Calorie Deficit Calculator
Your daily calorie target — and a timeline that tells you the truth.
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Enter your body stats to get your maintenance calories (TDEE), a daily calorie target at three deficit levels, your protein/carb/fat targets, and a week-by-week projection to your goal weight. Unlike calculators that divide your weight loss by a fixed 7,700 kcal per kilo, this one recalculates your metabolism at every step — so the timeline it shows you is one you can actually hit.
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Quick Answer
To calculate your calorie deficit: find your BMR with the Mifflin-St Jeor equation, multiply it by your activity factor (1.2–1.9) to get your TDEE, then subtract 250–750 kcal. A 500 kcal daily deficit is the standard recommendation. Enter your details above and the calculator returns your exact target, your macros, and a realistic timeline in about 45 seconds.
Shortcut
One caveat worth knowing before you start: a 500 kcal deficit does not produce 0.45kg of loss per week indefinitely. That figure is only true in week one. Your maintenance calories fall as your body mass falls, so the same daily intake creates a progressively smaller deficit — which is why this calculator projects your timeline week by week instead of dividing once.
How the Calculator Works
The Formulas Behind It
Every number this calculator shows you, and where it comes from
Formula
Step 1 — BMR (Mifflin-St Jeor): Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5 Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161 Step 2 — TDEE: TDEE = BMR × Activity Multiplier (1.2–1.9) Step 3 — Daily Target: Target = TDEE − Deficit (250 / 500 / 750 kcal) Step 4 — Timeline (iterated weekly, not divided once): For each week: TDEE_week = BMR(current weight) × Multiplier × (1 − Adaptation) Weekly loss = (TDEE_week − Intake) × 7 ÷ 7,700
Variables
Basal Metabolic Rate
The calories your body burns at complete rest to keep your heart beating, lungs working, brain running, and temperature stable. BMR is typically 60–75% of total daily expenditure. We use Mifflin-St Jeor because Frankenfield et al. (2005) compared five prediction equations across ten studies and found it accurate within 10% for the largest share of subjects — it replaced Harris-Benedict as the Academy of Nutrition and Dietetics' preferred equation.
Total Daily Energy Expenditure
Your full daily burn: BMR plus movement, exercise, and the thermic effect of food. This is your maintenance level — eat this and your weight holds steady. The activity multipliers are: sedentary ×1.2, lightly active ×1.375, moderately active ×1.55, very active ×1.725, extremely active ×1.9. Most people overestimate here; if you have a desk job and train three times a week, you are moderately active, not very active.
Energy Density of Body Fat
The energy stored in a kilogram of adipose tissue (3,500 kcal per pound), from Wishnofsky's 1958 work. The number itself is sound. The error most calculators make is treating it as a fixed divisor — assuming your deficit stays constant for the whole journey. It does not, which is what the adaptation term below corrects for.
Adaptive Thermogenesis Allowance
Sustained restriction reduces energy expenditure beyond what the weight loss alone predicts. Leibel et al. (1995) and Rosenbaum et al. (2010) measured reductions of roughly 10–15% after a 10% weight loss. We scale this allowance to the relative size of your chosen deficit and cap it at 10%, then apply it to each projected week — so a larger deficit carries a larger adaptation penalty, which is why aggressive deficits do not shorten your timeline proportionally.
Macronutrient Targets
Protein is set at 1.6g per kg of your goal weight — the level Morton et al. (2018) associated with preserving lean mass — and anchored to goal rather than current weight so people carrying substantial fat mass are not prescribed protein for tissue they intend to lose. Fat is 28% of total calories (protein 4 kcal/g, fat 9 kcal/g, carbohydrate 4 kcal/g), staying clear of the sub-15% range linked to hormonal disruption. Carbohydrates take the remaining calories.
Note: Mifflin-St Jeor carries roughly ±10% error for most adults, and more if your muscle mass is unusually high or low. Treat your result as a starting point: track for 2–3 weeks, compare your actual trend against the projection, and adjust intake by 100 kcal if they disagree. The calculator flags any target below 1,500 kcal/day for men or 1,200 kcal/day for women.
Worked Example: 80kg to 70kg
30-year-old man, 175cm, moderately active — and why his timeline is 38 weeks, not 22
Calculate BMR (Mifflin-St Jeor, male)
(10 × 80) + (6.25 × 175) − (5 × 30) + 5 = 800 + 1,093.75 − 150 + 5 = 1,749 kcal/day
Apply the activity multiplier (moderately active)
1,749 × 1.55 = 2,710 kcal/day maintenance (TDEE)
Apply a moderate deficit
2,710 − 500 = 2,210 kcal/day — this is the intake he holds constant
What the flat rule predicts
500 × 7 ÷ 7,700 = 0.45 kg/week → 10 ÷ 0.45 = 22 weeks
What the week-by-week projection predicts
At 80kg his deficit is the full 500 kcal. By 75kg his BMR has fallen ~50 kcal and a 4.6% adaptation allowance applies, so his real deficit is nearer 330 kcal. Recalculating each week gives 38 weeks.
