BMI & Body Metrics Calculator
Calculate Body Mass Index with a visual category scale, plus your BMR and estimated daily maintenance calories by activity level — metric or imperial.
Interactive Client Prototype Sandbox
For information only — not medical advice.
Disclaimer: This free tool is provided “as is,” without warranties of any kind, and is for general informational purposes only — not professional, legal, financial, medical, tax, or engineering advice. Results may contain errors; verify anything important independently and use at your own risk. We accept no liability for any loss or damage arising from its use. See our Terms of Use for details.
Step-by-Step Guide
Pick your unit system, enter height, weight, age, sex, and activity level. The BMI, its category, your BMR, and maintenance calories update instantly.
80 kg, 180 cm ➔ BMI 24.7 (Normal); BMR ~1,780 kcal; moderate activity ~2,760 kcal/day.
Who it's for
People tracking fitness, dieters, gym-goers, and anyone curious about body metrics.
Core Features
- BMI with a color-coded category scale (under / normal / over / obese) and a live marker.
- BMR via the accurate Mifflin-St Jeor equation.
- Estimated daily maintenance calories across five activity levels.
- Metric (kg/cm) and imperial (lb/ft·in) input; informational, not medical advice.
🛡️ No tracking — your inputs, keys, and details never leave this client sandbox.
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared. Enter metric or imperial values and it computes the number plus its category — underweight, normal, overweight, or obese — and marks it on a scale.
What is a healthy BMI range?
The standard adult ranges are: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30+ obese. BMI is a rough screening number — it doesn't account for muscle mass, so very athletic builds can read high without excess fat.
What are BMR and maintenance calories?
BMR (basal metabolic rate) is the energy your body uses at rest, estimated here with the Mifflin–St Jeor formula from your height, weight, age, and sex. Maintenance calories multiply BMR by an activity factor to estimate what you'd eat to hold your current weight.
Does it support both metric and imperial units?
Yes. Toggle between metric (kg / cm) and imperial (lb / ft·in) and the calculations convert automatically.
Is this medical advice?
No. BMI and calorie figures are for general information only and aren't a substitute for a doctor or dietitian. Everything is computed in your browser — your measurements are never uploaded.
Does BMI apply to athletes and highly muscular people?
BMI is often misleading for athletes and highly muscular people because muscle is denser than fat. A powerlifter or professional football player can have a BMI in the 'overweight' or 'obese' range despite having very little body fat. Conversely, someone with a 'normal' BMI can have excess fat and insufficient muscle (a condition called 'normal-weight obesity'). For a more accurate body composition assessment, options include body fat percentage measurement (via DEXA scan, hydrostatic weighing, or skinfold calipers), waist-to-hip ratio, or waist circumference.
What is the Mifflin-St Jeor equation and is it accurate?
The Mifflin-St Jeor equation (1990) estimates basal metabolic rate (BMR) as: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5 (for men) or −161 (for women). It is generally considered the most accurate of the simple predictive equations, with a mean error of about ±10% compared to indirect calorimetry. The activity multiplier (sedentary 1.2, lightly active 1.375, moderately active 1.55, very active 1.725, extra active 1.9) adds significant imprecision because actual activity levels are hard to self-assess accurately.
If BMI doesn't distinguish muscle from fat, why do doctors still use it?
BMI has well-documented limitations: it cannot tell the difference between a 200-pound powerlifter and a 200-pound person with excess body fat. Athletes and heavily muscular people routinely show up as 'overweight' or 'obese' by BMI despite very low body fat. Conversely, people with 'normal' BMI can have excess fat and insufficient muscle — a condition researchers call normal-weight obesity. Given these flaws, why is BMI still the standard screening metric in clinical settings worldwide?
What BMI is actually measuring
BMI was not designed as a clinical diagnostic tool. Belgian mathematician Adolphe Quetelet developed it in the 1830s as a population-level statistical descriptor — a way to characterize the weight distribution of a large group, not to assess any individual's health. Its adoption as a clinical screening tool happened because it requires only a scale and a measuring tape, it correlates reasonably well with body fat percentage at the population level, and those correlations hold well enough to predict group-level health outcomes. For identifying patterns across millions of people in epidemiological studies, it works. For assessing any individual, it is a rough proxy that should prompt further evaluation rather than serve as a final verdict.
BMR and the limits of calorie estimation
Basal Metabolic Rate (BMR) — the energy your body uses at rest — accounts for 60–75% of total daily calorie expenditure for most people. The Mifflin-St Jeor equation (1990) estimates it from height, weight, age, and sex, with a mean error of about ±10% compared to measured values via indirect calorimetry. Multiplying by an activity factor to get Total Daily Energy Expenditure (TDEE) adds more uncertainty: most people significantly overestimate their activity level. The combined error can easily reach 20–25%, which is why calorie-based weight management plans often need iterative adjustment rather than trusting a calculated TDEE as a precise target.
What this tool is for and what it is not
This calculator provides BMI, its WHO category, an estimated BMR, and a range of maintenance calorie estimates across activity levels. It is informational — a starting point for understanding roughly where you fall on standard screening metrics, not a medical assessment. For a meaningful health evaluation, body composition (body fat percentage, lean mass, fat distribution), bloodwork, and physical fitness assessments provide far more useful information than a height-weight ratio.