“Eat less, move more” is true and useless. The question people actually need answered is how much less — because a deficit that is too small feels like nothing is happening, and one that is too large costs you muscle, energy and sleep, and rarely survives contact with a normal week. This article covers the arithmetic behind a calorie deficit, what a sensible size looks like, and the reasons the number on the scale moves slower than the maths predicts.
Quick answer: a calorie deficit is eating fewer calories than your body uses in a day. For most adults, a deficit of about 10–20% below maintenance — often in the region of 300–500 kcal a day — is the range that balances steady progress with something you can actually stick to. Work out your own maintenance figure first with the free Calorie Calculator.
1What a calorie deficit actually is
Your body burns energy every day: keeping you alive at rest, digesting food, and everything you do on top of that. Add it up and you get your maintenance calories, more formally your Total Daily Energy Expenditure or TDEE. Eat that amount and your weight holds steady. Eat consistently below it and your body makes up the difference from stored energy — that gap is the calorie deficit.
Two numbers matter, and they are easy to confuse. BMR (Basal Metabolic Rate) is what you would burn lying still all day — for most adults it is the largest single slice of the total. TDEE is BMR multiplied by an activity factor. Your deficit is always measured against TDEE, never against BMR. If that distinction is fuzzy, our guide to BMR vs TDEE unpacks it properly.
2How to find your own maintenance number
The Calorie Calculator uses the Mifflin–St Jeor equation, which is the formula most dietitians reach for because it estimates resting energy needs more accurately than the older Harris–Benedict equation for the general population:
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
That result is then multiplied by an activity factor from 1.2 (desk job, little exercise) up to 1.9 (physical job or twice-daily training). Take a 32-year-old woman, 165 cm and 72 kg, who trains lightly two or three times a week:
| Step | Working | Result |
|---|---|---|
| BMR | (10 × 72) + (6.25 × 165) − (5 × 32) − 161 | ≈ 1,430 kcal |
| TDEE (lightly active, ×1.375) | 1,430 × 1.375 | ≈ 1,967 kcal |
| 15% deficit | 1,967 × 0.85 | ≈ 1,672 kcal a day |
| 500 kcal deficit | 1,967 − 500 | ≈ 1,467 kcal a day |
So her honest range sits somewhere between roughly 1,470 and 1,670 calories a day. Not one magic number — a band, which is exactly how you should treat your own.
3How big should the deficit be?
The traditional rule of thumb says a pound of body fat holds about 3,500 kcal, so a 500 kcal daily deficit should cost roughly a pound (0.45 kg) a week. Use it to set a starting point, not as a promise:
| Daily deficit | Per week | Predicted loss | Realistic for |
|---|---|---|---|
| 250 kcal | 1,750 kcal | ≈ 0.2 kg (0.5 lb) | Already lean, or in no hurry; easiest to sustain |
| 500 kcal | 3,500 kcal | ≈ 0.45 kg (1 lb) | The standard starting point for most adults |
| 750 kcal | 5,250 kcal | ≈ 0.7 kg (1.5 lb) | Higher starting body weight, with good protein intake |
| 1,000 kcal | 7,000 kcal | ≈ 0.9 kg (2 lb) | The upper limit — hard to sustain and rarely necessary |
A percentage-based deficit scales better than a flat number, which is why the calculator uses a 15% cut for its weight-loss goal. Five hundred calories is a modest trim for someone maintaining on 3,200; for someone maintaining on 1,700 it is close to a third of their intake, and that is the kind of cut that leaves people cold, irritable and unable to train.
Very low intakes are not a shortcut. Eating below roughly 1,200 kcal a day (women) or 1,500 (men) makes it genuinely difficult to meet basic protein, vitamin and mineral needs, and diets at that level are normally used only with medical supervision. If your calculated target lands below those figures, increase your activity instead of cutting further, and speak to a doctor or a registered dietitian.
