Calorie Deficit Explained: How Many Calories to Eat a Day

By Alex Lawrence, qualified nutritionist · Fact-checked against sources on 30 October 2026 · Updated 30 October 2026

A calorie deficit means eating fewer calories than your body uses, so it draws on stored fat for the rest. The NHS suggests aiming to lose 0.5 to 1 kg (1 to 2 lb) a week, and for most people a deficit of about 500–550 kcal a day gives roughly 0.5 kg (1 lb) a week at the start.

This guide explains how your daily calories are worked out, how big a deficit to pick, and how to keep your muscle while you lose fat. To get your own numbers straight away, use the calorie deficit calculator.

What is a calorie deficit?

A calorie deficit is the gap between the energy you take in from food and drink and the energy your body burns. The NHS puts it simply: you’re likely to lose weight if you eat and drink fewer calories than you use, because your body uses its stored fat for energy.

Your body burns energy in three main ways:

Resting burn (BMR): keeping you alive at rest. For most people this is the biggest share.
Daily movement: your job, walking, chores, fidgeting.
Exercise: your workouts, runs and classes.

Add these up and you get your maintenance calories, sometimes called TDEE (total daily energy expenditure). Eat at maintenance and your weight stays roughly steady. Eat below it and you’re in a deficit.

Energy on UK labels is shown in kcal (kilocalories), which is what people usually mean by “calories”.

How many calories should I eat a day?

There’s no single number, but the NHS reference figures are that an average man needs about 2,500 kcal a day and an average woman about 2,000 kcal. The NHS also notes these vary with your age, weight, height and how active you are.

To get closer to your own figure, most calculators (including ours) start with the Mifflin-St Jeor equation, which estimates your resting burn from your sex, age, height and weight:

Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

That’s from Mifflin et al. (1990). A systematic review by Frankenfield et al. (2005) found it was the most reliable of the common equations, predicting resting burn within 10% of the measured value for more people than the others. It still has real errors for some individuals, so treat any result as a starting point.

Next, the resting burn is multiplied by your day-to-day activity. The Get Gains calculator uses four levels based on your job and daily life, from 1.2 (desk job) to 1.7 (heavy manual work).

Why workouts aren’t in the multiplier

Our calculator gives your maintenance before exercise: what keeps your weight steady on a day you don’t train. Workouts vary too much from day to day to average into one number. Instead, you add what each session burns on the days you train. We call this base + burn, and it’s how the Get Gains app works too.

That’s also why the worked examples below sit below the NHS averages: they’re for people with desk jobs, on rest days.

How many calories should I eat to lose weight?

Take your maintenance calories and subtract a daily deficit that matches the rate you want to lose. Our calculator uses the common starting estimate of about 7,700 kcal per kg of body weight lost (roughly 3,500 kcal per lb):

Rate of loss	Daily deficit
0.25 kg (0.5 lb) a week	about 275 kcal
0.5 kg (1.1 lb) a week	about 550 kcal
0.75 kg (1.7 lb) a week	about 825 kcal
1 kg (2.2 lb) a week	about 1,100 kcal

Two worked examples

Here are the calculator’s results (rounded to the nearest 10 kcal) for two people with desk jobs, both aged 35:

Daily target	Woman, 165 cm (5 ft 5 in), 70 kg (154 lb)	Man, 180 cm (5 ft 11 in), 85 kg (187 lb)
Maintenance (before exercise)	1,670 kcal	2,170 kcal
Lose 0.25 kg a week	1,400 kcal	1,890 kcal
Lose 0.5 kg a week	1,120 kcal (too low)	1,620 kcal
Lose 0.75 kg a week	850 kcal (too low)	1,340 kcal (too low)
Lose 1 kg a week	570 kcal (too low)	1,070 kcal (too low)

“Too low” = below the safe minimum (see the next section).

Two things stand out. Smaller, less active people have a smaller “budget” to cut from, so faster rates quickly fall too low. And the calories from training are on top: if the woman above burns 300 kcal in a workout and eats them back, her 0.5 kg target on that day becomes about 1,420 kcal.

Use the calorie deficit calculator to see your own numbers, then the macro calculator to split them into protein, carbs and fat.

How low is too low?

Don’t go below about 1,200 kcal a day (women) or 1,500 kcal a day (men) without medical supervision. Below that it’s hard to get enough protein, vitamins and minerals, and it’s difficult to keep up. Our calculator flags any target under these figures. If yours is flagged, choose a slower rate, add your exercise calories back, or speak to your GP or a dietitian first.

