AkCalculators

From BMI to BMR: how and why to estimate your basal metabolic rate

Basal metabolic rate (BMR) is the number of calories your body needs at complete rest to maintain vital functions — breathing, circulation, cellular turnover and temperature regulation. BMR is a foundational number for nutrition planning, weight management, and sports programming. Often people know their BMI (body mass index) and height but not their weight (or want a quick estimate). This converter derives a weight estimate from BMI and height, then applies the widely used Mifflin–St Jeor equation to estimate BMR. Below we explain the logic, assumptions, practical use, and limitations so you can interpret results appropriately.

How BMI and height give weight

BMI is defined as weight (kg) divided by height in metres squared. Rearranging gives weight = BMI × height². For example, a person with BMI 24.5 and height 1.75 m would have estimated weight 24.5 × (1.75²) = 75.0 kg. This back-calculation assumes BMI accurately reflects the person’s body composition — an assumption that holds roughly at population level but can be off for muscular or particularly lean individuals.

Why Mifflin–St Jeor?

Many equations estimate BMR; Mifflin–St Jeor (1990) is widely used and shown in multiple studies to predict resting energy expenditure reliably for modern populations. The equations:

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

Because the formula includes height and age in addition to weight, converting BMI→weight and then applying the formula keeps height as a direct input rather than implicitly double-using it.

When this approach is useful

This converter is helpful when: you have BMI (from a chart or report) and height but not weight; you need a quick baseline BMR without weighing; you are building population-level estimates. It’s also useful for education: showing the direct relationship between BMI, height and metabolic requirements.

Limitations and important caveats

A few important cautions: BMI is an imperfect proxy for adiposity and does not capture muscle mass. Two people with the same BMI and height can have different body compositions and therefore different metabolic rates. Mifflin–St Jeor gives an estimate for resting metabolism — true measured resting metabolic rate (via indirect calorimetry) may differ by ±10% or more. Use the result as an initial guide, not a clinical measurement.

How to use the BMR estimate

To estimate daily energy needs, multiply BMR by an activity factor that matches your average daily movement:

• Sedentary (little or no exercise): ~1.2
• Light activity (1–3 days/week): ~1.375
• Moderate activity (3–5 days/week): ~1.55
• Very active (6–7 days/week): ~1.725
• Extremely active (hard manual work or twice-a-day training): ~1.9

Multiply to get estimated total daily energy expenditure (TDEE). For example: BMR 1,600 kcal × 1.55 ≈ 2,480 kcal/day.

Practical tips to improve estimates

• If you can measure weight directly, use a standard BMR calculator with the actual weight rather than deriving from BMI.
• If you have body composition data (fat-free mass), predictive equations using fat-free mass are more precise for athletes and clinical use.
• For weight management, combine TDEE estimates with gradual, sustainable adjustments (e.g., a 300–500 kcal/day deficit for weight loss) and monitor progress.

Disclaimer: This converter provides estimated values for informational use. For personalized metabolic testing or clinical nutrition planning consult a registered dietitian or medical professional.