What is BMR (Basal Metabolic Rate)?

Your Basal Metabolic Rate is the number of calories your body burns to keep vital functions running while at complete rest - breathing, circulating blood, regulating body temperature, growing and repairing cells, brain function. BMR accounts for 60-75% of your total daily calorie expenditure - much more than exercise! Knowing your BMR is the foundation of any weight management plan: eat above your TDEE (BMR + activity) to gain weight, eat below to lose. The Mifflin-St Jeor formula used in this calculator has been validated by multiple studies as the most accurate prediction equation for general populations, with a typical error margin of ±10%.

How to use this tool

1. Choose unit system — Metric (kg, cm) or Imperial (lbs, inches) - tab at the top.
2. Enter sex — Male or female. Men typically have higher BMR due to greater muscle mass.
3. Enter age — BMR declines about 1-2% per decade after 30 due to natural muscle loss (sarcopenia).
4. Enter weight and height — Be accurate - inaccurate inputs give inaccurate BMR. Weigh in the morning after using the bathroom.
5. Read your BMR + TDEE — BMR is your at-rest calories. The stats show TDEE for 4 different activity levels (sedentary to very active) - those are your maintenance calorie targets.

Mifflin-St Jeor formula

The most accurate BMR formula for general populations:

• Men: BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age + 5
• Women: BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age - 161

Example: 30-year-old man, 80 kg, 175 cm:

• BMR = 10(80) + 6.25(175) - 5(30) + 5 = 800 + 1094 - 150 + 5 = 1,749 calories/day

Activity multipliers for TDEE:

• Sedentary (desk job): BMR × 1.2
• Light activity (1-3 workouts/week): BMR × 1.375
• Moderate (3-5 workouts): BMR × 1.55
• Very active (6-7 workouts or physical job): BMR × 1.725

Examples

• 25-year-old male, 70 kg, 175 cm: BMR ~1,679. Moderate activity TDEE ~2,602 kcal.
• 30-year-old female, 60 kg, 165 cm: BMR ~1,345. Light activity TDEE ~1,849 kcal.
• 45-year-old female, 70 kg, 160 cm: BMR ~1,314. Sedentary TDEE ~1,577 kcal.
• 50-year-old male, 90 kg, 180 cm: BMR ~1,805. Light activity TDEE ~2,482 kcal.
• 20-year-old male athlete, 75 kg, 178 cm: BMR ~1,754. Very active TDEE ~3,025 kcal.

Tips & best practices

• Recalculate BMR every 5 kg of weight change - the number drops as you lose weight
• After age 30, BMR drops 1-2% per decade naturally - resistance training preserves muscle and metabolic rate
• Crash diets (below 1,200 women / 1,500 men kcal) slow your BMR - your body conserves energy in starvation mode
• Sleep affects BMR - chronic sleep loss reduces BMR by 5-10%, making weight loss harder
• BMR is heavily influenced by muscle mass - 1 kg of muscle burns 13 kcal/day at rest vs 4 kcal/day for fat
• Don't confuse BMR with RMR (Resting Metabolic Rate) - they differ slightly (RMR is ~10% higher than BMR) but the terms are often used interchangeably

Limitations & notes

Mifflin-St Jeor has a margin of error of about ±10%. Individual variation in genetics, hormones (especially thyroid), body composition, and gut microbiome can mean your actual BMR is 200-400 calories above or below the estimate. For exact measurement, indirect calorimetry (measuring oxygen consumption while at rest) is the gold standard - available at sports medicine clinics and university research labs.

Frequently Asked Questions

What's the difference between BMR and TDEE?

BMR is calories burned at complete rest (just to keep you alive). TDEE (Total Daily Energy Expenditure) is BMR + calories burned through daily activities and exercise. You need TDEE-level calories to maintain weight; eat below for weight loss, above for gain.

Why does BMR decrease with age?

Three main reasons: (1) loss of muscle mass (sarcopenia) - ~3-8% per decade after 30, (2) hormonal changes (lower testosterone, growth hormone), (3) lower physical activity reducing demand. Strength training can offset much of this decline.

How accurate is the Mifflin-St Jeor formula?

Most accurate equation for the general population per multiple meta-analyses, with typical error ±10%. Less accurate for: very lean athletes (underestimates), very obese individuals (overestimates), elderly (underestimates), pregnant women. For better accuracy in those groups, use Katch-McArdle which uses lean body mass.

Can I increase my BMR?

Yes, though slowly. Most effective methods: (1) strength training - more muscle = higher BMR (gain 5 kg muscle, BMR up ~65 kcal/day), (2) high protein diet (5-10% higher BMR from food thermogenesis), (3) good sleep (5-10% BMR boost), (4) NEAT (Non-Exercise Activity Thermogenesis - fidgeting, walking, standing). Stimulants like caffeine give a temporary 4-8% boost.

Should I eat at BMR for weight loss?

No - never eat below your BMR. Aim for a 500-750 kcal deficit from your TDEE (not BMR). Eating at or below BMR for extended periods slows your metabolism (adaptive thermogenesis), causes muscle loss, and is unsustainable. Always include exercise to maintain muscle while in a deficit.

How does muscle vs fat affect BMR?

Muscle is metabolically active tissue - 1 kg burns ~13 calories per day at rest. Fat is mostly storage - 1 kg burns only ~4 calories per day. Two people of the same weight can have different BMRs by 200-400 kcal based on body composition. This is why strength training is more important than cardio for long-term metabolism.

Is the activity multiplier accurate?

It's a rough estimate. Most people overestimate their activity level. Honest assessment: only count exercise (not daily walking) when picking the multiplier. If your job is desk-based and you exercise 3-4 times a week, that's 'moderate' (1.55), not 'very active'. Track for 2 weeks and adjust based on actual weight change.

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