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Calorie deficit — how to calculate it and how large it should be
A calorie deficit is the only mechanism by which the body loses fat tissue — everything else is a way of reaching one and staying there. Below: the formula, the numbers, and three things no calculator will tell you.
In this guide
What a deficit is, and what it is not
A deficit is the gap between the energy you spend and the energy you take in. Spend 2400 kcal, eat 1900, and the deficit is 500 kcal a day.
A rule of thumb says a kilogram of fat tissue is about 7700 kcal, so a 500 kcal daily deficit should give half a kilo a week. As a starting point that is fine, but as a forecast over several months it overshoots, and it is worth knowing why. As body mass falls, energy expenditure falls with it — a smaller body costs less at rest and less in motion. On top of that the body dials down spontaneous activity: less fidgeting, less walking for no reason, slower up the stairs. That is not willpower, it is regulation you cannot feel happening.
The practical consequence: a calorie target is not a number you set once. It is a number you recalculate every few kilograms.
Step 1: basal metabolic rate
BMR is the energy you burn lying still — heartbeat, breathing, holding your temperature. It is usually 60–70% of total daily expenditure. The equation that performs best across a general population is Mifflin-St Jeor:
Worked example: a woman, 32 years old, 168 cm, 70 kg.
The formula is a population estimate with a spread of roughly ±10% for any individual. At 1429 kcal that is about 140 kcal either way — so treat the steps that follow as a hypothesis to test, not as the result of a measurement.
Step 2: total daily energy expenditure
Multiply BMR by an activity factor to get TDEE:
| Lifestyle | Multiplier | What it means in practice |
|---|---|---|
| Sedentary | 1.2 | desk job, no training, under 5,000 steps |
| Lightly active | 1.375 | 1–3 sessions a week, or a lot of walking |
| Moderately active | 1.55 | 3–5 sessions a week |
| Very active | 1.725 | 6–7 sessions, or physical work |
| Extremely active | 1.9 | two sessions a day, competitive sport |
Our example trains three times a week and has a desk job, so 1.375:
The most common mistake here is an inflated multiplier. Three hours in the gym a week is about 900 kcal spread across seven days — under 130 kcal a day. If you are torn between two levels, take the lower one. Too high a multiplier gives you a target at which the scale does not move, and the impression that “the deficit is not working”.
Step 3: how large the deficit should be
Set the deficit as a percentage of TDEE, not as a fixed number — 500 kcal for someone spending 3200 is a completely different situation from 500 kcal for someone spending 1800.
| Deficit | Rate | Who it suits |
|---|---|---|
| 10% of TDEE | 0.25–0.4% of body mass / week | the last few kilos, protecting muscle |
| 15–20% of TDEE | 0.5–0.7% of body mass / week | the default for most people |
| 25% of TDEE | about 1% of body mass / week | higher starting mass, short blocks |
For our example, 20% of 1965 is 393 kcal, so a target of roughly 1570 kcal a day.
Why not go deeper
A very large deficit looks attractive on paper but costs you in three places at once: more of what you lose is muscle, training quality drops, and — most importantly — the odds of not finishing go up. A diet you abandon in week four loses to a slower one you run for six months.
The floor
Below roughly 1200 kcal for women and 1500 for men it becomes hard to cover protein, vitamin and mineral requirements from food alone. That is not a safety line carved in stone, but underneath it a diet belongs under supervision rather than under a calculator.
Why the scale does not fall in a line
This is where most people quit — usually needlessly, because the problem is how the measurement is read, not the diet.
Morning body mass is fat, muscle, bone, water, gut contents and glycogen. It swings 1–2 kg day to day for reasons that have nothing to do with energy balance:
- Glycogen. Each gram of glycogen binds about 3 g of water. A higher-carbohydrate day can add a kilo on the scale without adding a gram of fat.
- Sodium. A salty dinner holds water for a day or two.
- The menstrual cycle. Water swings in the luteal phase reach 1–2 kg.
- A new training stimulus. Inflammation after an unusual session is water in the muscle.
- Gut contents. Trivial, but real — a few hundred grams either way.
The practical conclusion: read the weekly average, not a single reading. Weigh yourself every morning, after the bathroom, before breakfast — and compare this week's average with last week's. The daily number is not there to judge the day. It is there so the average has something to be made of.
When and how to adjust the target
The formula gave you a hypothesis. Reality will test it in two or three weeks.
- Hold the target for two full weeks, logging everything. Anything shorter is too noisy to read.
- Compare weekly averages. If the drop sits between 0.5% and 0.7% of body mass per week, the target is right — leave it alone.
- If the scale has been flat for three weeks, take off 100–150 kcal or add movement. Do not cut 400 at once.
- If you are losing faster than 1% a week, add 100–150 kcal back. That is not success, it is a signal that muscle is going too.
- Recalculate TDEE every 4–5 kg. The equation for 70 kg no longer describes someone at 63 kg.
A calculator is a starting point, not an answer. The only number that genuinely describes your expenditure is the one you derive from your own data: what you ate and what the scale did. Two weeks of an honest diary are worth more than an arbitrarily precise formula.
Common questions
How do I calculate a calorie deficit?
Work out BMR with the Mifflin-St Jeor equation, multiply by an activity factor (1.2 for a desk job, 1.375 for 1–3 sessions a week, 1.55 for 3–5), then subtract 15–20%. For a woman of 32, 168 cm and 70 kg training three times a week, that comes to a target of roughly 1570 kcal a day.
How big should a calorie deficit be?
For most people 15–20% of total daily energy expenditure, which corresponds to losing 0.5–0.7% of body mass per week. A larger deficit buys speed at the cost of muscle mass, training quality and the odds of finishing the diet at all.
Why am I not losing weight in a deficit?
Usually one of three things: an inflated activity multiplier in the TDEE calculation, under-reported intake (missed drinks, cooking fats, snacks), or water fluctuations masking a real loss. Before changing the target, compare the weekly averages of two full weeks rather than individual readings.
Can you be in a deficit and not lose fat?
Not over any length of time — if the scale is flat for three weeks with consistent measurement, the deficit is not actually there. It usually means intake is higher than the log says, or expenditure is lower than the formula assumed. An honest diary settles both questions; another calculator will not.
How often should I weigh myself while cutting?
Every morning, after the bathroom, before food and drink. A single reading is not interpreted at all — it exists only to feed the weekly average, and it is averages that get compared with each other.
Work it out once, then let it run
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Open the app →This article is informational and is not medical advice. The formulas below are estimates for healthy adults. If you have a medical condition, take medication, are pregnant or are breastfeeding, set your targets with a clinician rather than a calculator.