eat; the fasting window decides when. Neither substitutes for the other — you can eat your full daily target inside four hours or spread it across fourteen, and the energy math does not change.

The practical win is friction. Fitting 1,715 kcal into a 16:8 window usually means two or three real, satisfying meals instead of constant snacking decisions across a 14-hour day. Most people find their protein target easier to hit, their hunger concentrated in fewer episodes, and their evenings — the hardest hours — simply closed for business.

One caution: do not stack a long fast with an aggressive deficit when starting out. Begin with 16:8 and a mild-to-moderate deficit, keep protein near 1.6-2.2 g per kg of body weight, and adjust one variable at a time. Plan your eating window with the Intermittent Fasting Calculator, or learn the basics with our guide on how to start intermittent fasting.

BMR uses the Mifflin-St Jeor equation (American Journal of Clinical Nutrition, 1990), the most accurate predictive formula for modern populations, multiplied by standard activity factors from 1.2 (sedentary) to 1.9 (extra active). Weight-loss timelines use the standard energy model — 3,500 kcal per pound (7,700 kcal per kg) of body fat — the same convention behind the NIH Body Weight Planner research program, simplified to a straight-line projection.

Treat every result as an estimate within roughly ±10%. The straight-line model does not include metabolic adaptation, so real-world loss usually trails the projection slightly in the later weeks of a long plan — the projected goal date is best read as the earliest realistic date. Your scale remains the referee: weigh in weekly, and if your average stalls for two to three weeks, recalculate with your new weight.

This tool provides planning estimates only and is not medical advice. If you are pregnant, managing a medical condition, or have a history of disordered eating, work with a qualified professional instead of self-prescribing calorie targets.

Last updated: November 2026. Formulas follow the Mifflin-St Jeor publication (1990) and standard energy-expenditure practice.