BMI & Body Weight Calculator
Your BMI category, healthy weight range, and daily calorie needs by activity level.
How this is calculated
BMI = weight (kg) ÷ height (m)². The WHO category boundaries (underweight, healthy weight, overweight, and three obesity classes) then place that figure on a standard scale. BMR uses the Mifflin-St Jeor equation, and TDEE multiplies BMR by an activity factor for each of five activity levels.
For example, someone 180cm tall weighing 80kg has a BMI of 80 / 1.8² ≈ 24.7 — within the healthy 18.5–24.9 range, close to its upper edge. Their BMR under Mifflin-St Jeor and TDEE at each activity level then show the calorie range relevant to maintaining, losing, or gaining weight at that size. Once you know your daily calorie needs, the Calorie Burn calculator estimates how many of those calories a specific activity or workout burns, and the Daily Water Intake calculator uses similar body-weight inputs to estimate hydration needs.
Frequently asked questions
- Is BMI an accurate measure of health?
- BMI is a population-level screening tool, not a diagnosis. It doesn't distinguish muscle from fat, so very muscular people can show as "overweight" despite low body fat, and it doesn't account for where fat is distributed, which matters for health risk. It's a useful starting point, not the full picture.
- How is BMR different from TDEE?
- BMR (Basal Metabolic Rate) is the calories your body burns at complete rest just to stay alive. TDEE (Total Daily Energy Expenditure) adds the calories burned through daily activity and exercise on top of that — it's the number to use for weight maintenance, loss, or gain targets.
- How accurate is the Mifflin-St Jeor formula?
- It's considered one of the more accurate predictive equations for BMR in the general population, typically within about 10% of measured values. Individual metabolism varies with genetics, muscle mass, and other factors a formula can't capture.
- What is a healthy BMI range?
- The WHO classifies a BMI of 18.5–24.9 as the "normal" or healthy weight range for most adults. Below 18.5 is classified as underweight, 25–29.9 as overweight, and 30+ as obese, split into three further classes.
- What are the limitations of BMI as a health measure?
- BMI doesn't distinguish between muscle and fat mass, doesn't account for where fat is distributed in the body (which matters for cardiovascular risk), and uses the same thresholds regardless of age, sex, or ethnicity — all of which can meaningfully affect how well BMI reflects an individual's actual health.
- Does BMI differ by age or ethnicity?
- The standard BMI thresholds were developed primarily from European population data and are applied universally, but research shows health risk at a given BMI can vary by ethnicity — some Asian population guidelines use lower overweight/obesity thresholds, for example. Body composition also naturally shifts with age even at a stable BMI.
- What is the difference between BMI and body fat percentage?
- BMI is a simple ratio of weight to height squared — it doesn't measure fat directly. Body fat percentage measures the actual proportion of the body that is fat tissue, typically via skinfold calipers, bioelectrical impedance, or DEXA scanning, and is a more direct (though harder to measure) indicator of body composition.
- What BMI is associated with health risks?
- Risk of conditions like type 2 diabetes, cardiovascular disease, and certain cancers rises at both extremes — BMI below 18.5 (underweight) and especially above 30 (obese), with risk generally increasing further within each successive obesity class.