Macro targets at 2,210 kcal
Protein 70 × 1.6 = 112g (448 kcal) · Fat 2,210 × 28% ÷ 9 = 69g (621 kcal) · Carbs (2,210 − 448 − 621) ÷ 4 = 285g
Read the milestones, not just the endpoint
The first 2.5kg arrives faster than the last 2.5kg. The projection spaces his milestones accordingly, so a slowdown at month four reads as expected rather than as failure.
Reference Guide
| unit | value | note |
|---|---|---|
| Maintenance (TDEE) | 2,710 kcal/day | Neither loss nor gain at this intake |
| Mild deficit (−250) | 2,460 kcal/day | ~120 weeks realistic · flat rule claims 44 |
| Moderate deficit (−500) | 2,210 kcal/day | ~38 weeks realistic · flat rule claims 22 |
| Aggressive deficit (−750) | 1,960 kcal/day | ~23 weeks realistic · flat rule claims 15 |
| Protein target | 112 g/day | 1.6g per kg of 70kg goal weight |
| Split across 3 meals | 737 kcal/meal | 2,210 ÷ 3 |
Which Deficit Level to Pick
The fastest deficit on paper is rarely the fastest in practice
Barely noticeable day to day: you can eat out, train hard, and stay social without much friction. The trade-off is real, though — because a small deficit is easily erased by falling maintenance calories, mild deficits can stall well before your goal on a long timeline. Best used to finish the last few kilos, or to lose weight while prioritising training performance.
Best for: The final 3–5kg, athletes in season, and anyone whose previous attempts collapsed under stricter diets.
The default recommendation, and the level most people should start at. Large enough to keep producing visible progress as your metabolism adjusts, small enough that hunger stays manageable and muscle is preserved when protein is adequate. Expect roughly 0.45kg per week early on, tapering as you get closer to goal.
Best for: Most people, most of the time. Start here unless you have a specific reason not to.
Faster, but with a sharper adaptation penalty, more hunger, worse training performance, and a higher risk of losing muscle alongside fat. Note what the calculator shows: going from a 500 to a 750 kcal deficit does not cut your timeline by a third, because the larger deficit triggers a larger metabolic response. Only worth it with high protein, resistance training, and a defined end date.
Best for: 10kg or more to lose with a hard deadline, combined with lifting and 2g/kg protein — not as an indefinite default.
The calculator flags this because it is where the trade-offs stop being about speed. Meeting your micronutrient needs becomes very difficult, metabolic downregulation accelerates, and lean-mass loss climbs. Very-low-calorie diets have a legitimate clinical role, but they belong under medical supervision rather than a web calculator.
Best for: Not recommended without supervision from a doctor or registered dietitian.
Why Our Timeline Differs From Other Calculators
If you have used two or three calorie deficit calculators, you have probably noticed they broadly agree on your daily calorie target and then disagree wildly on how long your weight loss will take. The daily target is the easy part — most tools use the same Mifflin-St Jeor equation and the same activity multipliers, so they land within a few dozen calories of each other. The timeline is where they diverge, and where most of them are wrong in the same direction. The usual approach divides the weight you want to lose by a fixed weekly rate derived from the energy density of body fat: 500 kcal a day is 3,500 kcal a week, which is about a pound of fat, so a pound a week. That arithmetic is fine for the first week. It fails over months because it assumes maintenance calories are a constant. They are not. A lighter body costs less energy to run, so your BMR falls as you lose — and sustained restriction reduces expenditure further still, beyond what the weight change alone accounts for. Thomas et al. put this plainly in the International Journal of Obesity in 2013, concluding that the 3,500 kcal rule grossly overestimates actual weight loss when checked against supervised trials where intake was genuinely controlled. Hall and Chow responded in the same journal that year, identifying the specific flaw: the rule's failure to model the dynamic changes in energy balance that occur during a diet. The practical consequence is a promise the body cannot keep — a calculator tells someone twelve weeks, their progress slows at week six exactly as physiology predicts, and they conclude the diet has stopped working. This calculator handles the timeline differently. Rather than dividing once, it steps forward a week at a time: recalculating your BMR at your new body weight, holding your food intake fixed, applying an adaptive-thermogenesis allowance scaled to your deficit, and accumulating the loss week by week until you reach your goal or the projection plateaus. It also shows you the flat-rule figure alongside its own, because the gap between the two is genuinely useful information — it is the difference between the timeline you were promised elsewhere and the one your metabolism will actually deliver. Two honest limitations. First, this is still an estimate built on a population-average BMR equation, not a metabolic ward measurement; treat it as a well-reasoned starting point and let two to three weeks of your own tracking data refine it. Second, adaptive thermogenesis varies considerably between individuals, so we deliberately use a conservative, capped allowance rather than pretending to model your personal response precisely.
