Calorie Calculator Methodology

Overview

The calculators run entirely in the visitor's browser. They take sex, age, height, weight, activity level and goal (plus body fat percentage for Katch-McArdle) and work through four steps:

1. BMR (basal metabolic rate): energy used at rest, from one of three published equations.
2. TDEE (total daily energy expenditure): BMR multiplied by an activity factor.
3. Goal calories: TDEE plus or minus a fixed number of kcal per day, with a minimum floor.
4. Macros: goal calories divided into protein, carbohydrate and fat by a percentage split, at 4, 4 and 9 kcal per gram.

Imperial inputs are converted first: 1 lb = 0.45359237 kg and 1 inch = 2.54 cm. The equations then work in kilograms, centimetres and years. Results are shown rounded to whole kcal and whole grams; rounding happens only for display, so the macros are calculated from the unrounded goal calories.

Accepted inputs: age 13 to 100 years, height 100 to 250 cm, weight 30 to 300 kg, and for Katch-McArdle a body fat percentage of 3 to 70.

Step 1: BMR equations

Mifflin-St Jeor (default)

Published by Mifflin and colleagues in 1990 and the default in the calculator. Weight W in kg, height H in cm, age A in years:

Men:    BMR = 10 × W + 6.25 × H − 5 × A + 5
Women:  BMR = 10 × W + 6.25 × H − 5 × A − 161

Harris-Benedict (revised, Roza & Shizgal 1984)

The 1918 Harris-Benedict equation as re-fitted by Roza and Shizgal in 1984. The calculator uses the revised coefficients, not the original 1918 ones:

Men:    BMR = 88.362 + 13.397 × W + 4.799 × H − 5.677 × A
Women:  BMR = 447.593 + 9.247 × W + 3.098 × H − 4.330 × A

Katch-McArdle (needs body fat %)

Katch-McArdle works from lean body mass instead of total weight, so it does not use sex or age. When a site owner selects it, the calculator adds a required body fat % field:

Lean body mass (kg) = W × (1 − body fat % / 100)
BMR = 370 + 21.6 × lean body mass

It is only as accurate as the body fat estimate it is given. A rough guess can move the result more than the choice of equation does.

Step 2: from BMR to TDEE

TDEE = BMR × activity factor. The default levels and factors are:

Moderately active is preselected. These are the conventional multipliers used with the Mifflin-St Jeor and Harris-Benedict equations. They are broad bands, and people often overestimate their own activity level.

Step 3: goal calories

Goal calories = TDEE + the goal's daily adjustment:

The weekly figures are the usual rule of thumb (roughly 7,700 kcal per kg of body fat, or 3,500 kcal per lb). Real weight change is not linear and usually slows as the body adapts, so treat them as a starting point.

Minimum floor. If the result falls below 1,500 kcal for men or 1,200 kcal for women, the calculator raises it to that minimum and says so: faster loss than this is not recommended without medical supervision.

Site owners can change these numbers

The values above are the plugin defaults. On a given website the owner can, under Settings > Your Move Nutrition, rename activity levels and set each multiplier between 1 and 2.5, and rename, enable or disable goals and set each adjustment between −1,500 and +1,500 kcal per day. At least one goal always stays enabled. A site owner can also hide the goal selector, which then calculates for maintenance. If the calculator you used shows different labels, the site has customised them.

Step 4: macro split

Goal calories are divided by percentage, then converted to grams:

Protein (g) = goal kcal × protein % / 4
Carbs (g)   = goal kcal × carbs %   / 4
Fat (g)     = goal kcal × fat %     / 9

Balanced is preselected. The split is a percentage of calories, so protein scales with the calorie target rather than with body weight.

Worked example

A 30-year-old woman, 165 cm, 60 kg, moderately active, choosing “Lose weight” and the Balanced split, with the default Mifflin-St Jeor equation:

BMR    = 10 × 60 + 6.25 × 165 − 5 × 30 − 161  = 1,320.25  → 1,320 kcal
TDEE   = 1,320.25 × 1.55                       = 2,046.39  → 2,046 kcal
Goal   = 2,046.39 − 500                        = 1,546.39  → 1,546 kcal  (above the 1,200 floor)

Protein = 1,546.39 × 30% / 4 = 116 g
Carbs   = 1,546.39 × 40% / 4 = 155 g
Fat     = 1,546.39 × 30% / 9 =  52 g

For the same person, the revised Harris-Benedict equation gives a BMR of about 1,384 kcal, and Katch-McArdle at 28% body fat gives about 1,303 kcal (lean mass 43.2 kg). The equations disagree by a few percent, which is normal and one reason to treat any result as an estimate.

The floor in action: a 60-year-old woman, 155 cm, 50 kg, sedentary, choosing “Lose weight” has a Mifflin-St Jeor BMR of 1,008 kcal and a TDEE of 1,209 kcal. TDEE − 500 would be 709 kcal, so the calculator shows 1,200 kcal with a note.

BMI calculator

The BMI calculator (and the optional BMI line on the calorie calculator) uses the standard formula, with height h in metres:

BMI = weight (kg) / h²

The result is shown to one decimal and placed in the WHO adult classification:

The category is assigned from the unrounded value. The healthy weight range shown by the BMI calculator is the weight that gives a BMI of 18.5 to 24.9 at the entered height: 18.5 × h² to 24.9 × h² kg (converted to pounds in imperial mode). For a height of 1.65 m the BMI calculator shows 50.4 to 67.8 kg, and 60 kg gives a BMI of 22.0, healthy weight.

BMI is a population screening measure. It does not distinguish muscle from fat or account for where fat is carried, and the WHO adult cut-offs do not apply to children and adolescents, who are assessed with age- and sex-specific growth charts. Some populations, including many Asian populations, have different suggested action points.

Limitations

• Estimates, not measurements. In a systematic review of these equations, Mifflin-St Jeor was the most reliable, yet no equation matched measured resting metabolic rate within 10% for everyone (Frankenfield et al. 2005). Adjust based on how your weight actually changes over a few weeks.
• Not medical advice. The calculators are general information tools. They do not diagnose anything or replace advice from a doctor or registered dietitian.
• Adults. The equations and BMI categories were developed for adults. The calculator accepts ages from 13, but results for under-18s should be read with caution and checked with a professional.
• Not suited to every situation. Pregnancy and breastfeeding, eating disorders, chronic illness, recovery from surgery and other medical conditions all change energy needs. Very muscular athletes and people with very high or low body fat are poorly described by weight-based equations; Katch-McArdle with a measured body fat percentage is usually a better fit for them.
• Two sex options. The Mifflin-St Jeor and Harris-Benedict equations were fitted separately for men and women, so the calculator asks for one of the two.

Where these calculators run

These formulas power the calorie and BMI calculators in the free Your Move Nutrition WordPress plugin. The calculators work without an account or API key and never contact an external server. The plugin's food tracker, barcode scanner, recipes and meal plans run on the Your Move Nutrition API.

References

1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241-247.
2. Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition. 1984;40(1):168-182.
3. Harris JA, Benedict FG. A biometric study of human basal metabolism. Proceedings of the National Academy of Sciences. 1918;4(12):370-373.
4. McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance. Lippincott Williams & Wilkins (source of the Katch-McArdle lean-mass equation).
5. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association. 2005;105(5):775-789.
6. World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO Consultation. WHO Technical Report Series 894. Geneva: WHO; 2000.