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BMR Calculator

Your basal metabolic rate from the Mifflin-St Jeor equation, with maintenance calories for your activity level.

Your numbers

Units
Biological sex
Age
Weight (kg)
Height (cm)
Activity level

Only the maintenance figure uses it; BMR is the same at every level.

Result

Basal metabolic rate (BMR)
1,748 kcal

BMR is the energy your body uses at complete rest (Mifflin-St Jeor). Maintenance multiplies it by your activity factor; the targets are maintenance − 500 and + 250 kcal.

Maintenance calories (TDEE)
2,709 kcal
Fat-loss target
2,209 kcal
Lean-gain target
2,959 kcal
Protein baseline
140.4 g

Your daily energy

BMR is the floor; activity lifts it to maintenance.

BMR
1,748 kcal
Maintenance
2,709 kcal
Fat-loss target
2,209 kcal
Lean-gain target
2,959 kcal

Formula from Mifflin-St Jeor (1990) and Frankenfield systematic review (2005), checked against the source on May 25, 2026.

Education, not medical advice. The result comes only from the published formula and your inputs. How we check our formulas.

What does the BMR Calculator calculate, and which formula does it use?

The BMR Calculator estimates basal metabolic rate with the Mifflin-St Jeor equation: 10 × weight (kg) + 6.25 × height (cm) − 5 × age, plus 5 for men or minus 161 for women. It then multiplies BMR by an activity factor from 1.2 (sedentary) to 1.9 (athlete) for maintenance calories, with fat-loss (−500 kcal) and lean-gain (+250 kcal) targets.

Last updated .

How far off can this be?

A systematic review of the four equations used most in practice found Mifflin-St Jeor, the equation this calculator uses, the most reliable: it predicted resting metabolic rate within 10% of the measured value in more non-obese and obese adults than any other, with the narrowest error range. Some individuals still fall outside 10%.

For the example inputs, the calculator gives a BMR of 1,748 kcal. Within 10%, the measured value would lie between 1,573 kcal and 1,922 kcal.

Older adults and US ethnic minorities were underrepresented in the studies behind the equations, so the error can be larger for them.

Source: Frankenfield, Roth-Yousey & Compher 2005, J Am Diet Assoc, “Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review” .

How to use it

  1. Pick Metric or US units, then enter sex, age, weight and height: the Mifflin-St Jeor equation needs all four.
  2. Read your BMR: the calories your body burns at complete rest (organs, circulation, cell repair). It is a floor, not a target.
  3. Choose your activity level to see maintenance calories (BMR × 1.2 for sedentary up to × 1.9 for twice-daily training), plus a fat-loss target (maintenance − 500) and a lean-gain target (maintenance + 250).
  4. Recalculate after weight changes: in Mifflin-St Jeor each kilogram is worth 10 kcal of BMR, so a 20 lb (9 kg) loss lowers it by about 90 kcal, and each year of age lowers it by 5 kcal.
  5. If you know your body fat, the TDEE Calculator adds Katch-McArdle, which uses lean mass and suits very muscular or very lean people better.

Questions people usually ask

What is BMR and why does it matter?

Basal Metabolic Rate is the minimum calories your body needs to sustain vital functions at complete rest — breathing, circulation, cell repair. The National Academies' 2023 Dietary Reference Intakes for Energy put basal and resting metabolism at 60 to 70 percent of total energy expenditure, the largest component of your energy budget.

Which BMR formula is most accurate?

The Mifflin-St Jeor formula (used here). In a 2005 systematic review (Frankenfield et al.) it predicted resting metabolic rate within 10% of measured values in more people than the older Harris-Benedict equations. The Katch-McArdle formula can be more accurate if you know your lean body mass, since it accounts for muscle mass directly.

Does muscle mass affect BMR?

Yes significantly. Tissue-level estimates used in resting-energy models (Elia's values, applied by Wang et al. 2010) put skeletal muscle at about 13 kcal per kg a day (about 6 per pound) and fat at about 4.5 kcal per kg (about 2 per pound). A person with 140 lb of lean mass has a meaningfully higher BMR than someone of identical weight with 115 lb of lean mass.

What happens to BMR during a calorie deficit?

BMR decreases as weight comes down: in Mifflin-St Jeor each kilogram lost lowers it by 10 kcal. Large or prolonged deficits add adaptive thermogenesis on top, so energy use falls further than the lost weight alone predicts. This is why gradual deficits and diet breaks help long-term fat loss, and why you should recalculate as your weight changes.

Can I increase my BMR?

Yes. Building muscle through resistance training is the most effective method — each pound of muscle raises daily BMR by approximately 6 calories. Adequate protein intake (0.7-1g per lb of body weight) also prevents BMR-reducing muscle loss during weight loss.

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Use it from an agent or code

Any agent can run this calculator by URL: put the inputs in the query string and get JSON back, with the inputs that used defaults and a line to cite (add format=md for markdown). MCP clients reach it through https://aifithub.io/mcp.

https://aifithub.io/api/run/bmr-calculator?sex=male&age=30&weight_kg=78&height_cm=178&activity_level=moderate

In code, import the same module; it runs where you import it, with no request or key.

import { compute } from "https://aifithub.io/engines/bmr-calculator.js";

Input and output contract, TypeScript types and the guide for agents.