BMR Calculator — Basal Metabolic Rate BMR BMR Calculator — Basal Metabolic Rate

Calculate your Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE). BMR is the energy your body burns at complete rest over 24 hours for vital functions: breathing, heartbeat, body temperature regulation, and brain function.

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⚡ Basal Metabolic Rate: what your body burns at rest

Your BMR represents 60 to 75% of your total caloric expenditure. It's the baseline energy your body needs to:

  • Keep your heart beating — ~8% of BMR
  • Power your brain — ~20% of BMR
  • Maintain body temperature at 98.6°F (37°C)
  • Breathe and ensure gas exchange
  • Filter blood through kidneys and liver

Think of BMR as your body's idle engine speed: even at complete rest, your body consumes an irreducible amount of fuel.

🔬 Three scientific formulas, one estimate

No formula can exactly measure your BMR (only indirect calorimetry in a lab can do that). We use three complementary approaches:

Formula Year Key feature
Mifflin-St Jeor 1990 Most accurate for the majority of adults. Recommended by the Academy of Nutrition and Dietetics.
Harris-Benedict revised 1984 Historical reference, tends to slightly overestimate in overweight individuals.
Katch-McArdle 1996 Uses lean body mass. More reliable if you know your body fat %.

⚠️ Why your actual BMR may differ from the calculation

The formulas above rely on 4 measurable parameters (weight, height, age, gender). In reality, BMR varies considerably between individuals due to factors that are difficult to quantify:

1. Body composition (muscle vs. fat)

This is the number one factor. Muscle is a metabolically active tissue, unlike fat which is essentially a passive energy store. At the same weight, a highly muscular person will have a significantly higher BMR than someone with a high body fat percentage. This is why the Katch-McArdle formula (which uses lean mass) is more reliable — but it requires knowing your body fat %.

2. Hormones and genetics

BMR is largely driven by your endocrine system, especially the thyroid. A person with a very active thyroid will burn calories like a high-temperature furnace. Conversely, a slight hormonal imbalance or genetic predisposition can result in a "slower" metabolism, even when doing everything right. These individual differences are invisible to formulas.

3. Dieting history (adaptive thermogenesis)

Your body has a metabolic memory. After prolonged very restrictive diets, the body learns to become extremely efficient: this is adaptive thermogenesis. Two identical people on paper (same weight, height, age, gender) won't have the same BMR if one has just gone through months of severe caloric restriction. This phenomenon can lower BMR by 10 to 15% compared to the theoretical calculation.

💡 "Fast" vs. "slow" metabolism: the reality

You often hear about fast or slow metabolism. In reality:

  • The difference between a "fast" and "slow" metabolism is about 200-300 kcal/day at equal weight, height and age
  • Muscle mass is the main modifiable factor: each kg of muscle burns ~13 kcal/day at rest vs ~4.5 kcal for fat tissue
  • Resistance training and running sustainably increase BMR through the EPOC effect (excess post-exercise oxygen consumption)

📋 Don't confuse BMI and BMR

BMI (Body Mass Index) BMR (Basal Metabolic Rate)
What it is Weight / height² ratio Energy spent at rest (kcal/day)
Measures Your body shape Your engine at idle
Purpose Categorize weight (underweight, normal, overweight) Adapt nutrition to actual needs

You can calculate your BMI here and compare it to the US population.

Frequently Asked Questions about BMR

What is Basal Metabolic Rate (BMR)?

Basal Metabolic Rate (BMR) is the amount of energy (in kcal) your body burns at complete rest over 24 hours to maintain vital functions: breathing, blood circulation, brain activity, and body temperature regulation.

What is the difference between BMR and TDEE?

BMR is the energy spent at total rest. TDEE (Total Daily Energy Expenditure) is your total energy expenditure which includes BMR + thermic effect of food + your physical activity. TDEE = BMR × activity factor.

Which formula is most accurate?

The Mifflin-St Jeor formula (1990) is considered the most accurate by the Academy of Nutrition and Dietetics. If you know your body fat percentage, the Katch-McArdle formula is even more reliable as it uses lean body mass.

Does metabolism slow down with age?

Yes, BMR decreases by approximately 1 to 2% per decade after age 20, mainly due to loss of muscle mass (sarcopenia). Resistance training and running can help limit this decline.

Why is BMR important for runners?

Knowing your BMR helps you adjust your nutrition to your actual needs. A caloric deficit too large relative to BMR leads to fatigue, muscle loss and poor performance. Runners should aim for a caloric intake at least equal to their TDEE.

How can I increase my BMR?

Several strategies: build muscle mass (each kg of muscle burns ~13 kcal/day at rest), maintain regular physical activity (the post-exercise EPOC effect increases BMR for 24-48h), sleep enough (7-9h) and don't chronically undereat (prolonged hypocaloric diets reduce BMR).

To go further in optimizing your nutrition and performance: