BMR Calculator

BMR Calculator — Mifflin–St Jeor, Harris–Benedict, and Katch–McArdle

Your Basal Metabolic Rate (BMR) is the number of calories your body burns every day at complete rest, just to keep you alive. These calories fuel essential processes: breathing, blood circulation, cell repair, temperature regulation, and hormone production. Use this BMR calculator to estimate your resting calorie requirement using the three most widely validated formulas — Mifflin–St Jeor, Harris–Benedict (Revised), and Katch–McArdle — and optionally estimate your Total Daily Energy Expenditure (TDEE) by applying an activity multiplier.

What Is BMR and Why Does It Matter?

BMR represents the floor of your daily calorie needs. Even if you spent the entire day lying still, your body would burn this many calories simply to sustain life. For most adults, BMR accounts for 60–75% of total daily calorie expenditure. Understanding your BMR is the foundation for any calorie-based nutrition strategy — whether your goal is fat loss, muscle gain, or maintenance.

BMR is influenced by several factors: body weight, height, age, and biological sex. Heavier and taller individuals tend to have higher BMRs because they have more tissue to maintain. BMR declines with age, partly because of gradual muscle loss (sarcopenia) and hormonal changes. Males typically have higher BMRs than females due to greater average muscle mass. Body fat percentage also plays a role — muscle is metabolically more active than fat, meaning two people of equal total weight can have meaningfully different BMRs if their body compositions differ.

The Three BMR Formulas: When to Use Each

Mifflin–St Jeor (Default Recommendation)

Developed in 1990, the Mifflin–St Jeor equation is considered the most accurate formula for estimating BMR in modern, general populations. It is recommended by the American Dietetic Association as the preferred method for clinical use. This formula uses weight (kg), height (cm), age (years), and sex.

Harris–Benedict (Revised)

The original Harris–Benedict equation was published in 1918 and revised by Roza and Shizgal in 1984. It tends to produce slightly higher estimates than Mifflin–St Jeor and may be more appropriate for people who are significantly above average weight. Both the original and revised versions remain widely used in clinical research and literature.

Katch–McArdle (Body Composition–Based)

The Katch–McArdle formula is unique in that it calculates BMR from lean body mass (LBM) rather than total weight. This makes it particularly accurate for lean or muscular individuals where body fat percentage is known. Because fat tissue has a much lower metabolic rate than muscle, two people with identical weight and height can have very different BMRs if their body compositions differ significantly.

From BMR to TDEE: The Activity Multiplier

BMR alone is not a useful daily calorie target — it represents calories burned at rest, not your actual daily need. To estimate how many calories you actually need each day, multiply your BMR by an activity factor to get your Total Daily Energy Expenditure (TDEE). TDEE is your true calorie maintenance level.

• Sedentary (×1.2) — Desk job, minimal exercise. Daily movement beyond routine tasks is low.
• Lightly Active (×1.375) — Light exercise 1–3 days per week, or an active daily commute.
• Moderately Active (×1.55) — Moderate exercise 3–5 days per week. The most common category for regular gym-goers.
• Very Active (×1.725) — Hard exercise or sports 6–7 days per week, or a physically demanding job.
• Extra Active (×1.9) — Twice-daily training, very physically demanding work, or competitive athletic training.

To lose body fat gradually, aim for a 10–20% calorie deficit below TDEE. To gain lean muscle, aim for a 10–15% surplus above TDEE. Eating at or near your raw BMR for extended periods is not recommended — it risks muscle loss and metabolic adaptation.

Practical Applications: Using BMR in Nutrition Planning

Once you know your BMR and TDEE, you have a data-driven baseline for nutrition decisions. Here is how to apply these numbers:

• Fat loss: Set a calorie target 300–500 kcal below your TDEE. This creates a moderate deficit that promotes fat loss while preserving muscle. Avoid deficits larger than 1,000 kcal/day without medical supervision.
• Maintenance: Eat at TDEE to maintain current weight. Recalculate every few weeks as your weight changes, since a lower body weight means a lower BMR.
• Muscle gain: Eat 200–300 kcal above TDEE with adequate protein (1.6–2.2 g per kg of body weight) to support muscle protein synthesis.
• Tracking progress: If weight is not changing as expected, recalculate your TDEE — activity estimates are often inaccurate, and metabolism adapts to prolonged deficits or surpluses.

Limitations of BMR Calculators

All BMR formulas produce estimates based on population averages. Your actual resting metabolic rate can vary by ±10–15% from these predictions depending on genetics, thyroid function, gut microbiome, chronic illness, medication use, and other factors. For clinical-grade accuracy, indirect calorimetry (measuring oxygen consumption) is the gold standard.

Additionally, the activity multipliers used to calculate TDEE are themselves estimates. Most people tend to overestimate their activity level, which leads to inflated TDEE values and slower-than-expected fat loss. When in doubt, start with a lower activity factor and adjust based on observed weight trends over 2–4 weeks.

This BMR calculator supports metric and imperial units, three scientifically validated formulas, and optional TDEE estimation with a full activity multiplier reference table — giving you everything you need to set an accurate calorie baseline.