The short answer: to lose weight, eat about 20% fewer calories than your body burns in a day. For most adults that comes to a deficit of 300–600 kcal a day, which usually means losing around 0.3–0.6 kg (roughly ¾–1¼ lb) a week. To find your own number, estimate your basal metabolic rate (BMR), multiply it by how active you are, then take 10–25% off.
That's really all a calorie deficit is. The rest of this guide covers how to get your number right, how to pick a pace you can live with, and what to do when the scales stop cooperating. It uses the same maths Feat uses to set your daily target.
What is a calorie deficit?
You're in a calorie deficit when you take in less energy than you use. Your body covers the gap from its own stores, which over time means mostly body fat, and your weight comes down.
Your body spends energy in four ways:
Basal metabolic rate (BMR): the energy it takes to keep you alive at rest, covering breathing, your heartbeat, your brain and staying warm. For most people this is the biggest share of the day, at around 60–70%.
Digesting food: roughly 10% of the calories you eat go on breaking that food down. Protein takes the most effort to digest.
Everyday movement: walking to the bus, standing at the hob, taking the stairs, fidgeting. Researchers call this NEAT (non-exercise activity thermogenesis), and it varies between people far more than most of us realise.
Exercise: the gym, a run, five-a-side.
Together these make up your total daily energy expenditure (TDEE), sometimes called your maintenance calories. Eat at your TDEE and your weight stays roughly where it is. Eat below it and you're in a deficit.
A quick note on wording: "calories" on food labels and in apps are technically kilocalories (kcal). They mean the same thing here.
How to work out how many calories you should eat
Step 1: Estimate your BMR
The most widely used formula is the Mifflin–St Jeor equation, published in 1990. A 2005 review for the American Dietetic Association compared the main equations against measured resting energy and found Mifflin–St Jeor the most reliable for healthy adults, both with and without obesity. It's the formula Feat uses.
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
Using stones and pounds? Multiply stones by 6.35 to get kilograms, and multiply your height in inches by 2.54 to get centimetres.
Step 2: Multiply by how active your day is
BMR only covers you at rest, so the next step is to scale it up for the rest of your day. These are the everyday movement levels Feat uses, with a rough guide based on how far you walk:
Your typical day | Rough guide | Multiply your BMR by |
|---|---|---|
Mostly seated | Under 5 km of walking a day (about 6,500 steps) | 1.15 |
Lightly active | 5–7 km a day (about 6,500–9,000 steps) | 1.25 |
Active day-to-day | 7–10 km a day (about 9,000–13,000 steps) | 1.40 |
Highly active | 10 km or more a day (13,000+ steps) | 1.65 |
You may notice these numbers are lower than the 1.2–1.9 factors many online calculators use. That's deliberate. They cover everyday movement only, so workouts get added separately on the days they actually happen (more on that below). And most of us overestimate how active we are, so a generous multiplier can quietly cancel out the deficit you thought you had.
Step 3: Take off 10–25%
Use a percentage rather than a flat number so the deficit fits your body. Cutting 500 kcal a day is gentle for a tall, active man and steep for a petite woman with a desk job.
Deficit | What it feels like | Best for |
|---|---|---|
10% | Barely noticeable | Slow, steady progress, the last few kilos, or if you've dieted a lot before |
20% | Noticeable but manageable | Most people. It's the default for Feat's "Lose weight simply" goal |
25% | Firm | Shorter, focused phases. Feat's "Cut / Get shredded" goal uses this |
30% or more | Hard going | Rarely worth it. Hunger, low energy and muscle loss tend to catch up |
A worked example
Let's take a 40-year-old man who is 180 cm (5 ft 11 in) and weighs 90 kg (about 14 st 2 lb). He's on his feet a lot and walks 7–10 km on a normal day.
BMR: (10 × 90) + (6.25 × 180) − (5 × 40) + 5 = 1,830 kcal
Daily burn: 1,830 × 1.40 = 2,562 kcal
Target at a 20% deficit: 2,562 × 0.8 ≈ 2,050 kcal a day
That's a deficit of about 510 kcal a day, or roughly 3,600 kcal a week. Using the usual rule of thumb that a kilogram of body fat holds about 7,700 kcal, he'd expect to lose a little under 0.5 kg (about 1 lb) a week. That sits comfortably within the NHS's guidance of 0.5–1 kg a week.
Quick reference: calories to lose weight
Here are daily targets at a 20% deficit for some typical profiles, calculated the way Feat does it and rounded to the nearest 10 kcal. They're before any workouts, so add exercise on the days you do it.
