Quick definition

How it actually works

The Mifflin-St Jeor equation is the formula most clinicians use to estimate BMR. For men it's 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 5. For women you subtract 161 at the end instead of 5. The result is calories per day at total rest. A 35-year-old, 5'8", 165 lb woman lands around 1,420 BMR; a 45-year-old, 6'0", 200 lb man lands around 1,830.

The single biggest lever on BMR is your lean body mass. A pound of muscle burns roughly 6 calories a day at rest; a pound of fat burns about 2. That difference is why two people the same weight can have BMRs 100 to 200 calories apart. It's also why sarcopenia matters — losing muscle past 40 drops your BMR even if the scale stays the same.

BMR also drifts down with age. Most studies show roughly a 1 to 2 percent decline per decade after 30, mostly driven by muscle loss rather than a mysterious "slowing metabolism." A landmark 2021 Science study by Pontzer et al. actually found BMR stays remarkably stable from age 20 to 60 — the post-40 weight creep is almost always a calorie problem, not a furnace problem.

Why it matters for weight loss

BMR is the starting point for any honest weight-loss math. You can't build a calorie deficit without first knowing roughly what your body burns. Most viral plans skip this step and sell you a number — 1,200 calories, the famous "1,500 for women" — that has no relationship to your actual physiology.

Plug your real numbers into our calorie deficit calculator first. Whatever BMR comes out, multiply it by your activity factor (roughly 1.2 sedentary, 1.55 moderately active, 1.725 very active) to estimate your TDEE. Subtract 300 to 500 from that. That's your daily target. Anything dramatically lower is a starvation diet, not a sustainable plan.

Common misconceptions

The biggest myth is that some people have a "naturally slow metabolism." Real measurement studies (using a metabolic chamber, the gold standard) show BMR variance between individuals of the same age, sex, and body size is small — usually within ±200 calories. The person who claims they "gain weight on 1,000 calories a day" is almost always under-reporting intake by 30 to 50 percent. That's been replicated in multiple studies dating back to the 1990s.

The second myth: spicy food, green tea, or apple cider vinegar will meaningfully raise your BMR. None of them do, in any clinically relevant way. The honest path to a higher BMR is to add muscle, not to chase a supplement aisle.

Related terms

• TDEE · Total Daily Energy Expenditure Everything you burn in 24 hours — BMR plus movement, digestion, and fidgeting.
• NEAT · Non-Exercise Activity Thermogenesis Calories burned by everything that isn't exercise — fidgeting, walking, standing, chores.
• Thermic Effect of Food · TEF The 10% or so of calories your body burns just digesting what you ate.
• Metabolic Adaptation The drop in calorie burn that follows sustained dieting — real, but smaller than TikTok claims.
• Calorie Deficit Eating fewer calories than you burn — the only mechanism that produces fat loss.

Read next on Real Easy Diet

• Calorie deficit calculator
• BMI calculator
• How much weight can you lose in a month?
• Back to the full glossary

Sources

1. [01]
   Estimating energy needs — NIDDK NIDDK
2. [02]
   Basal metabolic rate — MedlinePlus NIH MedlinePlus
3. [03]
   Metabolism and weight loss — Mayo Clinic Mayo Clinic