Key Features
💡 Pro Tips
- →Weigh yourself daily but only judge the 7-day average. Day-to-day scale weight swings 1–2kg on water, sodium, and digestive contents alone. The average removes that noise; a single morning's number tells you almost nothing.
- →Re-run this calculator after every 5kg lost. Your TDEE has genuinely fallen by then, so the target that was creating a 500 kcal deficit at your starting weight is creating noticeably less. This single habit prevents most 'my diet stopped working' plateaus.
- →Ignore the first fortnight's rate entirely. Glycogen holds roughly three times its weight in water, so cutting carbohydrates dumps water fast. It looks like spectacular progress and then it looks like a stall. Neither is the real trend, which only becomes visible from week three.
- →If you only track one thing, track protein. It is the variable that decides whether the weight you lose comes mostly from fat or partly from muscle, and it has the highest satiety per calorie of the three macros. Calories decide how much you lose; protein decides what you lose.
- →Weigh your cooking oil. A tablespoon is around 120 kcal and it is the single most commonly untracked item in home cooking — three tablespoons across a day can quietly erase a moderate deficit before you have counted anything else.
- →Consider a planned maintenance break every 6–8 weeks rather than dieting straight through. Byrne et al. (2018) found intermittent restriction produced greater fat loss than continuous restriction over the same period, with better preservation of metabolic rate.
Common Mistakes
Trusting a linear timeline and quitting when reality diverges
This is the mistake the calculator is built to prevent. A flat-rule estimate of 22 weeks for 10kg sets you up to conclude something is broken around week eight, when in fact your slowing progress is exactly what a falling TDEE predicts. Expect the taper, plan for it, and recalculate rather than abandoning the deficit.
Overstating your activity level
Selecting 'very active' (×1.725) instead of 'moderately active' (×1.55) adds roughly 300 kcal to a typical TDEE — which can wipe out a moderate deficit entirely. Desk job plus three or four gym sessions is moderately active. Very active means hard training six or seven days a week, or physical work on top of it.
Eating back everything your watch says you burned
Consumer trackers overestimate exercise energy expenditure substantially, with errors varying widely by activity and device. If your watch credits a 45-minute run at 600 kcal and you eat all of it, you may have cancelled your deficit. Eat back at most half, then let your weekly weight trend tell you whether that was right.
Under-tracking liquids and condiments
Self-reported intake is systematically lower than actual intake — Lichtman et al. (1992) documented substantial underreporting even among people trying to report accurately. Coffee drinks, alcohol, dressings, and sauces are the usual culprits because they do not register as 'food'. Weighing everything for the first two to four weeks calibrates your estimates for good.
Starting at an aggressive deficit to 'get it over with'
A 750 kcal deficit abandoned after ten days produces nothing. A 400 kcal deficit held for six months produces a transformation. Adherence dominates every other variable in weight loss, including deficit size, macro ratios, and meal timing — and the calculator's own timelines show the aggressive option returns less time saved than its extra difficulty suggests.
Setting a goal weight without checking what it requires
A goal weight is a number; the deficit needed to reach it by a given date may be one that falls below your safe minimum. Enter your goal here first and read the flagged warnings — adjusting the deadline is almost always better than adopting an unsafe intake.
Research & Citations
All factual claims on this page are sourced from peer-reviewed research
- [1]
Mifflin, M.D., St Jeor, S.T., Hill, L.A., Scott, B.J., Daugherty, S.A., Koh, Y.O. (1990). A new predictive equation for resting energy expenditure in healthy individuals. The American Journal of Clinical Nutrition, 51(2), pp. 241–247.
Source of the BMR equation implemented in this calculator
View source - [2]
Frankenfield, D., Roth-Yousey, L., Compher, C. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 105(5), pp. 775–789.
Systematic comparison that established Mifflin-St Jeor as the preferred BMR equation over Harris-Benedict
View source - [3]
Thomas, D.M., Martin, C.K., Lettieri, S., Bredlau, C., Kaiser, K., Church, T., Bouchard, C., Heymsfield, S.B. (2013). Can a weight loss of one pound a week be achieved with a 3500-kcal deficit? Commentary on a commonly accepted rule. International Journal of Obesity, 37(12), pp. 1611–1613.
