How your daily calorie needs are worked out
The calculation happens in two stages. First it estimates your Basal Metabolic Rate — the energy your body burns simply staying alive, with no movement at all. Then it multiplies that figure by an activity factor to give your Total Daily Energy Expenditure, or TDEE: the calories you burn across a typical day.
This calculator uses the Mifflin-St Jeor equation for the first stage. It replaced the older Harris-Benedict formula in the 1990s and is generally accepted as the more accurate of the two for the modern population.
Take a 35-year-old woman who is 5 ft 5 in (165 cm) and weighs 10 st (63.5 kg). Her BMR works out at roughly 1,330 kcal. If she exercises three to five days a week, the moderate activity factor of 1.55 gives a TDEE of about 2,060 kcal a day. That is what she burns; it is also what she would eat to maintain her current weight.
Choosing your activity level honestly
This is where most calorie estimates go wrong. The activity multiplier has a larger effect on the final number than your height or age does, and people consistently pick a level above the one that matches their actual week.
| Level |
Factor |
What it actually looks like |
| Sedentary |
1.2 |
Desk job, little or no deliberate exercise. Most office workers who do not train belong here, even if they feel busy. |
| Lightly active |
1.375 |
Exercise one to three days a week, or a job involving some walking. |
| Moderately active |
1.55 |
Genuine exercise three to five days a week — a run, a gym session, a hard cycle. Not a gentle stroll. |
| Very active |
1.725 |
Hard training six or seven days a week, or a physically demanding job on top of exercise. |
| Extra active |
1.9 |
Manual labour combined with training, or twice-daily sessions. Genuinely uncommon. |
If you are unsure, pick the lower option. Overstating your activity level by one step adds roughly 200–300 kcal to your daily target — enough to quietly cancel out a deliberate deficit and leave you wondering why nothing is changing.
Setting a target for your goal
Once you have your TDEE, a goal is just an adjustment either side of it. The four figures the calculator shows above follow the standard steps.
- −500 kcal a day — aims at roughly 1 lb (0.45 kg) of loss a week.
- −250 kcal a day — roughly half a pound a week. Slower, but easier to sustain and gentler on muscle mass.
- Maintenance — your TDEE, for holding steady.
- +250 kcal a day — a modest surplus for building muscle alongside resistance training, without excessive fat gain.
Those loss figures come from the familiar rule that a pound of body fat holds about 3,500 kcal. It is a reasonable starting point, but it flatters the long run: as you lose weight your body becomes smaller and burns less, and unconscious daily movement tends to drop too. Expect the rate to slow after the first couple of months. That is normal, not failure.
The NHS considers 0.5 kg to 1 kg (1–2 lb) a week a safe rate of loss for most adults. Going faster rarely holds.
There is also a floor worth knowing about, and it is not the same for everyone. International clinical guidelines for managing overweight typically prescribe 1,200–1,500 kcal a day for women and 1,500–1,800 kcal for men — the higher male range reflects greater average body size and the difficulty of meeting vitamin and mineral needs at low intakes. Below those levels it becomes hard to eat adequately without professional support. The NHS describes a diet of under 800 to 1,200 calories a day as a very low calorie diet, says such diets are “not suitable or safe for everyone”, and describes them as something a dietitian supports rather than something you set for yourself.
For that reason the calculator above will not display a weight-loss figure below 1,200 kcal a day for women or 1,500 kcal a day for men. Where the arithmetic lands lower, it shows that limit instead, says that it has done so, and names the guidance. What a figure at or near the limit means for any one person is a question for a GP or dietitian, not for a calculator.
Why your number is an estimate, not a measurement
Mifflin-St Jeor lands within about 10 per cent of true metabolic rate for most people. On a 2,000 kcal target that is a swing of 200 kcal in either direction — which matters, and is worth knowing before you treat the output as precise.
The equation cannot see your body composition. Muscle burns more at rest than fat does, so two people of identical height, weight, age and sex can have genuinely different needs. This is the one situation where a different equation is clearly better: if you are notably muscular or carry very little body fat, an equation based on lean mass rather than height and weight — Katch-McArdle is the common one — will estimate you more accurately, provided you have a reliable body fat measurement to feed it. Thyroid conditions, some medications, pregnancy and breastfeeding all shift the figure as well.
The practical approach is to treat the number as a starting hypothesis. Eat to it for two or three weeks, track your weight at the same time of day, and adjust based on what actually happens rather than what the formula predicted. Your own data beats any equation.