Faster isn’t better. The NHS advice is to avoid losing weight suddenly with diets, and for lean people who lift, a review of contest-prep research suggests losing about 0.5–1% of body weight a week to keep as much muscle as possible (that review looked at natural bodybuilders, so take it as a guide rather than a rule).

If you’re worried about your eating, your weight or how much you exercise, talk to someone. Beat, the UK eating disorder charity, can help: call 0808 801 0677 or visit beateatingdisorders.org.uk. This guide is general information, not medical advice (terms).

Why “3,500 kcal = 1 lb” oversimplifies

The 3,500 kcal (or 7,700 kcal per kg) rule is a fine starting estimate, but it assumes your burn stays the same while you lose weight, and it doesn’t. When Thomas et al. (2013) compared the rule against seven tightly controlled weight loss experiments, they found it overestimated how much weight people actually lost.

The reason is that as you get lighter, you burn less. Hall et al. (2011) modelled this and showed the body weight response to a change in calories is slow, with a half-time of about a year: weight loss is fastest at the start and tapers off. On top of that, dieting brings adaptations in hormones and energy use that shrink the deficit over time (Trexler et al., 2014).

In practice: expect weight loss to slow over the months, and re-run your numbers every time you’ve lost a few kilos.

Should you eat back exercise calories?

Yes, at least some of them, if your target is “before exercise” like ours. Eating back workout calories keeps your deficit where you planned it and helps you train well.

Be cautious with the number on your watch, though. In a Stanford study of seven wrist-worn devices, none estimated energy burn with an error below 20%, even though most measured heart rate well. A common starting point is to eat back about half of a tracked workout, then adjust based on how your weight trends.

How to keep your muscle in a deficit

Lift weights and eat plenty of protein. Diet-only weight loss takes some muscle with it, but a review by Cava et al. (2017) found that a higher protein intake helps preserve lean mass during weight loss, and that exercise, particularly resistance training, helps preserve muscle and also improves strength.

How much protein?

A meta-analysis of 49 studies (Morton et al., 2018) found gains in muscle from resistance training levelled off at about 1.6 g per kg of body weight a day, with 2.2 g covering most people.
In a deficit, needs may be higher. The Helms et al. review suggests 2.3–3.1 g per kg of lean mass for lean, trained people. Our protein calculator converts this to 1.8–2.7 g per kg of body weight for “losing fat while lifting”.

If hitting that sounds hard, high-protein snacks help fill the gap without many extra calories. See our list of high-protein snacks from UK supermarkets, ranked by protein per 100 kcal. New to lifting? Start with a beginner workout plan.

The best diet is the one you’ll stick to

How you create the deficit matters less than whether you can keep it up. In a one-year trial comparing four popular diets (Dansinger et al., 2005), weight loss was linked to how well people stuck to their diet, not which diet they were on. A meta-analysis of 48 trials (Johnston et al., 2014) found only small differences in weight loss between named diets, and the authors backed recommending whichever diet a person will stick to.

A few things that help most people:

Pick a rate you can live with. 0.25–0.5 kg a week is easier to sustain than 1 kg.
Think in weeks, not days. Your weekly average matters more than any single day, so a slightly bigger weekend is fine if the week still balances.
Keep protein high and meals filling, with plenty of vegetables, fruit and wholegrains.
Log honestly. Weighing and logging food for a few weeks shows you where the calories actually go.

What to do when weight loss stalls

Judge progress over two to three weeks, not day to day. Scale weight swings with water, salt and the food in your gut, so a flat few days means little.

If your trend has been flat for two to three weeks:

Re-run the calculator with your current weight. A lighter body burns less.
Check your logging, including drinks, oils, sauces and weekends.
If both are right, reduce your target by 100–200 kcal a day or add some daily walking.

Some people also take a planned week or two at maintenance during a long diet. It can make dieting easier to stick to and won’t undo your progress.

Let the numbers keep up with you

Your calorie needs change as your weight does. Get Gains logs your food (with a barcode scanner and food database on the free plan), sets a base target and adds the calories from each workout you log. On the Titan plan, Adaptive TDEE adjusts your target from your real logging and weight data, so you don’t have to keep re-calculating by hand.

How we wrote this: drafted with AI assistance, then fact-checked against the sources below and edited by Alex Lawrence. Read our editorial policy.