How we calculate
The exact formulas, thresholds and rounding rules behind every calculator on the site.
Resting energy and daily calories
Resting metabolic rate uses the Mifflin-St Jeor equation, the formula most dietitians and the Academy of Nutrition and Dietetics favour: 10 x weight in kilograms + 6.25 x height in centimetres - 5 x age, plus 5 for men and minus 161 for women.
Total daily energy expenditure multiplies that figure by an activity factor: 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active and 1.9 for hard training twice a day. The comparison figure on the BMR page uses the older Harris-Benedict equation, published in 1919, purely so you can see how far the two estimates drift apart.
| Formula | Used for | Note |
|---|---|---|
| Mifflin-St Jeor | BMR and every derived number | current standard for resting energy |
| Harris-Benedict | comparison on the BMR page only | tends to read higher than Mifflin |
| Katch-McArdle | not used | needs body fat, so it is not safe as a default |
Body composition
Body fat uses the US Navy circumference method: a log-based equation on neck, waist and hip measurements, with a BMI-based cross-check shown beside it. The two rarely agree exactly, and the page says which is which rather than averaging them.
BMI uses the World Health Organization cut-offs - under 18.5, 18.5 to 24.9, 25 to 29.9 and 30 and over. Waist-to-height ratio uses the half-your-height rule promoted by NICE: below 0.4 is unusually low, 0.4 to 0.49 healthy, 0.5 to 0.59 increased risk and 0.6 or above high risk.
Fat-free mass index divides lean mass by height squared and normalises to 1.8 metres with the standard +6.1 x (1.8 - height) adjustment used in the body composition literature.
Training and hydration
One-rep max is reported as the average of the Epley, Brzycki and Lombardi formulas, with each estimate printed separately because they diverge above about eight reps.
Exercise energy cost uses MET values from the Compendium of Physical Activities: kilocalories = MET x 3.5 x kilograms / 200 x minutes. Water intake uses 35 ml per kilogram of body weight plus 350 ml for every 30 minutes of training.
Rounding and display
Energy figures are rounded to the nearest calorie for daily totals and to one decimal place for per-kilogram figures. Body fat, BMI, waist-to-height and FFMI are shown to one or two decimals. Rounding happens only at display time; every internal calculation keeps full precision.
What we deliberately do not do
We do not diagnose, we do not treat, and we do not give medical advice. We do not store the measurements you type: the calculator runs in your browser and the only copy of your inputs is the optional link you copy yourself.
We do not publish condition-specific calculators for pregnancy, PCOS or clinical populations. Those need a clinician's judgement and a different evidence base, so they are out of scope.
Every formula used on this site
The table below is the complete list. Nothing is calculated by a method that is not named here, and each calculator page repeats the formula that applies to it.
| Calculator | Method |
|---|---|
| TDEE | Mifflin-St Jeor resting energy x activity factor (1.2, 1.375, 1.55, 1.725, 1.9) |
| BMR | Mifflin-St Jeor: 10 x kg + 6.25 x cm - 5 x age + 5 (men) or -161 (women) |
| Calorie deficit | TDEE minus a percentage deficit, converted to a weekly rate |
| Macro split | Protein in g per kg of body weight, fat at 25% of calories, carbohydrate as the remainder |
| Body fat | US Navy circumference method, with a tape-measurement note on its limits |
| Ideal weight | Devine, Hamwi, Robinson and Miller formulas shown side by side |
| One-rep max | Epley and Brzycki estimates from a set of reps at a known weight |
| Water intake | 35 ml per kg of body weight, plus a training allowance |
| Heart rate zones | Tanaka maximum (208 - 0.7 x age) with Karvonen zone percentages |
| Calories burned | MET value x 3.5 x body weight in kg / 200, per minute of activity |
| Steps to calories | Step count to distance, then walking MET values for the pace |
| Waist-to-height ratio | Waist circumference divided by height, in the same unit |
| FFMI | Fat-free mass in kg divided by height in metres squared, plus a height normalisation |
Units, rounding and precision
Imperial entries are converted to metric before any formula runs, because every equation here was derived in metric units: pounds to kilograms at 0.45359237, inches to centimetres at 2.54. The conversion happens first so that the arithmetic you can reproduce by hand matches what the page shows.
Results are displayed rounded but calculated unrounded. Weight changes, energy totals and macro grams are rounded to the nearest whole number, body fat and FFMI to one decimal, and distances to two. If you re-run a calculation on a calculator yourself, expect the last digit to differ.
Where a formula has a documented valid range, the calculator does not silently extrapolate. Body-fat estimates say so next to the result when the inputs sit outside the range the method was validated for, rather than returning a confident number from a method that does not apply.
Where the numbers can still be wrong
An equation that predicts resting energy within 10% for most people still misses badly for some individuals, and the activity multiplier is a coarser instrument than the equation it multiplies. Two people with identical inputs can differ by several hundred calories a day through differences in muscle mass, gut bacteria, spontaneous movement and the accuracy of their own food logging.
Circumference methods for body fat assume a population-average fat distribution, so they are least reliable at the extremes: very lean, very muscular and very heavy bodies all break the assumption in different ways. The page says so next to the number.
MET values are population averages for a labelled activity and ignore how hard you were actually working. Step counts miss cycling and swimming entirely. Water targets ignore climate, illness and medication. None of these are reasons to discard the numbers; they are reasons to treat them as a starting point and correct them against observed results.
Reproducing a result by hand
Every number on the site can be checked with a calculator and the formula printed on the page. Worked example: a 35-year-old woman, 165 cm, 68 kg, lightly active. Resting energy is 10 x 68 + 6.25 x 165 - 5 x 35 - 161, which is 1,375 kcal. Multiply by 1.375 for light activity and maintenance is about 1,891 kcal.
A 20% deficit from that is 1,513 kcal, and at 1.7 g of protein per kg the protein target is 116 g, or 464 kcal. Fat at 25% of calories is 42 g, which leaves 131 g of carbohydrate. Running those steps in the same order the page uses will land on the same figures, apart from rounding.
Where the thresholds come from
The cut-offs on this site are not ours. They come from published guidance, and the table below names the body responsible for each one.
- BMI categories - World Health Organization adult cut-offs
- Waist-to-height ratio - NICE guideline CG189 and Ashwell's validation work
- Activity multipliers - ACSM and standard dietetic practice
- Exercise energy cost - the Compendium of Physical Activities
- Protein ranges - position stands on protein and exercise
- Maximum heart rate - Tanaka and Monahan's age formula
Last reviewed September 12, 2026. How we calculate · Editorial policy