A calorie deficit is simply eating fewer calories than your body uses, so it has to draw on stored energy — which is how you lose weight. This calculator works out how many calories you burn in a day, how big a deficit is safe, and roughly how long reaching your goal would take. It uses the UK’s own method — the Henry equations and activity levels adopted by SACN — rather than the American formula most online tools default to.
What you’ll need
Your age, sex, height and weight, an honest read on how active you are, and the rate you’d like to lose weight (or a target weight and a date). Sex and age genuinely change the answer here — unlike BMI, the energy equations use both — so the calculator asks for them.
Be realistic about activity. Most people overestimate it, which inflates the maintenance figure and quietly wipes out the deficit. The lower SACN band (PAL 1.49) covers a desk job with little exercise; the higher band is for physical work or serious daily training, not a couple of gym sessions a week. These are population activity levels, not the gym-day multipliers (1.2–1.9) used by most US-style calculators.
How your calories are worked out
There are two steps. First, your basal metabolic rate (BMR) — the energy your body uses at complete rest to keep you alive: heartbeat, breathing, brain, keeping warm. This is the bulk of what you burn. The calculator estimates it with the Henry (Oxford) equations, which the UK’s Scientific Advisory Committee on Nutrition (SACN) adopted in 2011 and the British Dietetic Association recommends for clinical use. They use your weight, age and sex.
Second, we multiply BMR by a physical activity level (PAL) to get the calories you actually burn in a normal day — your total daily energy expenditure (TDEE), sometimes called maintenance calories:
Maintenance calories = BMR × activity level
SACN sets the middle of the range at a PAL of 1.63, with 1.49 for less active people and 1.78 for more active ones, plus small increments for regular exercise. As a sense-check, SACN’s average requirement for a UK adult at a healthy weight is about 2,605 kcal/day for men and 2,079 kcal/day for women — close to the familiar NHS “2,500 and 2,000” guide figures.
Turning a deficit into weight lost
To lose weight you eat below your maintenance figure. The rule of thumb is that roughly 7,700 kcal is stored in a kilogram of body fat, so a deficit of about 7,700 kcal over time equals about a kilo lost. That means a 600 kcal daily deficit works out at roughly 0.5 kg a week — which is exactly the pace the NHS recommends.
Weekly loss (kg) ≈ (daily deficit × 7) ÷ 7,700
The calculator does this both ways: tell it a rate and it shows the daily calories to eat; tell it a target weight and date and it shows the deficit that implies, and flags it if that’s faster than is sensible.
What a safe deficit looks like
The NHS and NICE both point to a deficit of around 600 kcal a day, producing 0.5 to 1 kg of loss a week. That’s the sweet spot: fast enough to see progress, slow enough to protect muscle and keep the weight off.
There’s a floor as well as a target. The NHS 12-week plan sets daily intakes of about 1,900 kcal for men and 1,400 kcal for women as a sensible lower bound for self-directed dieting. Going below roughly 800 kcal a day — a very-low-calorie diet — risks nutrient deficiencies, gallstones and electrolyte problems, and should only be done under medical supervision. If your target date forces the numbers below these levels, the honest answer is to extend the timeline, not to starve the gap. Keeping your protein up and staying active while you diet both help you lose fat rather than muscle.
Common mistakes
Trusting the maintenance number to the calorie. Every equation is an estimate for an average person; yours could sit 10% either side. Treat it as a starting point, then adjust to what the scales actually do over two or three weeks.
Over-reporting activity. The single biggest source of error. When in doubt, pick the lower band — you can always eat a little more once the real trend is clear.
Eating your exercise back. Fitness trackers are notoriously generous with “calories burned”. If you add all of it back as food, the deficit disappears.
Expecting the loss to be linear. Weight swings day to day with water and salt, and slows as you get lighter because a smaller body burns less. A stall of a few days means nothing; judge by the multi-week trend.
When the numbers can mislead
This is a healthy-adult estimate. It isn’t designed for pregnancy or breastfeeding (energy needs rise — speak to your midwife), for under-18s (who are still growing and shouldn’t be put in a deliberate deficit without professional input), or for people with a medical condition or medication that affects weight or metabolism. Very muscular people burn more than the equation predicts; older or less mobile people, sometimes less.
If counting calories is stirring up anxiety, or you’ve had a difficult relationship with food or an eating disorder, please give this tool a miss and talk to your GP or the eating-disorder charity Beat. A number on a screen isn’t worth your wellbeing.
When to see a professional
For most people a modest deficit, a bit more movement and some patience is all it takes. But see your GP or a registered dietitian if you have a lot of weight to lose, an underlying health condition, or you’ve dieted carefully for a couple of months with nothing to show for it — the NHS runs free weight-management support you can be referred to. This calculator is a planning aid, not a medical assessment.
Frequently asked questions
What is BMR? Basal metabolic rate is the energy your body uses at complete rest — heartbeat, breathing, brain and keeping warm. This calculator estimates it with the Henry (Oxford) equations. Maintenance calories are BMR multiplied by your activity level (PAL).
Is a 600-calorie deficit good? For most healthy adults, yes — it’s the level the NHS and NICE point to, and it produces about half a kilo of loss a week without being punishing.
Why is my maintenance number different from another calculator? Most tools — including calculator.net and many UK retail calculators — use the American Mifflin-St Jeor equation plus gym-style activity multipliers (about 1.2 to 1.9). This one uses the Henry equations and SACN population activity bands (PAL 1.49 / 1.63 / 1.78) recommended for the UK. The gap is often hundreds of kcal for the same body, from both the BMR equation and the activity scale — not a CalcLab bug. Neither estimate is exact; adjust to what the scales confirm over a few weeks.
How fast can I lose weight? Safely, about 0.5 to 1 kg a week. Faster than that usually means losing muscle and water, and it’s much harder to keep off.
Do I have to count calories forever? No. Counting for a few weeks teaches you roughly what your meals contain; most people then keep it up loosely rather than logging every bite.
Sources & how this is kept current
Maintenance calories use the Henry (Oxford) BMR equations and physical activity levels from the SACN Dietary Reference Values for Energy (2011). Safe-deficit and minimum-intake guidance follows the NHS weight-loss plan and NICE public health advice; the 7,700 kcal-per-kilogram figure is the long-standing Wishnofsky estimate for body-fat energy density. See the review date at the top. This is general information to help you plan, not medical or dietary advice — for anything specific to you, speak to your GP or a registered dietitian.