Health & Fitness Calculators

Which calculator answers which question

BMR, RMR and TDEE are three different numbers

They are used interchangeably in everyday talk and they are not the same thing, which is why two calculators can hand you figures 300 calories apart and both be right.

• BMR — basal metabolic rate. What the body burns completely at rest, fasted, in a thermally neutral room. A laboratory measurement.
• RMR — resting metabolic rate. The same idea under ordinary conditions; typically runs about 10% higher than BMR.
• TDEE — total daily energy expenditure. BMR multiplied by an activity factor: everything you actually burn in a day.

Only TDEE is useful for planning intake. The calorie calculator works through the distinction with numbers, and the calorie calculator applies the multiplier for you.

Activity multipliers

Most people overestimate here. Three gym sessions a week alongside a desk job is “lightly active”, not “very active”, and picking the wrong row moves the daily target by 400 calories or more.

Which BMR formula should you use?

Three equations are in common use, and they disagree. All three, applied to the same 35-year-old man, 180 cm, 80 kg, 20% body fat:

Mifflin-St Jeor (men): 10W + 6.25H − 5A + 5

Mifflin-St Jeor (women): 10W + 6.25H − 5A − 161

Harris-Benedict (men): 88.362 + 13.397W + 4.799H − 5.677A

Harris-Benedict (women): 447.593 + 9.247W + 3.098H − 4.330A

Katch-McArdle: 370 + 21.6 × lean body mass in kg

W is weight in kilograms, H height in centimetres, A age in years. Mifflin-St Jeor is the sensible default: it was derived from a more representative sample than the 1919 Harris-Benedict data and generally tracks measured values more closely. Katch-McArdle ignores age and sex entirely and works from lean mass, which makes it the better choice for athletes and the worse choice for anyone guessing at their body fat percentage.

BMI ranges, and what BMI cannot see

BMI = weight in kg ÷ (height in m)²

BMI uses two numbers and therefore cannot distinguish muscle from fat, or tell you where fat is carried. A muscular athlete can land in the “overweight” band in perfect health; someone with low muscle mass can sit inside the healthy band with a high fat percentage. It also reads differently across ages and ethnic groups — several health authorities apply lower thresholds for South and East Asian populations. Pair it with the body fat calculator and the body fat ranges and measurement methods before drawing conclusions from a single figure.

Estimating a one-rep max without attempting one

Two standard equations, from a set taken close to failure:

Epley: 1RM = weight × (1 + reps ÷ 30)

Brzycki: 1RM = weight × 36 ÷ (37 − reps)

Both are reliable up to about 10 reps and drift apart beyond that, because endurance rather than strength starts to set the limit. A set of 5 at 100 kg estimates 117 kg by Epley and 113 kg by Brzycki — close enough to program from, not close enough to treat as a tested max.

Reading these results honestly

• Every number here is an estimate from a population equation, not a measurement of you. Individual metabolic rate varies by roughly ±10% around any formula's prediction.
• Track outcomes, not predictions. If the calorie target and real weight change disagree over three or four weeks, the real change is the correct one — adjust the target.
• Due dates are a 280-day convention, not a forecast. Only about 4% of births occur on the estimated date.
• None of this is medical advice. Pregnancy, medication, thyroid conditions and eating-disorder history all change what these numbers mean, and that is a conversation for a doctor or registered dietitian.

The formulas behind this category, and the published sources they come from, are listed on the methodology page.