BMI Calculator
What is BMI?
Body Mass Index (BMI) is a quick screening tool that relates your weight to your height. It is used worldwide to categorize weight status and to flag potential health risks at the population level.
Here is the calculation end to end. A person who weighs 70 kg at 1.75 m tall divides 70 by 1.75 squared - that is 70 divided by 3.0625, giving a BMI of 22.9, comfortably in the normal range. The same formula defines their healthy weight window: 18.5 × 3.0625 = 56.7 kg at the bottom and 24.9 × 3.0625 = 76.3 kg at the top, a corridor of roughly 20 kg. Meanwhile, someone 1.78 m tall weighing 82 kg lands at 25.9 - just over the line into overweight, which is exactly the kind of borderline reading worth double-checking with a tape measure.
Using it is deliberately simple. Enter your height in centimeters and your weight in kilograms - no other information is needed. The calculator returns your BMI to one decimal place, places you in one of four categories (underweight below 18.5, normal from 18.5 to 24.9, overweight from 25 to 29.9, obese at 30 and above), and shows the weight range that would put you in the normal band for your height. The point is a fast, repeatable baseline you can recheck every month or two.
BMI earns its place as a first-pass screen at clinics, insurance checkups and population studies because it is free and consistent. Where it fails is the individual level: it cannot separate muscle from fat, so lifters and athletes frequently read as overweight, while older adults with low muscle can read as normal despite carrying risky fat. It also says nothing about where fat sits - visceral fat around the waist matters more than fat elsewhere. For a sharper picture, pair BMI with waist circumference (under 102 cm for men and 88 cm for women is the common guideline) or a body fat estimate.
Frequently Asked Questions
How is BMI calculated?
BMI = weight in kilograms divided by height in meters squared (kg/m²). For example, a 70 kg person who is 1.75 m tall has a BMI of 70 / 1.75² = 22.9, which is in the normal range.
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered normal for most adults. Below 18.5 is underweight, 25-29.9 is overweight, and 30 or above is obese. For older adults and certain ethnic groups, slightly different cutoffs are sometimes used.
Is BMI accurate?
BMI is accurate at the population level but imperfect for individuals: it does not distinguish fat from muscle, so athletes can show a high BMI while being lean. It also does not show where fat is stored. Use it alongside waist measurement or body fat percentage for a fuller picture.
How often should I check my BMI?
Monthly is plenty for tracking; weight fluctuates day to day with water and food. Weigh yourself at the same time of day - morning, after the bathroom, before eating - for comparable readings, and watch the trend over weeks rather than any single result.
Does BMI work for children?
Not with adult cutoffs. Children and teenagers are assessed with age- and sex-specific BMI percentiles that compare them against growth charts, because body composition changes dramatically through development. Use a pediatric BMI percentile calculator or ask a pediatrician for anyone under 20.
Does age change healthy BMI ranges?
Slightly. Research suggests that for adults over 65, a BMI in the 23-29 range may carry lower mortality risk than the standard 18.5-24.9 band, likely because some reserves protect against illness. Cutoffs also differ across ethnic groups - some Asian populations use lower thresholds for overweight.