Calorie Deficit: How to Calculate It and Stick to It

A calorie deficit is the one non-negotiable mechanism of fat loss: body fat is only spent when the body takes in less energy than it uses. The interesting question was never whether that is true, but how big the gap should be and how anyone sustains it past week three. The working answer is 10–20 percent below maintenance — for most adults, 300 to 600 kcal a day.

Working out your maintenance calories#

The calculation has two stages: your resting burn, then a correction for how you actually live.

Stage one: resting metabolic rate#

Resting metabolism is what your organs cost to run while you do nothing at all. It accounts for 60–70 percent of daily expenditure, which is worth sitting with for a moment — most of your calorie budget is spent before you stand up. The most accurate of the simple equations is Mifflin–St Jeor:

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

The two halves are identical apart from the constant: add 5 for men, subtract 161 for women.

Worked example. A woman of 35, 72 kg (159 lb), 168 cm (5 ft 6 in): 10 × 72 + 6.25 × 168 − 5 × 35 − 161 = 720 + 1,050 − 175 − 161 = 1,434 kcal.

If you think in pounds and inches, convert first — divide pounds by 2.2, multiply inches by 2.54 — rather than hunting for an imperial version of the formula.

Stage two: the activity multiplier#

Activity level	Multiplier	Maintenance for the example above
Desk job, no training	1.2	1,721 kcal
Light activity, 1–3 sessions a week	1.375	1,972 kcal
Moderate, 3–5 sessions a week	1.55	2,223 kcal
High, 6–7 sessions a week	1.725	2,474 kcal
Very high, physical job plus training	1.9	2,725 kcal

Almost everyone overrates their own category. Three gym visits a week on top of a desk job and 5,000 steps a day is 1.375, not 1.55. When in doubt, pick the lower one. Adjusting upward once you see real data is trivial; working out why the scale has been stuck for a month is not.

Stage three: subtract the deficit#

Take 15–20 percent off. For the example at a 1.375 multiplier: 1,972 − 15% ≈ 1,675 kcal.

Deficit size	Weekly loss	How it feels	Muscle loss risk
10%	0.2–0.3 kg (0.5–0.7 lb)	Barely noticeable	Minimal
15–20%	0.4–0.7 kg (0.9–1.5 lb)	Comfortable — the sweet spot	Low if protein is adequate
25–30%	0.8–1.0 kg (1.8–2.2 lb)	Real hunger, energy dips	Moderate
Over 30%	1 kg (2.2 lb) and up	Rarely survives a month	High

The calculator below runs all three stages on your own numbers — and goes further than the arithmetic above, splitting the result into protein, fat and carbs and refusing to let the deficit fall below your basal rate.

About you

kg / cmlb / ft

Sex

MaleFemale

Age, years

Height, cm

Weight, kg

Body fat, %

Optional. If you know it from DEXA or a good scale, the estimate gets better

ActivitySedentary — desk job, little walkingLight — 1–3 workouts a weekModerate — 3–5 workoutsHigh — 6–7 workoutsVery high — athlete or physical job

Torn between two levels? Pick the lower one. Activity is what people overestimate most

Goal

Lose fatMaintainGain muscle

Rate

0.41 kg per week · 0.5% of body weight

Goal weight, kg

Optional. Shows how long it takes at this rate

Calories from fat: 28%

Carbs are whatever is left. Move it if you know what suits you

Your daily target

1 972kcal per day

Basal rate

1 763 kcal

Mifflin-St Jeor

Daily burn

2 423 kcal

× 1.375

Difference

−451 kcal

deficit

Protein, fat, carbs

Protein

158 g

32% · 634 kcal · 1.9 g/kg

Fat

63 g

29% · 570 kcal · 0.8 g/kg

Carbs

192 g

39% · 768 kcal · 2.3 g/kg

Fibre

28 g

14 g per 1000 kcal

Fluids

2.5–2.9 L per day

30–35 ml per kg

BMI

25.9

healthy for your height: 59–79 kg

Every formula here carries about a 10% error — that is 200–250 kcal. Use the number as a starting point: eat it for two weeks, watch the scale, and correct from what actually happened rather than from the maths.

Читайте также: Calorie and Macro Calculator: your daily target

The floor you should not go below#

For women, 1,200 kcal is the conventional lower bound; for men, 1,500 kcal, absent medical supervision. Below that it becomes genuinely difficult to hit protein, vitamin and mineral needs with ordinary food, however carefully you shop.

A second, more individual floor: do not sit below your resting metabolic rate for long stretches. The deficit properly comes out of total expenditure, not out of the number that keeps your organs running.

Very-low-calorie diets under 800 kcal do exist in clinical practice, used for a limited course under supervision in significant obesity, with monitoring and often meal replacements. That is a medical intervention with a defined endpoint, not something to improvise from a podcast.

