What is BMR (Basal Metabolic Rate)?

BMR vs RMR vs TDEE

Three terms get used interchangeably and shouldn't be. Knowing the difference matters because most online calculators give you one number labeled as another.

• BMR (Basal Metabolic Rate): Calories burned at complete rest, measured under strict lab conditions — 12 hours fasted, lying down, in a thermoneutral room, awake but not moving. The textbook number.
• RMR (Resting Metabolic Rate): Calories burned at rest under more realistic conditions — sitting quietly, not fasted, in normal room conditions. RMR runs roughly 10% higher than BMR. Most consumer fitness apps and "BMR calculators" actually estimate RMR and call it BMR. For practical purposes, treat them as the same number.
• TDEE (Total Daily Energy Expenditure): BMR + activity + thermic effect of food + NEAT. The actual total your body burns in 24 hours. This is the number you build a calorie target off of, not BMR.

The relationship: BMR is the largest single chunk of TDEE (~60-75%), but it's not the whole picture. See what is TDEE for how the pieces add up.

Calculating BMR — Mifflin-St Jeor Formula

The Mifflin-St Jeor equation, published in 1990, is the modern standard. It outperforms the older Harris-Benedict equation against real-world calorimetry data and is what every reputable nutrition app (MacroMate included) uses.

For males:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5

For females:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

Worked example: 30-year-old male, 180 lb, 5'10". Convert: 180 lb = 81.6 kg, 5'10" = 178 cm.
BMR = (10 × 81.6) + (6.25 × 178) − (5 × 30) + 5 = 816 + 1,113 − 150 + 5 = 1,784 cal/day.

Worked example: 30-year-old female, 140 lb, 5'5". Convert: 140 lb = 63.5 kg, 5'5" = 165 cm.
BMR = (10 × 63.5) + (6.25 × 165) − (5 × 30) − 161 = 635 + 1,031 − 150 − 161 = 1,355 cal/day.

What Raises and Lowers BMR

Raises BMR:

• More muscle mass. Muscle tissue is roughly 2-3x more metabolically active than fat at rest. Adding 10 lb of muscle adds ~50-70 cal/day to BMR. Not huge, but real.
• Larger body size. More body to maintain = more calories burned at rest, regardless of composition. A 200-lb person has higher BMR than a 150-lb person, full stop.
• Being male. Males average more lean mass per pound of bodyweight, which is why the Mifflin-St Jeor formula adds +5 for males and −161 for females.

Lowers BMR:

• Age. BMR drops roughly 2-3% per decade after 20, mostly because of lean-mass loss. The "metabolism slowing with age" effect is mostly muscle loss masquerading as metabolic decline.
• Severe calorie restriction. Chronic deep deficits (>30% below TDEE for months) trigger metabolic adaptation — the body downshifts BMR to defend bodyweight. The fix is refeeds and reverse diets, not more cardio.
• Very low body weight. Smaller body, less to maintain, lower BMR.

Why BMR Alone Isn't Your Calorie Target

The single most common mistake in self-directed dieting: people calculate BMR, set their daily intake at BMR, and then crash in 3 weeks. Eating at BMR means eating below TDEE by 30-50% for anyone who isn't bedridden — a deficit so aggressive that energy, sleep, training, and adherence all collapse.

To get to a real target, you have to add activity. Multiply BMR by an activity factor: 1.2 sedentary, 1.375 light activity, 1.55 moderate, 1.725 active, 1.9 very active. That gives you TDEE. Then subtract for a deficit or add for a surplus.

Worked example, same 180-lb male, moderately active: BMR 1,784 × 1.55 = TDEE ≈ 2,765 cal. A 20% deficit puts the cutting target at ~2,212 cal — not 1,784. The difference between "ate at BMR" and "ate at a 20% deficit off TDEE" is the difference between metabolic crash and a sustainable cut.

For the deficit math itself, see what is a calorie deficit. For the full TDEE breakdown, see what is TDEE.

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