4Why the scale moves slower than the maths
The 3,500-calorie rule is a straight line drawn through a curved reality. Three things bend it:
- You get smaller as you lose. A lighter body costs less to run, so the same meals that once created a 500 kcal deficit slowly stop doing so. Recalculate every 4–6 kg of loss.
- You move less without noticing. In a deficit, unconscious activity — fidgeting, walking pace, standing up — tends to drop. It is one of the biggest reasons real-world results undershoot predictions.
- Water masks fat loss. Glycogen, salt, hormones and an unfamiliar training load can swing scale weight by a kilo or two in either direction within days. Weigh at the same time under the same conditions and judge the weekly average, not any single morning.
The practical answer to all three: track the trend over two to three weeks. If your weight has not moved at all in that window, trim about 10% from your intake or add activity — do not slash 500 more calories in a panic.
5Protecting muscle while you lose fat
A deficit costs you weight; how you run it decides how much of that weight is fat rather than muscle. Two things do most of the work.
Protein. The calculator scales its protein target with your activity level, from about 1.2 g per kg of body weight for sedentary people up to around 2.0 g per kg for very active ones. For the 72 kg woman above, that is roughly 86–144 g a day depending on how she trains. Higher protein in a deficit is also more filling, which quietly makes the deficit easier to hold.
Resistance training. Giving your muscles a reason to stay is the strongest signal you can send while eating less. Two or three sessions a week is enough to make a visible difference to how you look at the same body weight.
The rest of the plate should still follow ordinary healthy-eating guidance — the World Health Organization’s healthy diet advice puts adults at a minimum of 400 g of fruit and vegetables a day, at least 25 g of fibre, free sugars under 10% of energy and salt under 5 g. A deficit made of biscuits technically works and will make you feel terrible.
6Run your own numbers
The Calorie Calculator gives you BMR, TDEE, a target for weight loss, maintenance or muscle gain, and a macronutrient split in one screen. Pair it with the BMR & TDEE Calculator if you know your body-fat percentage — it adds the Katch-McArdle estimate, which is more accurate for lean or muscular bodies — and the BMI Calculator for a quick population-level check on where your weight sits. Everything runs in your browser; nothing you type is sent anywhere.
Educational information, not medical advice. Calorie equations are population estimates and can be off by 10% or more for any individual. If you are pregnant or breastfeeding, under 18, managing diabetes, thyroid or kidney conditions, taking medication that affects appetite or metabolism, or have a history of disordered eating, plan changes with a doctor or registered dietitian rather than a calculator.
7Frequently Asked Questions
How many calories should I eat to lose weight?
Start from your maintenance calories (TDEE) and subtract 10–20%, which for most adults works out somewhere between 300 and 500 kcal a day. Enter your age, sex, height, weight and activity level into the Calorie Calculator to get your own figure rather than using a generic 1,200 or 2,000.
Is a 500 calorie deficit enough to lose weight?
For most people, yes — it predicts roughly 0.45 kg (1 lb) a week. Actual loss is usually a little slower because your energy needs fall as you get lighter and unconscious daily movement tends to drop. Judge it over two to three weeks, not day to day.
Why am I in a calorie deficit but not losing weight?
The three usual reasons are under-reported intake (drinks, cooking oil, tastes while cooking), an overestimated activity level in the calculation, and normal water-weight fluctuation hiding real fat loss. Recheck your activity setting honestly, weigh under the same conditions, and compare weekly averages before changing anything.
What is the fastest safe rate of weight loss?
About 0.5–1 kg (1–2 lb) a week is the range generally considered sustainable for adults. Faster loss increases the share of muscle lost and is harder to maintain. The NHS healthy weight guidance is a good starting point, and larger or faster changes belong under medical supervision.
Do I need to count calories forever?
No. Most people count for a few weeks to learn what their usual portions actually contain, then move to a lighter approach — consistent meals, a protein target and a weekly weigh-in. The calculator is there to reset the target whenever your weight or activity changes noticeably.