Profile | Mostly seated | Lightly active | Active day-to-day | Highly active |
|---|---|---|---|---|
Woman, 30, 163 cm, 70 kg | 1,300 | 1,410 | 1,580 | 1,860 |
Woman, 45, 165 cm, 80 kg | 1,330 | 1,450 | 1,620 | 1,910 |
Man, 30, 178 cm, 85 kg | 1,670 | 1,820 | 2,040 | 2,400 |
Man, 40, 180 cm, 90 kg | 1,680 | 1,830 | 2,050 | 2,420 |
For context, the NHS says an average woman needs about 2,000 kcal a day to maintain her weight and an average man about 2,500. Your own numbers can sit well above or below that depending on your size, age and how much you move, which is why a personal estimate beats a generic one.
How fast will you lose weight?
The 7,700 kcal rule (3,500 kcal per pound) is a useful starting point, so a 500 kcal daily deficit works out at about 0.45 kg a week on paper. Real life is a bit messier:
The first week or two often moves faster. Eating less means storing less carbohydrate and the water that goes with it, so early losses are partly water.
Progress slows over time. A smaller body burns less energy, so the same intake becomes a smaller deficit. Research from the US National Institutes of Health showed the classic rule overestimates long-term weight loss for exactly this reason.
Daily weigh-ins bounce around. Salt, hormones, a late curry or a hard workout can shift the scales by 1–2 kg overnight. Watch the trend across weeks rather than any single morning.
A pace of 0.5–1 kg a week is what the NHS recommends, and for many people the lower end is easier to keep up. Slower isn't failing. It's often the version that lasts.
Should you eat back your exercise calories?
It depends on whether your target already includes exercise. The real risk is counting the same activity twice: an app sets your target assuming you're active, a fitness tracker then adds another 600 kcal for your spin class, and the deficit disappears without you noticing.
A simple rule: if your target is based on everyday movement only (like the multipliers above), add your workouts on top on the days you do them. If your target already assumes regular exercise, don't add them again.
This is how Feat handles it. Your base target starts from a mostly-seated day. Then, if you connect Apple Health, Feat adds the energy from your actual steps and walking distance, and from each workout, as it happens. Feat works those numbers out itself instead of using Apple's combined "Active Energy" figure, and steps taken during a workout are only counted once. You can switch the movement or exercise bonus off, or use your average from the previous seven days if you'd rather your target didn't swing from one day to the next.
What to eat in a calorie deficit
Calories decide whether you lose weight. What makes up those calories decides how you feel along the way, and whether you lose fat or muscle.
Put protein first
Protein keeps you fuller for longer, costs more energy to digest, and helps you hold on to muscle while you're eating less. On the "Lose weight simply" goal, Feat sets protein at 1.6 g per kg of body weight. If you're over 50 it's never below 1.5 g per kg, because we tend to lose muscle more easily with age. In a larger body, Feat bases protein on a healthy reference weight rather than your full weight, so the number stays realistic. And if you have kidney or liver disease, Feat caps protein lower, at 0.8 g per kg. Speak to your GP or specialist about what's right for you.
Lean on fibre and volume
Vegetables, fruit, beans, lentils, oats and wholegrains fill your plate without using up much of your budget. UK guidelines recommend 30 g of fibre a day, but most adults only manage about 20 g, so there's usually room to add some.
Don't cut fat too far
Fat is calorie-dense, which makes it an easy thing to slash. But your body needs it for hormones and for absorbing vitamins A, D, E and K. Feat keeps fat at no less than 20% of your calories when you're losing weight.
Keep the food you love
A deficit you can't stick to isn't much use. If a Friday sausage roll is part of your week, keep it and log it. Feat knows what's in a Greggs sausage roll and a fair few other UK chain menus, so eating out still counts. Nothing is off limits, and there are no cheat days, because there's nothing to cheat on.
Watch the drinks, though. Alcohol has 7 kcal per gram, and lattes, juices and pints add up quicker than food usually does.
Signs your deficit is too big
More isn't always better. You might be cutting too hard if you're:
hungry all the time, or thinking about food constantly
tired, sleeping badly, or finding your workouts suddenly hard work
still losing more than 1 kg a week after the first couple of weeks
irritable, low, or noticing changes to your periods
regularly ending up eating far more than planned at weekends
If that sounds familiar, move your deficit 5–10 percentage points closer to maintenance. You'll lose a bit more slowly and probably a lot more happily. Very low-calorie diets (under 800 kcal a day) should only be done with medical support.
Why weight loss slows, and what to do about it
Almost everyone hits a plateau. Before you cut calories again, work through this list:
Recalculate. Your BMR drops as your weight does, so a target that was right at 90 kg will be too generous at 82 kg. Unless you've set your own target, Feat recalculates it as your weight updates, including when it syncs from Apple Health.
