Which BMR formula is most accurate?

The Mifflin-St Jeor equation (1990) is the most accurate of the common predictive formulas for healthy adults — about ±10% standard error. It outperforms older formulas like Harris-Benedict (which tends to overestimate) and is the formula recommended by the Academy of Nutrition and Dietetics.

Does BMR change with age?

BMR drops roughly 1–2% per decade after age 20, mostly because we lose lean muscle mass over time. Resistance training is the most effective way to slow this decline.

Can I lose weight by eating below my BMR?

Eating below BMR for short periods is medically reasonable under supervision (e.g., very low calorie diets for severe obesity), but for most people it's neither necessary nor sustainable. A 500 kcal deficit from your TDEE (which includes activity) is the standard approach and produces about 0.45 kg of weight loss per week.

How is BMR different from RMR?

RMR (resting metabolic rate) is measured a few hours after eating and any activity, while BMR requires fasted, post-sleep, lying-down measurement in a lab. The difference is small (~10%); the terms are often used interchangeably.

Why is my BMR different from someone the same size?

Two people with identical weight and height can have different BMRs because of body composition (more muscle = higher BMR), age, sex, and individual variation. Predictive equations capture the average; you may run faster or slower than predicted.

BMR Calculator — Mifflin-St Jeor

BMR Calculator

Mifflin-St Jeor equation · Basal metabolic rate at rest

Implements a published clinical formula — see citation

Maintained by TheHealthTools team. Not medical advice.

Weight (lbs)

Height (ft)

Inches

Age

Sex

Activity level (for TDEE)

What BMR is — and why it matters

Basal metabolic rate is the energy your body uses to keep you alive at complete rest. It accounts for 60–75% of total daily energy expenditure in most sedentary adults. Knowing your BMR is useful because it's the floor: cutting calories below BMR for extended periods slows metabolism, breaks down lean tissue, and usually doesn't produce the long-term weight loss people are after.

We use the Mifflin-St Jeor equation, published in 1990 and now the standard recommendation from the Academy of Nutrition and Dietetics. It's accurate to about ±10% in healthy adults, outperforming older formulas like Harris-Benedict (which tends to overestimate BMR by 5–15%).

For most weight-management goals, BMR alone isn't the number you want — TDEE (BMR × activity factor) is. We compute both, and the activity multiplier dropdown lets you tune the second based on how active you actually are.

Frequently asked questions

Basal metabolic rate is the energy your body uses at complete rest — to keep your heart beating, brain running, and temperature stable. It's the floor below which intentional weight loss becomes risky.