Mifflin-St Jeor Calculator

What Is the Mifflin-St Jeor Equation?

Published in 1990 by Mifflin, St Jeor, and colleagues, this equation estimates resting energy expenditure (REE/BMR) from weight, height, age, and sex. It was derived from indirect calorimetry in 498 healthy adults and is widely used as a default BMR formula in nutrition software and clinical references. Frankenfield et al. (2005) found it within ~10% of measured RMR for roughly 82% of non-obese adults and ~70% of obese adults — better than many older equations, though still a population estimate, not your measured metabolic rate. Mifflin is validated mainly in adults — for ages under 15 or WHO age-band workflows, use the Schofield Calculator. When height is unknown, the weight-only Owen Calculator (1986/1987) offers a simple RMR alternative. For all seven equations with auto-select and TDEE, see the REE Calculator.

The Mifflin-St Jeor Formula

Male:
BMR = (10 × kg) + (6.25 × cm)
    − (5 × age) + 5

Female:
BMR = (10 × kg) + (6.25 × cm)
    − (5 × age) − 161

Every variable is required: weight and height scale resting metabolism, age captures gradual decline, and the sex constant (+5 men / −161 women) reflects average body-composition differences at the population level.

How the Formula Works

Your calculator results show these contributions as bars — useful for understanding how weight, height, age, and sex combine in your estimate.

Worked Examples

BMR vs RMR vs TDEE

Activity Multipliers

TDEE ≈ BMR × activity factor. Standard multipliers (1.2–1.9) are simplified planning values that approximate activity, NEAT, and typical thermic effect of food — not a separate TEF add-on. Pick the level that matches an average week — not your most active day. When unsure, use the Activity Level Quiz for a precise multiplier.

Mifflin-St Jeor vs Harris-Benedict

Mifflin-St Jeor vs Katch-McArdle

Katch-McArdle uses lean body mass (from body fat %) and can be more appropriate when composition is far from average — but only with a reliable body-fat measurement. See the Katch-McArdle Calculator for Navy estimation, LBM breakdown, and TDEE planning. If your assessment already lists lean mass, the Cunningham RMR Calculator accepts direct LBM entry.

Lean mass (kg) = weight (kg)
    × (1 − body fat % / 100)

BMR = 370 + (21.6 × lean mass kg)

Accuracy & Error Sources

• Individual thyroid, medication, and illness effects are not modeled.
• Activity self-report is often a larger error source than the BMR equation itself.
• Recalculate after ~5–10 kg (10–15 lb) weight change or every 4–6 weeks.

Factors Affecting BMR

• Age — gradual decline; captured by the −5 × age term.
• Body size & composition — more lean mass generally means higher REE.
• Sex — population constant; individual variation still large.
• Genetics & hormones — not in the equation; part of individual spread.
• Sleep, stress, illness — can shift expenditure temporarily.
• Diet history — prolonged very-low intake may affect measured REE over time.

Supporting Metabolic Health

No food or habit permanently "fixes" a slow metabolism in the way marketing often claims. Evidence-supported habits that help maintain lean mass and daily expenditure include:

• Resistance training to preserve muscle during fat loss.
• Adequate protein (often ~1.6–2.2 g/kg/day for many active adults — see macro tool).
• Regular daily movement (NEAT) beyond structured gym sessions.
• Consistent sleep and recovery.
• Hydration and balanced energy intake over crash dieting.

Common Mistakes

Safety & Limitations