Check your logging. Most of us underestimate what we eat, sometimes by a lot. In one well-known study, people who struggled to lose weight under-reported their intake by nearly half. Cooking oil, sauces, nibbles while cooking and drinks are the usual culprits.
Look at your weekends. Five careful days and two relaxed ones can add up to maintenance.
Move a little more each day. Extra everyday steps often do more for your weekly total than one extra gym session.
Take a break. A week or two eating at maintenance can make the next stretch easier. In one trial, men who alternated two-week blocks of dieting with two weeks at maintenance lost more fat than those who dieted continuously.
How Feat works out your calorie target
You can do all of the above with a calculator and a notebook. Feat just does the sums for you and keeps them up to date:
Your starting target comes from the Mifflin–St Jeor equation, your goal and your body. On "Lose weight simply" that's 20% below a mostly-seated day. You can also set a target weight and timeframe, or enter your own calorie target whenever you like.
Your activity counts once. Steps, walking distance and workouts from Apple Health, or workouts you log by type and intensity, raise your target on the days you move more.
Your macros are set for you: protein, fat, carbs, plus a fibre minimum and a sugar limit, adjusted for your goal, age, sex and any health conditions you share.
Logging takes seconds. Snap a photo of your plate, scan a barcode or nutrition label, describe what you ate in your own words, or search, including restaurant menus. Nothing is logged until you confirm it.
You can see the trend. Weight and body-fat charts show the direction you're heading, and Weekly Wrapped turns your week into a short story every Sunday.
Looking for meals that fit your numbers? Browse over 1,200 Feat recipes, each with its calories and macros worked out.
Download Feat for free on the App Store and get your personal calorie target in a couple of minutes.
Frequently asked questions
How many calories should I eat to lose weight?
Work out your BMR with the Mifflin–St Jeor equation, multiply it by your activity level (1.15–1.65), then eat 10–25% less than the result. For most adults that lands somewhere between 1,300 and 2,400 kcal a day, depending on size, sex, age and activity.
How many calories should I eat to lose 1 kg a week?
Losing 1 kg of fat a week takes a deficit of around 1,100 kcal a day. That's a lot for most people and only realistic if you're quite large or very active. Aiming for 0.5 kg a week, a deficit of about 550 kcal a day, is easier to keep up and still within NHS guidance.
Is 1,200 calories a day enough?
For a smaller, mostly seated woman, 1,200–1,300 kcal can be close to a sensible weight-loss target. For most men, and most active women, it's too low and likely to leave you hungry and tired. If your calculation comes out below this, choose a smaller deficit or move a little more rather than eating less.
Is it OK to eat less than my BMR?
Your BMR is only part of what you burn, so a target close to or slightly below it isn't harmful in itself. It often happens on a mostly-seated day. Going far below it for long periods makes it harder to get enough protein and nutrients, and harder to stick with.
Will I lose muscle in a calorie deficit?
You can, especially with a large deficit and low protein. To protect muscle, keep your deficit moderate, eat enough protein (around 1.6 g per kg is a good target for most people), and do some strength training two or three times a week.
Why am I not losing weight in a calorie deficit?
The usual reasons are water retention hiding fat loss for a week or two, under-counting what you eat and drink, a target that hasn't been updated as your weight has dropped, or exercise calories being counted twice. Give it three to four weeks of consistent logging before changing anything.
Do I have to count calories to be in a deficit?
No. Smaller portions, more vegetables and fewer drinks can get you there. But logging for a few weeks is the fastest way to learn what a deficit actually looks like on your plate, even if you stop later.
Is a calorie deficit safe for everyone?
It isn't suitable if you're pregnant or breastfeeding, under 18, or have a history of eating disorders. If you have a medical condition or take medication that affects your blood sugar, talk to your GP before you start.
The bottom line
A calorie deficit is simply eating a bit less than you burn. Estimate your BMR, multiply it by how active you really are, take off around 20%, and give it a few weeks. Put protein first, leave room for food you enjoy, count your workouts once, and recalculate as you go. Steady and boring usually beats fast and miserable.
This article is for general information and isn't medical advice. If you have a health condition, are pregnant, or are unsure whether weight loss is right for you, speak to your GP or a registered dietitian.
Sources
Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990.
Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 2005.
Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011.
Leidy HJ, et al. The role of protein in weight loss and maintenance. American Journal of Clinical Nutrition, 2015.
Lichtman SW, et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. New England Journal of Medicine, 1992.
Byrne NM, et al. Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. International Journal of Obesity, 2018.
NHS. Understanding calories.