How our calculations work

This page sets out every formula, multiplier and assumption behind the calculators on this site, including the ones we are not fully confident in. If you want to check our working, everything is here.

Units and conversion

Internally, every calculation works in kilograms and centimetres. Other units exist only for display, and switching the unit shown never changes the stored value, so moving between stone and kilograms cannot introduce drift.

• 1 pound = 0.45359237 kg exactly (international avoirdupois pound)
• 1 stone = 14 pounds exactly
• 1 inch = 2.54 cm exactly
• 1 foot = 12 inches exactly

Displayed values are rounded, weights to a tenth of a unit, heights to the nearest inch or centimetre. Where rounding the pounds part of a stone-and-pounds figure would produce 14, it carries to the next stone, so you will never see "11 st 14 lb".

Body mass index

Categories for most adults follow the NHS^[NHS]. The category is derived from the BMI after rounding to one decimal place, so a displayed BMI of 25.0 is never labelled healthy.

Where a user selects an Asian, Black African, African-Caribbean or Middle Eastern family background, we use the lower NHS thresholds: 23 to 27.4 is overweight and 27.5 or above is obese. The NHS does not publish adjusted class 2 and class 3 obesity boundaries, so we do not invent them, everything at or above 27.5 is reported simply as obese.

Basal metabolic rate

We use the Mifflin-St Jeor equation^{[American Journal of Clinical Nutrition]}. It requires no body-fat measurement, which makes it usable from height, weight, age and sex alone.

Total daily energy expenditure

These are the conventional consumer-calculator values. Their original provenance is not clearly documented, and self-reported activity is the largest single source of error in any TDEE estimate, plausibly ±20%.

Calorie deficit and rate

The NHS suggests losing 1 to 2 lb, or 0.5 to 1 kg, a week, and reducing intake by around 600 kcal a day^{[NHS Better Health]}. All three paces we offer sit inside that band.

The rate is capped at the lower of 1 kg a week or 1% of body weight a week, so someone lighter is never pushed into a large relative deficit.

How the timeframe is estimated

Most calculators divide the total weight to lose by a daily deficit and print a date. That assumes your energy needs never change. They do, a lighter body burns less, so a fixed calorie target yields a shrinking deficit.

Instead we run a week-by-week simulation, recalculating BMR and TDEE at the new weight each week. The calorie target is re-derived every 4 weeks, modelling someone who revisits the calculator as they lose weight. The simulation stops at a plateau, at the target, or after 260 weeks.

It is run three times. The scenarios differ in exactly three named parameters, so every part of the range we report is attributable to one stated assumption:

The reported range runs from the optimistic scenario rounded down to the conservative scenario rounded up, so displaying it can only widen the span, never narrow it into false confidence.

Waist-to-height ratio

We report a single threshold of 0.5, following NHS advice to keep your waist to less than half your height^[NHS]. Other calculators show four or five risk bands; the NHS publishes one threshold, so we show one.

Safety limits

The calculators refuse to produce a plan in some situations, rather than returning a number we do not think should be acted on:

• A target below the healthy BMI range produces a neutral explanation and a suggestion to speak to a healthcare professional, not a calorie target or a date.
• A calorie target is never suggested below your estimated BMR, and never below 800 kcal a day under any circumstances.
• A target above your current weight switches to maintenance calories rather than producing a negative timeframe.
• The tools are for adults; under 18 we do not calculate at all.

Where the numbers are less solid

Every source cited on this site has now been read at source. What follows is a different thing: figures that are read, cited and still not certain, either because they are a convention rather than guidance, or because they are a choice this site made. Publishing them without saying so would be worse than not publishing them.

Calorie floors of 1,200 and 1,500 kcal

NICE defines a low-calorie diet as 800 to 1,200 kcal a day, and recommends one only within a specialist weight-management service^[NICE]. The lower figure is where this calculator stops: below it you are in territory NICE places under specialist supervision, which is not somewhere a calculator should send you unaccompanied. The 1,500 kcal floor for men is our own and more cautious still, since NICE gives no figure by sex. Below 800 kcal, the very-low-calorie threshold in the same guideline, we produce no plan at all.

7,700 kcal per kilogram

The familiar "3,500 kcal per pound" rule. It is a static approximation that assumes all weight lost is fat and that expenditure never changes, and it is known to overstate long-run weight loss. We vary it across scenarios rather than treating it as fixed.

Adherence and metabolic adaptation figures

Our own modelling choices, used to produce an honest range instead of a single false-precision number. Not fitted to data.

What happens to your measurements

Every calculation runs in your browser. Your height, weight, age and target are never sent to a server, never logged, and never included in analytics. Entries are saved to your own device so they are still there if you come back, and the "Start again" button clears them.