BMR Calculator Formula

Understand the math behind the bmr calculator. Each variable explained with a worked example.

Formulas Used

BMR (Calories/day)

bmr = 10 * weight_kg + 6.25 * height_cm - 5 * age + 5

Calories/hour at rest

bmr_per_hour = (10 * weight_kg + 6.25 * height_cm - 5 * age + 5) / 24

Variables

Variable	Description	Default
`weight_lbs`	Weight(lbs)	170
`height_inches_total`	Height (total inches)(inches)	70
`age`	Age(years)	30
`weight_kg`	Derived value`= weight_lbs * 0.453592`	calculated
`height_cm`	Derived value`= height_inches_total * 2.54`	calculated

How It Works

What BMR Actually Measures

Basal Metabolic Rate is the number of calories your body burns doing nothing. Lying in bed, not moving, not digesting food, just keeping organs running and cells alive. For most people, BMR accounts for 60-75% of total daily calorie burn. The rest comes from physical activity and the thermic effect of food (energy spent digesting).

The Formula (Mifflin-St Jeor)

Men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5 Women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) - 161

This equation was published in 1990 and is considered the most accurate for adults who aren't extremely muscular or extremely obese. The older Harris-Benedict equation (1919) overestimates by about 5% for most people.

BMR vs. TDEE

BMR is not your calorie target. It's the floor. Your Total Daily Energy Expenditure (TDEE) is BMR times an activity multiplier. A sedentary person's TDEE is about 1.2x their BMR. Someone exercising 5 days a week is closer to 1.55x. If your BMR is 1,600, your TDEE as a moderately active person is roughly 2,480 calories.

What Affects Your BMR

Muscle mass is the biggest variable. A pound of muscle burns about 6 calories per day at rest, while a pound of fat burns about 2. This is why two people at the same weight can have different BMRs. Age also matters. BMR drops roughly 1-2% per decade after 20, mostly because of muscle loss.

Common Mistakes

Eating at or below your BMR thinking it will speed up weight loss. Your body needs BMR just to function. Eating significantly below it slows metabolism and causes muscle loss.

Using BMR as a calorie target instead of TDEE. If you eat only your BMR calories, you're in a deficit equal to all your daily activity, which is too aggressive for most people.

Ignoring the accuracy range. The Mifflin-St Jeor formula is accurate to within 10% for most adults. Your actual BMR could be 150-200 calories higher or lower than calculated.

Worked Example

A 30-year-old, 170 lbs, 70 inches tall.

weight_lbs = 170height_inches_total = 70age = 30

01Weight: 77.1 kg, Height: 177.8 cm
02BMR = 10(77.1) + 6.25(177.8) - 5(30) + 5
03= 771 + 1,111.25 - 150 + 5 = 1,737 cal/day

When to Use This Formula

Setting a calorie baseline for a weight loss plan — your BMR tells you the minimum calories your body burns at complete rest, which you should generally not eat below.
Estimating your Total Daily Energy Expenditure (TDEE) by multiplying your BMR by an activity factor, to determine how many calories you need to maintain, lose, or gain weight.
Adjusting your calorie intake after significant weight change, since BMR shifts as body weight and composition change.
Comparing metabolic rates before and after building muscle mass, as lean mass is a key driver of BMR and gains should show measurable increases.
Understanding why two people of different ages, heights, and weights need different calorie intakes even at the same activity level.

Common Mistakes to Avoid

Confusing BMR with TDEE — BMR is the calories burned at absolute rest (lying in bed all day), while TDEE includes all daily activity; eating at your BMR level while exercising creates too steep a deficit for most people.
Using the wrong equation for your situation — the Mifflin-St Jeor equation (10 x weight in kg + 6.25 x height in cm - 5 x age - 161 for women, +5 for men) is the most accurate for most adults, while the older Harris-Benedict equation tends to overestimate by 5-15%.
Entering weight in pounds instead of kilograms or height in inches instead of centimeters without converting — the Mifflin-St Jeor coefficients assume metric units, and using imperial values without conversion gives drastically wrong results.
Treating BMR as a precise number rather than an estimate — individual variation due to genetics, hormones, body composition, and metabolic adaptation means your actual BMR can differ from the calculated value by 10-15%.
Overestimating activity level when calculating TDEE from BMR — most people with desk jobs should use "sedentary" or "lightly active" multipliers, not "moderately active," even if they exercise 3 times per week.

Frequently Asked Questions

What is BMR used for?

BMR tells you the minimum calories your body needs at rest to sustain vital functions like breathing and circulation. Multiply by an activity factor to get your total daily energy expenditure (TDEE) and plan your calorie intake.

What is the difference between BMR and TDEE?

BMR (Basal Metabolic Rate) is calories burned at complete rest. TDEE (Total Daily Energy Expenditure) is your actual daily calorie burn including all activity. TDEE = BMR multiplied by an activity factor ranging from 1.2 (sedentary) to 1.9 (very active).

Does muscle increase BMR?

Yes. Muscle tissue burns more calories at rest than fat tissue. Building lean muscle mass through strength training is one of the most effective long-term strategies for increasing your BMR and supporting weight management.

How accurate is the Mifflin-St Jeor equation?

The Mifflin-St Jeor equation is considered the most accurate formula for estimating BMR in most adults, with studies showing it predicts within 10% of measured BMR for most people. It is more accurate than the older Harris-Benedict equation.

Learn More

Guide

How to Calculate Daily Calories: A Complete Guide

Learn how to calculate your daily calorie needs using BMR, TDEE, and activity multipliers. Understand calorie deficits, surpluses, and how to set targets for your goals.

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