A BMI Calculator computes your body-mass index from height and weight. It applies the standard formula to the values you enter and returns the result instantly, without sending any data to a server. Useful when tracking fitness goals or planning a diet.
Calculates Body Mass Index using metric or imperial units.
Metric: BMI = weight(kg) / height(m)^2. Imperial: BMI = 703 x weight(lb) / height(in)^2.
Body Mass Index (kg/m²) with WHO categorization.
The BMI Calculator computes your Body Mass Index - a simple ratio of weight to height squared (kg/m²) - and classifies the result using the World Health Organization (WHO) categories. BMI is the most widely used screening tool for assessing whether an adult's weight falls within a healthy range.
This calculator supports both metric (kg, cm) and imperial (lbs, inches) units. It instantly displays your BMI value, your WHO category, and highlights where you fall in the classification table - from underweight through normal to obese class III.
While BMI is a useful population-level indicator, it does not measure body fat directly. It cannot distinguish between muscle and fat mass, which means muscular individuals may be classified as "overweight" despite being healthy. For a more comprehensive assessment, consider using our Body Fat Calculator or Ideal Weight Calculator.
Height in meters = 1.70 m. BMI = 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2. This falls in the Normal range (18.5-24.9).
Height = 1.65 m. BMI = 90 ÷ (1.65 × 1.65) = 90 ÷ 2.7225 = 33.1. This falls in the Obese Class I range (30-34.9). The person would need to reach about 67 kg to enter the normal BMI range.
Using imperial formula: BMI = (154 × 703) ÷ (70 × 70) = 108,262 ÷ 4,900 = 22.1. Normal range.
| Category | BMI Range (kg/m²) | Health Risk |
|---|---|---|
| Underweight | < 18.5 | Malnutrition risk, osteoporosis |
| Normal | 18.5 - 24.9 | Lowest health risk |
| Overweight | 25.0 - 29.9 | Moderate risk |
| Obese Class I | 30.0 - 34.9 | High risk |
| Obese Class II | 35.0 - 39.9 | Very high risk |
| Obese Class III | ≥ 40.0 | Extremely high risk |
Three numbers often confused; they answer different questions:
| Metric | Formula | What it tells you |
|---|---|---|
| BMI (Body Mass Index) | weight kg / height m^2 | Rough screen for under/normal/over weight. Bad for athletes. |
| BMR (Basal Metabolic Rate) | Mifflin-St Jeor equation | Calories your body burns AT REST in 24 hours |
| TDEE (Total Daily Energy Expenditure) | BMR x activity multiplier | Total calories burned per day, including activity |
| Body fat % | DEXA / calipers / bioimpedance | More accurate than BMI for athletes and very lean people |
| Category | BMI range | Notes |
|---|---|---|
| Underweight | < 18.5 | Increased risk of malnutrition, osteoporosis |
| Normal | 18.5 - 24.9 | Lowest health risk for most adults |
| Overweight | 25.0 - 29.9 | Increased risk of cardiovascular disease, diabetes |
| Obese class I | 30.0 - 34.9 | High risk; lifestyle change recommended |
| Obese class II | 35.0 - 39.9 | Very high risk; clinical intervention often warranted |
| Obese class III | >= 40.0 | Extreme risk; bariatric options often discussed |
South Asians have higher cardiovascular risk at lower BMI. India, Singapore, and many Asian countries use:
The simplified energy balance:
To lose 0.5 kg / 1 lb per week, eat ~500 kcal/day below TDEE. To gain muscle, eat 200-500 kcal/day above TDEE with adequate protein (~1.6g per kg body weight per day) and resistance training.
Caveats:
| Macronutrient | Daily target | Energy density |
|---|---|---|
| Protein | 0.8 g/kg sedentary; 1.6-2.2 g/kg if exercising | 4 kcal/g |
| Fat | 20-35% of total calories; minimum ~0.5 g/kg | 9 kcal/g |
| Carbohydrate | Fills the rest; minimum 130 g/day for brain function | 4 kcal/g |
| Fiber | 25-38 g/day | ~2 kcal/g (variable) |
| Water | 30-40 ml/kg body weight | 0 kcal/g |
A healthy BMI is between 18.5 and 24.9 kg/m² according to the World Health Organization (WHO). Below 18.5 is underweight, 25-29.9 is overweight, and 30+ is classified as obese.
BMI = weight (kg) ÷ height (m)². For example, a person weighing 70 kg and 1.70 m tall has a BMI of 70 ÷ (1.70 × 1.70) = 24.2, which falls in the normal range.
BMI does not distinguish between muscle and fat mass. Muscular athletes may have a high BMI (25-30) while having low body fat. For athletes, body fat percentage or DEXA scans are more accurate measures of body composition.
The BMI formula is the same for both genders, but women naturally carry more body fat. A BMI of 24 might represent different body compositions for men and women. The WHO categories apply equally, but clinical interpretation may vary.
BMI is a simple height-to-weight ratio. Body fat percentage directly measures the proportion of your weight that is fat tissue. BMI is easier to calculate but less precise. Body fat percentage requires calipers, circumference measurements (US Navy method), or DEXA scans.
BMI is a population screening tool, not an individual diagnosis. It overestimates body fat in muscular athletes and underestimates it in people with low muscle mass. Body fat percentage (DEXA, calipers) is more accurate. Waist-to-height ratio under 0.5 is another useful single number.
0.5-1 kg (1-2 lb) per week is sustainable for most adults. Faster loss is possible but typically comes from water, glycogen, and muscle - not just fat. Crash diets also slow metabolism and rebound when normal eating resumes.
BMR estimates are within ±10% of measured rates for most people. TDEE depends on self-reported activity, which is usually overestimated by 20-50%. Track actual weight change over 2-4 weeks and adjust intake to match.
Almost all daily fluctuation is water (sodium, carbohydrate storage as glycogen which holds 3-4x its weight in water) and digestive contents. Fat loss is on the order of 100-150g per day at most. Weigh weekly at the same time for trend, not daily.
For weight loss, the two are equivalent if total calories are the same. Some people find fasting easier to comply with (skipping breakfast = automatic deficit). Others find it harder. The best diet is the one you can sustain.
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.