What is a calorie deficit, and how do I start one?
A calorie deficit means eating fewer calories than your body burns in a day. When you do that, your body makes up the shortfall from stored fat, and you lose weight. That is the whole mechanism behind every diet, whatever it calls itself. To start one, you work out roughly how many calories you burn a day (your maintenance number), then eat a bit less than that. A modest deficit of around 300 to 500 calories a day is the sweet spot for most people. Here is how to find your number and set the gap sensibly.
Why a deficit works
Your body needs energy to run, and it measures that energy in calories. It burns calories keeping you alive, keeping you warm, moving you around, and digesting food. If you eat exactly what you burn, your weight holds steady. Eat more, and the surplus gets stored, mostly as fat. Eat less, and your body dips into its fat stores to cover the gap.
For scale: a kilogram of body fat holds roughly 7,700 calories. So a daily deficit of about 500 calories works out at around 3,500 a week, which is close to half a kilogram of fat lost over seven days. That is a healthy, sustainable pace. Slow and boring beats fast and miserable, because the slow version is the one you can actually keep going.
Step one: find your maintenance calories
Maintenance is the number of calories that keeps your weight steady. You need it before you can set a deficit, because the deficit is just maintenance minus a bit. There are two parts to it.
Your BMR (basal metabolic rate) is what you burn at complete rest, just being alive. The standard way to estimate it is the Mifflin-St Jeor equation, which uses your weight, height, age and sex. It is the same formula most calculators run under the bonnet.
Your activity multiplier scales that up for how much you move. Nobody lies still all day, so you multiply BMR by a figure between 1.2 and 1.9:
| Activity level | Multiplier |
|---|---|
| Sedentary, desk job, little exercise | 1.2 |
| Lightly active, some walking or light exercise | 1.375 |
| Moderately active, exercise a few times a week | 1.55 |
| Very active, hard training most days | 1.725 |
| Extremely active, physical job plus training | 1.9 |
BMR multiplied by your activity figure gives your maintenance calories, often called your TDEE (total daily energy expenditure). For a rough anchor, UK reference intakes put maintenance around 2,000 calories a day for women and 2,500 for men, though your real number depends on your size, age and how much you move.
Step two: set a modest deficit
Once you have your maintenance number, subtract a sensible amount. For most people, 300 to 500 calories a day is the range to aim for.
- A 500-calorie deficit loses roughly half a kilogram a week. This suits most people who want steady progress.
- A 300-calorie deficit is gentler and easier to live with, losing weight a little slower. Good if a bigger cut leaves you starving.
Say your maintenance is 2,400 calories. A 500-calorie deficit puts your daily target at 1,900. That is your number. You eat to it, log as you go, and let the weeks do the work.
Do not cut too hard
The temptation is to slash calories and lose weight fast. Resist it. A very aggressive deficit backfires in three ways: you feel awful and give up, you lose more muscle alongside fat, and your energy and mood take a hit. As a floor, most guidance suggests not dropping below about 1,500 calories a day for men or 1,200 for women without professional supervision. A smaller deficit you can hold for months beats a brutal one you quit in a fortnight.
Two things make a deficit far easier to stick to. Keep your protein high, because it keeps you full and protects muscle while you are eating less. And weigh yourself weekly rather than daily, since day-to-day water changes bounce the scale around and can be disheartening for no real reason.
Let the app do the maths
Working out your BMR, applying an activity multiplier, and then tracking every meal against your target is a lot of arithmetic to do by hand. Fettle (stayfettle.com) calculates your maintenance calories and sets a deficit target from your details and goal, then adds up what you eat as you log it. It has UK foods and barcode scanning, it is free, and there is no account to create.
One honest note
This is general guidance for healthy adults, not medical advice. If you have a health condition, are pregnant or breastfeeding, have a history of disordered eating, or are unsure what a safe calorie target is for you, speak to your GP or a registered dietitian before starting a deficit.