Why a deficit "stops working"#

You are eating more than you recorded#

This is the first explanation to consider and usually the correct one. A classic New England Journal of Medicine study looked at people who believed themselves resistant to weight loss and reported eating about 1,200 kcal a day. Objective measurement put actual intake roughly 47 percent higher than reported, while physical activity was overestimated by about half again. The participants were not lying. They were sincerely, systematically wrong.

The usual leaks:

• Cooking oil — a tablespoon is 120 kcal, not "a splash"
• Sauces, dressings and mayonnaise — 150–250 kcal on a plate without effort
• Drinks: lattes, juice, alcohol
• Grazing while cooking, which almost never gets logged
• Weighing food raw and looking up the cooked entry, or the reverse
• Weekends, which quietly stop being recorded around Friday evening

A blunt diagnostic: if your tracker says you have run a 500 kcal deficit for a month and your weight has not moved, the deficit is not real. Physics is not the variable here.

Expenditure adapts downward#

A lighter body costs less to move. Every step, every flight of stairs is cheaper than it was 8 kg ago. On top of that, spontaneous movement falls — people in a deficit fidget less, stand up less, take the shorter route. This component can run to several hundred kilocalories a day and is essentially invisible from the inside.

Читайте также: Metabolism Myths: What Actually Speeds It Up

Water is masking the fat#

Scale weight is not fat. Glycogen binds water at roughly three grams of water per gram of glycogen, so a swing in carbohydrate intake moves the number 1–2 kg (2–4 lb) in either direction within days. Salt, stress, menstrual cycle phase and a hard session the day before all push it around. Only a rolling weekly average shows what is actually happening to your fat mass.

How to not quit#

Falling off the plan is not a character defect. It is the predictable output of a badly designed one. A few principles do most of the work.

Protein and food volume#

Protein produces the strongest satiety of any macronutrient and protects lean tissue while you are in a deficit. Aim for 1.6–1.8 g per kg of target body weight.

Читайте также: How Much Protein Do You Need Per Day?

Fibre and water add bulk without calories. A plate of vegetables before the main course is unglamorous and works — it displaces a meaningful share of the meal that follows.

Читайте также: Fiber: How Much You Need and Where to Get It

Ban nothing outright#

Forbidding a food raises its subjective value and turns a binge into a scheduling question rather than a possibility. The practical alternative is to budget for it: 200–300 kcal a day of whatever you actually want does not derail a deficit, and it removes the sense of deprivation that makes people abandon the whole project.

Sleep#

The most underrated variable in the list. Restricted sleep raises ghrelin, lowers leptin and sharply increases the pull of calorie-dense food. People sleeping under six hours eat more while reporting the same subjective sense of control — which is precisely what makes it dangerous.

Читайте также: Sleep Hygiene: 12 Rules That Actually Work

Lift something#

Without a training stimulus, a deficit takes muscle along with fat — up to a quarter of the weight lost. Two or three resistance sessions a week, combined with adequate protein, shift the balance toward fat and protect your resting metabolic rate for the maintenance phase that follows.

Читайте также: Strength Training for Beginners: Your First 3 Months

Diet breaks#

On a long weight-loss run, spending one or two weeks at maintenance every 8–12 weeks is worth building into the plan. It lowers the psychological load and partially restores the hormonal picture. Such breaks do not harm the final outcome, and they measurably improve the odds of reaching it rather than abandoning the attempt in month four.

Tracking progress properly#

Weigh daily under identical conditions — morning, after the bathroom, before eating — and use the weekly average. Compare week to week, never day to day. Individual mornings are noise wearing the costume of data.

Add slower-moving measures: waist circumference every two weeks, photographs monthly in the same light and clothing, and how your clothes fit. In people who train, the scale can sit still for weeks while body composition changes visibly.

Target pace is 0.5–1 percent of body weight per week. At 80 kg (176 lb) that is 400–800 g, roughly 1–1.75 lb. Slower is perfectly acceptable. Consistently faster over months is a signal to shrink the deficit.

What to do in practice#

1. Run the equation, apply your activity multiplier, subtract 15–20 percent.
2. Weigh everything for two weeks. This calibrates your eye — it is not a life sentence.
3. Set a protein target of 1.6–1.8 g per kg of goal weight and let the remaining calories fall where they like.
4. Track the weekly average, not the daily number.
5. Recalculate maintenance every 5 kg (11 lb) lost.
6. Plan the exit before you need it: return to maintenance gradually, adding 100–150 kcal a week.

A calorie deficit is not a diet with a start and a finish. It is a temporary state you need an exit strategy from. A regimen you would not be willing to run for six months is unlikely to produce a durable result, however precise the arithmetic behind it.

Point six is the one that decides whether any of this lasts, and it is a separate skill rather than a longer version of the deficit. What people who keep the weight off actually do — and what the biology pushes back with — is worth reading before you finish losing, not after.

Читайте также: Keeping Weight Off: What Successful Maintainers Do