Making the number useful day to day
A calorie target on its own says nothing about what you eat, and composition matters for how the target feels. Protein is the most useful lever: it is more satiating than carbohydrate or fat, and eating enough of it protects muscle while you are in a deficit. Around 1.6 g per kilogram of body weight a day is a well-supported target if you are losing weight or training.
For context on the numbers you see on packaging, UK reference intakes are 2,000 kcal a day for women and 2,500 for men. Those are population averages for labelling purposes, not personal targets — which is precisely the gap this calculator exists to close.
Frequently asked questions
How many calories should I eat to lose weight?
Take your TDEE from the calculator above and subtract 250–500 kcal a day. That produces roughly half a pound to a pound of loss a week, which is the range the NHS considers sustainable. Larger deficits work faster at first but are harder to maintain and cost more muscle.
Is 1,200 calories a day enough?
For most adults it is quite low, and it is close to the floor at which meeting your nutritional needs becomes difficult. Some shorter people with sedentary lifestyles will have a legitimate target near that figure, but if your calculated deficit lands below about 1,200 kcal it is worth speaking to a GP or dietitian rather than pressing on.
Why is my TDEE different from other calculators?
Different sites use different equations — Harris-Benedict and Katch-McArdle are common alternatives — and they group activity levels differently. Katch-McArdle uses body fat percentage, so it needs a measurement most people do not have. Differences of 100–200 kcal between calculators are normal and reflect the inherent imprecision rather than one being wrong.
Should I eat back the calories I burn exercising?
Generally no, if you selected an activity level that already reflects your training. Doing both counts the same exercise twice. Fitness trackers also tend to overestimate calories burned, sometimes considerably, so eating back what they report is a common reason a deficit quietly disappears.
Does my calorie target change as I lose weight?
Yes. A smaller body burns fewer calories, so your TDEE falls as you lose. It is worth recalculating every 4–6 kg (roughly half a stone to a stone) of change, otherwise a target set at your starting weight gradually stops producing a deficit.
Sources and further reading
- Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990;51(2):241–247.
pubmed.ncbi.nlm.nih.gov/2305711
— the original paper behind the equation used above. Record checked 19 October 2026.
- 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;105(5):775–789.
pubmed.ncbi.nlm.nih.gov/15883556
— the review that established Mifflin-St Jeor as the most reliable of the age-and-weight equations. Record checked 19 October 2026.
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011;378(9793):826–837.
pubmed.ncbi.nlm.nih.gov/21872751
— why the 3,500 kcal-per-pound rule overstates long-term loss. Record checked 19 October 2026.
- Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine, 2018;52(6):376–384.
pubmed.ncbi.nlm.nih.gov/28698222
— the source of the 1.6 g per kilogram protein figure. Record checked 19 October 2026.
- National Health Service. Overweight and obesity in adults. NHS, page last reviewed 29 April 2026.
nhs.uk/conditions/overweight-and-obesity
— the definition of a very low calorie diet as “under 800 or 1200 calories”, that such diets are “not suitable or safe for everyone” and are followed with dietitian support, and “aim to lose 1 to 2lbs, or 0.5 to 1kg, a week”. This is the basis of the lower limit the calculator applies to its weight-loss figures. Checked 18 October 2026.
- National Health Service. Understanding calories. NHS, 2024.
nhs.uk/live-well/healthy-weight/managing-your-weight/understanding-calories
— the 2,000 kcal (women) and 2,500 kcal (men) UK reference intakes quoted above. Checked 18 October 2026.
- American Heart Association / American College of Cardiology / The Obesity Society. 2013 AHA/ACC/TOS Guideline for the Management of Overweight and Obesity in Adults. Circulation, 2014;129(25 Suppl 2):S102–S138.
pmc.ncbi.nlm.nih.gov/articles/PMC5819889
— the source of the 1,200–1,500 kcal (women) and 1,500–1,800 kcal (men) prescribed ranges above, whose lower bounds the calculator uses as its lower limit. Checked 18 October 2026.
Guidance on this page follows NHS recommendations for adults in the UK. BMR is calculated using the Mifflin-St Jeor equation. Last reviewed 18 October 2026 by the KnowYourBody health content writer;
no clinician has reviewed this page. Each source above was opened on the date shown beside it. For a fuller explanation, see our article on
how many calories you need.
How to read the dates above. Checked means we opened that page and confirmed it still carries the wording quoted beside it.
Record checked means we confirmed the citation against the PubMed record — authors, journal and year — but the article itself sits behind a publisher paywall and we have not re-read the full text. The dates are when a person actually opened the source; they are not bumped when a release is packaged.