How the calorie calculator works

Who it is for

Healthy men aged 19 to 78: the age range of the 498 healthy adults (251 men) the resting-energy equation was built on. Not for anyone under 19, women, people with a medical condition, on medicine that affects weight or appetite, recovering from injury or surgery, or with a history of an eating disorder. They should see a doctor or registered dietitian.

Step 1: resting energy (Mifflin–St Jeor)

Resting energy (kcal a day) = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5. Source: Mifflin et al., American Journal of Clinical Nutrition, 1990 (PubMed).

Accuracy: a systematic review found it the most reliable of the common equations (Frankenfield et al., 2005, PubMed); in a later study of 337 adults it was within 10% of measured resting burn for 82% of people, with no average bias (Frankenfield, 2013, PubMed).

Step 2: activity level (FAO/WHO/UNU)

Daily energy = resting energy × physical activity level (PAL). The FAO/WHO/UNU report on human energy requirements (2004) defines PAL as daily energy ÷ basal energy, with lifestyle bands of 1.40–1.69 (sedentary or light), 1.70–1.99 (active) and 2.00–2.40 (vigorous) (FAO, chapter 5).

Your day outside training sets the base: mostly sitting 1.55, on your feet a lot 1.64, hard physical work 1.85 (from the report’s worked examples). Each weekly training day adds 0.2 ÷ 7, because FAO notes one hour of daily exercise raises PAL by about 0.2. PAL is capped at 2.40.

Step 3: a safe pace

Fat loss: at most 1% of body weight or 0.98 kg a week, whichever is lower (Helms et al., 2014, PMC; NIH/NHLBI guidelines, 1998, PDF). Muscle gain: at most 0.5% of body weight a week (Iraki et al., 2019, PMC). If your timeframe is shorter, it is lengthened.

Floors: never below 1,500 kcal a day, the lower end of the men’s diet level in the 2013 AHA/ACC/TOS guideline (PMC); diets under 800 kcal need medical supervision. A target weight below BMI 18.5 (WHO) is refused.

Step 4: the projection (Hall et al., 2011)

The rule that 7,700 kcal equals 1 kg is wrong over time because the body adapts. We use the linearised form of the body-weight model in Hall et al., The Lancet, 2011 (PMC), with every constant quoted from the paper’s web appendix (NIDDK PDF): ρF 39.5 MJ/kg, ρL 7.6 MJ/kg, γF 13 and γL 92 kJ/kg/day, ηF 750 and ηL 960 kJ/kg, βTEF 0.1, βAT 0.14, Forbes constant 10.4 kg, starting fat mass from the Jackson equation.

Weight at day t = starting weight + (ΔEI ÷ ε) × (1 − e^(−t/τ)), where ΔEI is the daily change in intake and τ = ρ ÷ ε. The band shows the same projection if your true maintenance is 10% higher or lower than estimated.

The full model also tracks water and stored carbohydrate, which needs your usual carbohydrate and salt intake. Without them we use the linear form; in tests the two differed by under 1 kg over 6 months for moderate plans.

Step 5: protein, fat, carbohydrate and fibre

Protein: 1.6 g per kg of body weight to maintain or build muscle, 2.0 g per kg to lose fat (ISSN position stand, Jäger et al., 2017, PMC: 1.4–2.0 g/kg; Morton et al., 2018, PMC: benefit plateaus around 1.6 g/kg), never more than 35% of calories.

Fat 25% of calories, moved only to keep carbohydrate between 45% and 65%; fat stays within 20–35% and carbohydrate is the rest (Institute of Medicine ranges, Health Canada DRI table). Fibre: 14 g per 1,000 kcal; the adequate intake for men is 38 g a day (19–50) and 30 g (51+). Grams use 4, 9 and 4 kcal per gram (FAO, 2003).

How we check it

The code is tested against six worked examples computed step by step from these equations, and against Hall’s own example in the paper (a 100 kg man cutting 2 MJ a day settles at about 75 kg, half the change in about a year). All tests pass.

Limits

Every number is an estimate. Equations predict groups well and individuals less well; weigh yourself for 2 to 3 weeks and adjust. BMI misreads very muscular people. The calculator gives no meal plans and is not medical advice.

Review

Written by our nutrition coach. Reviewer: Qualified nutrition professional (to be named). Last reviewed 29 October 2026.