Methodology: every formula, assumption and rounding rule

1. Units and parsing

Imperial input is converted before any equation runs: pounds are divided by 2.2046226218 to reach kilograms, and feet and inches are combined into total inches and multiplied by 2.54 to reach centimetres. Input is parsed as a finite decimal number; commas are accepted as decimal separators, and anything that resolves to NaN, infinity or a non-numeric string is rejected with a message tied to that field rather than silently coerced to zero.

2. Validation ranges

• Age: 14 to 100 years for the BMR, TDEE, Calorie, and Macro calculators — the prediction equations were developed in adolescents and adults from age 14.
• Age: 20 to 100 years for the BMI calculator — adult BMI categories do not apply below age 20.
• Height: 100 to 250 cm (39 to 98 in).
• Weight: 30 to 300 kg (66 to 660 lb) for all calculators.
• Custom deficit: capped and flagged rather than blocked, with the reason shown.

3. Body mass index

Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)². Reported to one decimal place. Adult categories: under 18.5 underweight, 18.5 to 24.9 healthy range, 25.0 to 29.9 overweight, 30.0 and above obesity. The healthy-range weights we display are the weights that correspond to BMI 18.5 and 24.9 at your height — they are a statistical range, not a personal target.

4. Basal metabolic rate

We use the Mifflin-St Jeor equation, with weight in kilograms (W), height in centimetres (H) and age in years (A):

• Male equation: BMR = 10W + 6.25H − 5A + 5
• Female equation: BMR = 10W + 6.25H − 5A − 161

The equation takes one of two biological variants. That input is the variable the equation needs, not a statement about gender identity. Results are rounded to the nearest whole calorie and presented with a ±5% band, which reflects the typical spread of prediction equations against measured resting energy expenditure.

5. Total daily energy expenditure

TDEE = BMR × activity multiplier, rounded to the nearest 5 kcal:

• Sedentary — 1.20
• Lightly active — 1.375
• Moderately active — 1.55
• Very active — 1.725
• Extremely active — 1.90

The multiplier is the single largest source of error in any calorie estimate, because it compresses planned exercise, daily steps, occupation, training intensity and training duration into one number. That is why we show a maintenance range plus the neighbouring multipliers instead of one figure alone.

6. Calorie targets

Goal calories scale maintenance by a percentage, so the adjustment stays proportional to body size:

• Maintain — TDEE × 1.00
• Lose, conservative — TDEE × 0.90; moderate — × 0.85; larger estimate — × 0.80
• Gain, slow — TDEE × 1.05; moderate — × 1.10; faster — × 1.15

Targets are rounded to the nearest 10 kcal. Weekly change is estimated with the simplified 7,700 kcal per kilogram (about 3,500 kcal per pound) convention. That convention is a static approximation: real weight change slows as body mass falls, and water, glycogen and gut contents move the scale independently of fat mass.

7. Deficit intensity labels and safeguards

A deficit is labelled small (up to 10% of maintenance), moderate (10 to 20%), larger (20 to 25%) or as requiring professional guidance (above 25%). We add a visible explanation — never a silent change to your number — when the deficit exceeds 25% of maintenance, when the target falls below estimated BMR, when the implied pace exceeds the 1 to 2 lb per week guidance, when inputs sit near validation limits, when you are under 14, when you indicate pregnancy or breastfeeding, or when you tell us you have a history of disordered eating.

8. Macronutrients

Protein and carbohydrate are 4 kcal per gram; fat is 9 kcal per gram. Percentage method: each macro percentage of the calorie target is divided by its energy density. The general preset is protein 25%, carbohydrate 45%, fat 30%, shown against the adult reference ranges of 10 to 35% protein, 45 to 65% carbohydrate and 20 to 35% fat. Those ranges are context, not a requirement that everyone match one split.

Protein-first method: protein is set from body weight at 1.4, 1.6, 1.8 or 2.0 g/kg, fat is set as a percentage of calories, and the remaining calories become carbohydrate. Grams are rounded to whole numbers and percentages to whole numbers, so the reconciled total can differ from your target by a few calories. We display that difference rather than hiding it, and we warn you when carbohydrate would fall below zero instead of printing a negative number.

9. What we deliberately do not do

• We do not describe any output as your exact, perfect or guaranteed number.
• We do not publish an “ideal weight”.
• We do not diagnose a slow or damaged metabolism.
• We do not market the largest deficit as the best option.
• We do not put a result behind an email address, account, survey or payment.

Sources

• Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990. Accessed 2026-01-18.
• Centers for Disease Control and Prevention. Losing weight: steady, gradual weight loss of 1 to 2 pounds per week. CDC, reviewed 2023. Accessed 2026-01-18.
• Institute of Medicine. Dietary Reference Intakes: Acceptable Macronutrient Distribution Ranges for protein, carbohydrate and fat. National Academies Press, 2005. Accessed 2026-01-18.
• Jäger R, Kerksick CM, et al. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition, 2017. Accessed 2026-01-18.
• World Health Organization. Body mass index classification for adults. WHO, 2024. Accessed 2026-01-18.
• National Institute for Health and Care Excellence. Obesity: identification, assessment and management (CG189). NICE, updated 2023. Accessed 2026-01-18.

Use the calculators

• All-in-one fitness calculator
• BMR Calculator
• TDEE Calculator
• Calorie Calculator
• Calorie Deficit Calculator
• Macro Calculator
• BMI Calculator