Demonstrates that the flat 3,500 kcal rule grossly overestimates actual weight loss — the basis for our adaptive timeline
View source - [4]
Hall, K.D., Chow, C.C. (2013). Why is the 3500 kcal per pound weight loss rule wrong?. International Journal of Obesity, 37(12), pp. 1614.
Identifies failure to model dynamic energy balance as the rule's central error
View source - [5]
Hall, K.D., Sacks, G., Chandramohan, D., Chow, C.C., Wang, Y.C., Gortmaker, S.L., Swinburn, B.A. (2011). Quantification of the effect of energy imbalance on bodyweight. The Lancet, 378(9793), pp. 826–837.
Validated dynamic model of weight change underpinning the iterative projection approach
View source - [6]
Rosenbaum, M., Hirsch, J., Gallagher, D.A., Leibel, R.L. (2008). Long-term persistence of adaptive thermogenesis in subjects who have maintained a reduced body weight. The American Journal of Clinical Nutrition, 88(4), pp. 906–912.
Quantifies the adaptive-thermogenesis reduction our timeline allowance is calibrated against
View source - [7]
Morton, R.W., Murphy, K.T., McKellar, S.R., Schoenfeld, B.J., Henselmans, M., Helms, E., Aragon, A.A., Devries, M.C., Banfield, L., Krieger, J.W., Phillips, S.M. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine, 52(6), pp. 376–384.
Basis for the 1.6g/kg protein target used in the macro calculation
View source - [8]
Byrne, N.M., Sainsbury, A., King, N.A., Hills, A.P., Wood, R.E. (2018). Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. International Journal of Obesity, 42(2), pp. 129–138.
Evidence for planned maintenance breaks preserving metabolic rate during extended deficits
View source
This calculator is a reference tool and does not constitute medical advice. For personalised sleep health guidance, consult a qualified healthcare provider.
Last updated: September 16, 2026
Creators
Computer Scientist & Health Tools Developer
Reviewers
Reviewer — Health & Science
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Frequently Asked Questions
How do I calculate my calorie deficit?
Calculate your BMR with Mifflin-St Jeor — for men, (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5; for women the same but −161 instead of +5. Multiply that by your activity factor (1.2 sedentary to 1.9 extremely active) to get your TDEE, then subtract 250–750 kcal. Entering your details above does all of this and adds your macros and a week-by-week timeline.
Why does this calculator show a longer timeline than others?
Because most calculators divide your target weight loss by a fixed weekly rate, which assumes your maintenance calories never change. They fall as you lose weight, and sustained restriction lowers expenditure further. This tool recalculates your BMR for each projected week and applies an adaptive-thermogenesis allowance, then shows both figures side by side so you can see the difference. Thomas et al. (2013) found the flat rule grossly overestimates real-world loss.
Is a 500 calorie deficit enough to lose weight?
Yes, and it is the level most evidence supports for sustainable loss — roughly 0.45kg in the first week, tapering as your metabolism adjusts. It is large enough to keep producing progress without the hunger, performance loss, and muscle-loss risk of steeper cuts. The calculator shows what a 500 kcal deficit means for your specific body and goal, including where it might plateau.
Should my protein target use my current weight or my goal weight?
This calculator uses your goal weight at 1.6g per kg, so you are not prescribed protein for mass you intend to lose — which matters most for people carrying substantial fat mass. If you are lifting weights or already lean, scaling to current weight at 1.6–2.2g/kg is a reasonable alternative and will give you a higher target.
What does it mean if my result says the deficit plateaus before my goal?
It means that at the intake implied by that deficit, your maintenance calories would fall to meet your intake before you reach your goal weight, and loss would stop at the weight shown. The fix is not to eat less indefinitely — it is to recalculate at your new weight partway through, or choose a slightly larger deficit or more activity, both of which the calculator lets you test instantly.
How accurate is this calorie deficit calculator?
The Mifflin-St Jeor equation carries about ±10% error for typical adults, wider if your muscle mass is unusually high or low, and individual adaptive responses vary — so treat every figure as a well-founded starting estimate rather than a measurement. Track your intake and weight for two to three weeks, compare your actual trend against the projection, and adjust intake by 100 kcal in the indicated direction.
Can I use this calculator to gain weight instead?
No — it projects weight loss only and will tell you if your goal weight is above your current weight. Gaining requires a calorie surplus and a different rate model, since the energy cost of adding lean mass differs from the energy released by losing fat.
Is it safe to use this if I have a medical condition?
Treat the output as general information, not clinical guidance. The calculator applies population-average equations and does not account for pregnancy or breastfeeding, thyroid disorders, diabetes, medications affecting metabolism or appetite, or a history of disordered eating. If any of these apply, work from your doctor's or dietitian's numbers rather than these.