BMI Calculator

What Is BMI and How Should You Use It?

Body Mass Index (BMI) is a number from your height and weight that gives a quick screening of whether your weight is in a range often linked to health. The formula — weight in kilograms divided by height in metres squared — is used worldwide by public health bodies. It needs no special equipment and at the population level relates reasonably well to body fat.

Common BMI bands are: underweight (below 18.5), normal (18.5–24.9), overweight (25.0–29.9), and obese (30.0 and above). People outside the normal band have, on average, higher risks of some diseases — but these are group-level trends, not personal diagnoses. BMI cannot tell muscle from fat, bone density, or where fat is stored (visceral fat matters more than subcutaneous fat).

Treat BMI as a starting point. If yours is outside the healthy range, discuss it with a clinician; they may use waist size, body composition, or blood tests. Together with fitness, diet, blood pressure, and glucose, BMI is one part of the picture — sustainable habits matter more than any single number.

BMI is a screening index, not a diagnosis—muscle mass, bone density, and fat distribution vary widely. Athletes may read “overweight” while metabolically healthy.

Pair with the TDEE calculator when moving from screening to rough calorie planning; the unit converter helps swap kg/lb without rework.

Worked examples

• Metric: 70 kg at 170 cm → 70 ÷ (1.70 × 1.70) = 24.2, in the normal range.
• Imperial: 154 lb at 5 ft 7 in (67 in) → 703 × 154 ÷ (67 × 67) = 24.1. The factor 703 converts pounds and inches into the metric formula; computing the same person in exact metric gives 24.12.
• Healthy range for a given height: at 170 cm, a BMI of 18.5–24.9 corresponds to roughly 53.5–72.0 kg (multiply each BMI limit by height in metres squared).

Who the standard categories do not fit

• Children and teenagers: for ages 2–19, BMI is compared with growth charts for the same age and sex (BMI-for-age percentiles), not with the adult cut-offs used here.
• Asian populations: the World Health Organization has noted higher health risks at lower BMI in many Asian populations, and several countries use lower thresholds — China, for example, treats 24 as overweight and 28 as obese.
• Athletes and muscular builds: muscle is denser than fat, so a strength athlete can be classed as overweight while having low body fat.
• Older adults: muscle mass falls with age, so a “normal” BMI can hide high body fat; some research suggests slightly higher BMI ranges are not harmful in later life.
• Pregnancy: BMI is not meaningful during pregnancy; pre-pregnancy BMI is what clinicians use for weight-gain guidance.

Useful measures alongside BMI

• Waist-to-height ratio: waist circumference divided by height, both in the same unit. Keeping it below 0.5 — a waist less than half your height — is the guidance used by the UK’s NICE. At 170 cm, that means a waist under 85 cm.
• Waist circumference alone is a simple marker of abdominal fat, which carries more health risk than fat elsewhere.
• Body-fat percentage estimates (see the body fat calculator) separate fat from lean mass, which BMI cannot do.

BMI is a screening number, not a diagnosis. If yours is outside the normal range, or you have concerns about your weight, talk to a doctor, who can look at the whole picture. Your height and weight are processed only in your browser and are not stored.

Tracking BMI over time

Body weight varies by a kilogram or more over a single day with food, fluids and salt intake. If you are tracking a trend, weigh at the same time of day — first thing in the morning is easiest — and look at the weekly average rather than any single reading. A change of 0.3–0.4 in BMI corresponds to about 1 kg at average adult heights.