Health Metrics
BMI & BMR Calculator
Calculate Body Mass Index, Basal Metabolic Rate, estimated daily calorie needs, and water intake target.
Last reviewed: August 2026
Result
Fill in the fields to see your result.
How this is calculated
BMI divides weight in kilograms by height in meters squared. BMR uses the Mifflin-St Jeor equation (Mifflin et al., 1990, Am J Clin Nutr), generally considered one of the more accurate resting-metabolism formulas in clinical use. Estimated daily calorie needs (TDEE) multiply BMR by an activity factor ranging from 1.2 (sedentary) to 1.9 (extremely active) — these are the standard Harris-Benedict activity multipliers used across most clinical and fitness calculators. Water target uses a common clinical rule of thumb of roughly 33 ml per kilogram of body weight.
Category thresholds
BMI category cutoffs (underweight, typical, overweight, obese) follow the World Health Organization's obesity and overweight classification.
What these numbers don't capture
None of these figures account for muscle mass, medical conditions, medications, or specific activity levels beyond a rough multiplier. They're a reasonable starting estimate, not a personalized nutrition plan.
Worked example
Example
A 35-year-old woman, 165 cm, 68 kg, light activity (1–3 sessions/week):
- BMI: 68 ÷ (1.65 × 1.65) ≈ 25.0 — right at the boundary between "typical range" and "overweight" on the WHO scale.
- BMR (Mifflin-St Jeor): (10 × 68) + (6.25 × 165) − (5 × 35) − 161 ≈ 1,326 kcal/day at complete rest.
- TDEE: 1,326 × 1.375 (light activity) ≈ 1,823 kcal/day to maintain current weight.
- Water target: 68 × 0.033 ≈ 2.2 L/day.
Note how close the BMI sits to a category boundary — this is common, and it's exactly why BMI on its own is treated as a screening signal rather than a verdict.
Common mistakes when reading these results
- Treating BMI as a body-fat measurement. It isn't — it can't distinguish muscle from fat, which is why a muscular athlete can land in "overweight" despite low body fat. Pair it with the Body Fat calculator for a fuller picture.
- Confusing BMR with TDEE. BMR is calories burned lying still; it's never a calorie target on its own. The TDEE figure (BMR × activity factor) is the one relevant to weight maintenance.
- Picking the wrong activity level. "Light activity" means light exercise 1–3 days a week — not a generally active lifestyle. Overestimating this is the single most common reason people plateau or gain weight against expectations.
Why do BMI, BMR, and TDEE all appear on one page?
Because they're related steps in the same question — "roughly how many calories does my body use?" — starting from a general population screening number (BMI) and narrowing to a personal estimate (TDEE).
Is the Mifflin-St Jeor equation the most accurate one available?
Among widely used formulas it performs well for most adults, but no equation-based estimate replaces indirect calorimetry (measuring actual gas exchange), which isn't practical outside a lab or clinical setting.
My BMI is "overweight" but I feel healthy — should I worry?
BMI is one screening data point among many (blood pressure, waist circumference, bloodwork, family history). A single metric in an unfavorable range isn't a diagnosis; a healthcare provider can interpret it alongside the rest of your health picture.
Sources
- Mifflin MD, St Jeor ST, et al. "A new predictive equation for resting energy expenditure in healthy individuals." Am J Clin Nutr, 1990.
- World Health Organization. Obesity and overweight fact sheet.