BMR Calculator

How to calculate BMR manually

The Mifflin–St Jeor formula this calculator uses is: for men, BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5. For women, subtract 161 instead of adding 5. For example, a 30-year-old man weighing 75kg at 175cm: (10 × 75) + (6.25 × 175) − (5 × 30) + 5 = 750 + 1094 − 150 + 5 = 1,699 calories/day at complete rest. A woman with the same stats uses −161 instead of +5, landing closer to 1,533 calories/day.

What BMR actually measures

Basal Metabolic Rate is the energy your body spends keeping you alive while completely at rest — not sleeping, not digesting a meal, not moving at all. It covers the baseline cost of breathing, circulating blood, maintaining body temperature, and repairing cells at the tissue level. For most adults, BMR accounts for roughly 60–70% of total daily calorie burn, which is why it's the foundation every other calorie estimate builds on top of.

Clinically, BMR is measured in a lab under strict fasted, motionless conditions. What most calculators — including this one — actually estimate is closer to Resting Metabolic Rate (RMR), a slightly less strict measurement that allows for normal daily conditions rather than a controlled fast. In practice the two numbers are close enough that the distinction rarely matters for everyday use.

Why this calculator uses Mifflin–St Jeor

Several formulas exist for estimating BMR. The Harris-Benedict equation, first published in 1919 and revised in 1984, was the standard for decades but tends to overestimate calorie needs for much of the modern population. The Mifflin–St Jeor equation, published in 1990, has consistently been found in later research to be more accurate for a broader range of body types, which is why it's now the formula most commonly recommended by dietitians and used in clinical practice.

What changes your BMR

• Muscle mass. Muscle tissue is metabolically active and burns more at rest than fat tissue — two people at the same weight can have meaningfully different BMRs depending on body composition.
• Body size. Taller, heavier bodies have more tissue to maintain, which raises baseline energy needs.
• Age. BMR tends to decline gradually with age, driven largely by the muscle loss that often comes with it rather than age itself.
• Sex. Men typically carry more muscle and less body fat than women at the same height and weight, which is why the formula uses different constants.
• Hormones. Thyroid function in particular regulates metabolic rate — thyroid conditions can shift BMR up or down independent of diet or activity.
• Genetics. Baseline metabolic rate varies between individuals for reasons research hasn't fully mapped, even after controlling for size and body composition.

BMR vs TDEE

BMR only covers resting energy needs — it doesn't include walking, exercise, or daily activity. For a realistic total daily calorie target, add an activity multiplier on top with the TDEE calculator. Curious how your weight and height compare generally first? Check the BMI calculator.

Typical BMR by age (reference examples)

These are worked examples at a fixed weight and height (75kg, 175cm) across different ages, to show how much age alone shifts the number when everything else stays constant:

The pattern is consistent: exactly 50 calories/day lower per decade at the same weight and height, for both sexes, under this formula — the equation subtracts 5 calories per year of age. In practice, people who maintain muscle mass through their 40s and 50s tend to see a smaller real-world decline than the formula alone predicts, since the formula doesn't directly account for body composition — only age, weight, height, and sex.

How BMR fits into a full nutrition plan

BMR by itself isn't something you "hit" as a daily target — it's an input into a larger calculation. The typical planning sequence looks like this: calculate BMR → multiply by an activity factor to get TDEE → adjust TDEE up or down based on your goal (cut, maintain, or bulk) → split that calorie target into protein, carbs, and fat with a macro calculator → build actual meals that fit inside those numbers. Skipping straight from BMR to a meal plan without the TDEE step is one of the most common mistakes people make with these calculators — it usually results in a target that's far too low, since BMR excludes all daily activity.

Common misconceptions about BMR

"Eating below my BMR will make me lose weight faster." Eating at or below your BMR for extended periods is generally too aggressive — BMR is what your body needs at complete rest, not a safe target to eat toward. Weight-loss targets should be based on TDEE (BMR × activity factor), not BMR alone.

"My metabolism crashed the moment I turned 40." BMR does decline with age, but research suggests the change is slower and steadier than commonly assumed, and much of it traces back to gradual muscle loss rather than age itself — muscle loss that resistance training can meaningfully slow.

"BMR is a precise, fixed number." Any formula produces an estimate, not a lab-exact measurement. Actual BMR can vary by roughly 10% from the formula's output depending on individual factors like genetics and hormone status. Use your result as an informed starting point, not a fixed target.

Ways to support a healthy metabolism

• Build and maintain muscle. Resistance training adds metabolically active tissue, which raises resting energy needs over time.
• Eat adequate protein. Protein has a higher thermic effect than carbs or fat (meaning your body spends more energy digesting it) and helps preserve muscle, especially during a calorie deficit.
• Avoid extreme, prolonged restriction. Very aggressive, long-term calorie cuts can lead to muscle loss, which lowers BMR further — the opposite of what most people are trying to achieve.

Frequently Asked Questions

Does BMR change day to day?
It's relatively stable but can shift slightly due to illness, hormonal fluctuation, or extreme temperature exposure.

Can I increase my BMR?
Yes — building muscle mass through resistance training is the most reliable lever, since muscle tissue burns more at rest than fat tissue.

Is BMR the same as RMR?
They're very close. BMR is measured under strict lab conditions; RMR is measured under more relaxed real-world conditions. For everyday purposes, the two are effectively interchangeable.

How accurate is a BMR calculator?
Formula-based estimates are typically within about 10% of a lab measurement (indirect calorimetry) for most people, though individual body composition can shift the real number further from the estimate.

Should I eat only my BMR to lose weight?
No — BMR is the bare minimum your body needs at rest. Weight-loss targets should be based on TDEE with a moderate deficit, not BMR itself.

Who should be cautious with formula-based BMR

Mifflin–St Jeor was developed and validated on a general adult population, which means it's less reliable for some groups:

• Very muscular individuals (competitive athletes, bodybuilders) — the formula tends to underestimate BMR for people with significantly more muscle mass than average, since it doesn't account for body composition directly.
• Pregnant or breastfeeding people — energy needs shift substantially and shouldn't be estimated with a general-population formula; follow guidance from a healthcare provider instead.
• People with diagnosed metabolic or thyroid conditions — conditions like hypothyroidism or hyperthyroidism can shift real BMR well outside what any formula predicts.
• Older adults with significant muscle loss — the formula's age adjustment is a population average and may not reflect an individual's actual muscle mass and metabolic rate.

For any of these situations, a formula-based estimate is a reasonable starting point for a conversation with a doctor or dietitian, not a substitute for individualized guidance or, where needed, an actual metabolic rate test (indirect calorimetry).

Sources & References

• Mifflin MD, St Jeor ST, et al. — A new predictive equation for resting energy expenditure (American Journal of Clinical Nutrition, 1990)
• U.S. Departments of Agriculture and Health & Human Services — Dietary Guidelines for Americans, 2020–2025
• National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) — Energy balance and weight management