BMR Calculator
Your basal metabolic rate from the Mifflin-St Jeor equation, with maintenance calories for your activity level.
Result
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.
Your daily energy
BMR is the floor; activity lifts it to maintenance.
Formula from Mifflin-St Jeor (1990) and Frankenfield systematic review (2005), checked against the source on May 25, 2026.
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 to use it
- Pick Metric or US units, then enter sex, age, weight and height: the Mifflin-St Jeor equation needs all four.
- Read your BMR: the calories your body burns at complete rest (organs, circulation, cell repair). It is a floor, not a target.
- 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).
- 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.
- 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. It accounts for 60-75% of total daily calorie burn for most people, making it 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. Skeletal muscle burns approximately 6 calories per pound per day at rest, compared to 2 calories per pound for fat. 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.
The formula, worked through
- Formula Katch-McArdle Formula
Katch-McArdle BMR = 370 + 21.6·LBM_kg. Uses lean body mass instead of total mass — more accurate when reliable body-fat data is available.
- Formula BMR Formula (Mifflin-St Jeor)
BMR = 10·mass_kg + 6.25·height_cm − 5·age + sex_constant. Mifflin-St Jeor 1990. Sex constant: +5 male, −161 female. Used as the baseline for TDEE and deficit math.
- Formula TDEE Formula
TDEE = BMR × activity factor. Factors: 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 very active, 1.9 athlete. Drives fat-loss deficit and lean-gain surplus math.
Related calculators
- Nutrition TDEE Calculator
Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.
- Nutrition Macro Calculator
Convert calorie targets into protein, carbs, and fat grams for your goal.
- Nutrition Protein Intake Calculator
Get daily protein targets based on training level and goal.
- Nutrition Intermittent Fasting Window Planner
Plan protocol-based eating and fasting windows with a 24-hour timeline and 5:2 day picker.
Background on the BMR Calculator
- Examples BMR Examples
Explore diverse BMR examples to understand your baseline metabolism for effective fat loss. Learn how age, weight, and lifestyle impact your daily calorie.
- Learn What Is BMR? Simply Explained
Understand BMR (Basal Metabolic Rate), the minimum calories your body needs at rest. Learn how it impacts weight loss, how to calculate it, and why it's.
- Learn What Is Metabolic Rate? Simply Explained
Discover what metabolic rate is, how it's calculated using BMR, and its critical role in effective weight loss. Learn to manage energy expenditure.
- Formula Female Athlete Formula Suite
Female athlete energy availability (RED-S): EA = (intake_kcal − exercise_kcal) / FFM_kg, where EA below 30 kcal/kg is clinically low per IOC 2018 consensus.
Articles
- 11 min read TDEE Formulas Compared: Mifflin vs Harris vs Katch
TDEE formulas compared: Mifflin-St Jeor, Harris-Benedict, Katch-McArdle, WHO/FAO. Population, error bands, and when each estimator is appropriate.
- 7 min read TDEE for Athletes: Why Formulas Understate You
TDEE for athletes: why standard equations under-predict for hybrid and endurance training. Activity-factor adjustments and exercise-energy add-ons.
- 8 min read Mifflin, Harris-Benedict, Cunningham: Resting Metabolism Methodology
Mifflin, Harris-Benedict, and Cunningham RMR equations: how each was derived, who the calibration cohort was, and the 5% Harris-Benedict overestimate.
By goal
Use it from code
The same calculation as a JavaScript module you can import. It runs where you import it, with no request, key or rate limit.
import { compute } from "https://aifithub.io/engines/bmr-calculator.js";Input and output contract, TypeScript types and the guide for agents.