BMI Calculator

What BMI Does and Doesn't Measure

BMI (Body Mass Index) is calculated from height and weight alone — it has no direct knowledge of body composition. Two people with identical BMIs can have very different health profiles: a muscular athlete and a sedentary person may land in the same "overweight" category, while someone with high visceral fat might fall in the "normal" range. The CDC and WHO classify BMI as a population-level screening tool, not a diagnostic measurement for individuals. It's a starting point, not a verdict.

The BMI Formula

BMI is weight over height squared, using either metric or US units:

Metric: BMI = weight (kg) ÷ height (m)²
US units: BMI = 703 × weight (lb) ÷ height (in)²

The multiplier 703 converts pounds and inches to metric-equivalent units. The result is rounded to one decimal place and classified against the standard adult ranges: below 18.5 underweight, 18.5–24.9 healthy, 25.0–29.9 overweight, and 30.0 or above obese.

A Worked Example

A 70 kg adult at 1.75 m tall: height squared is 1.75 × 1.75 = 3.0625, so BMI = 70 ÷ 3.0625 = 22.9 — the healthy range. The same person in US units (154 lb, 70 inches, or 5'10"):

BMI = 703 × 154 ÷ 70² = 108,262 ÷ 4,900 = 22.1

The two methods agree because they are the same calculation in different unit systems — the small difference (22.9 vs 22.1) comes from rounding the 70 kg height conversion. Both land mid-healthy-range.

How to Use This Calculator

1. Enter your weight and height — the calculator accepts both metric and US units and converts for you.
2. Read your BMI value, category label, and the healthy weight range for your height shown alongside it.
3. Compare only adults aged 20 or older. For under-20s, BMI must be plotted on age- and sex-specific growth charts, which this tool does not do.

Known Limitations by Population

BMI tends to overestimate adiposity in muscular individuals and may underestimate it in older adults who have lost muscle mass. It also performs less accurately across ethnic groups: research shows that people of Asian descent may carry higher metabolic risk at lower BMI values than the standard cutoffs suggest, while some studies find the reverse for people of African descent. For children and teenagers, BMI is plotted on age- and sex-specific growth charts, not compared against adult thresholds.

Better Alternatives and Complements

Waist circumference is a stronger predictor of cardiovascular risk than BMI alone. Risk increases above 35 inches (89 cm) for women and 40 inches (102 cm) for men. Waist-to-height ratio — your waist divided by your height — is considered one of the simplest and most accurate single-number health indicators; values below 0.5 are associated with lower metabolic risk across most populations. Body fat percentage, measured by DEXA scan or hydrostatic weighing, gives the most direct picture but requires specialist equipment.

When to Talk to a Doctor

If your BMI falls below 18.5 or above 30, or if it has changed significantly without intentional dietary or exercise changes, it's worth discussing with a healthcare provider. BMI alone rarely drives clinical decisions — your doctor will consider blood pressure, blood glucose, cholesterol, family history, and waist measurements alongside it. Use this calculator to track trends over time and as one input among several, not as a standalone health score.