BMI Calculator

Metric (kg, cm)
Imperial (lb, ft/in)

What Is BMI?

BMI (body mass index) is a measure that uses your height and weight to work out if your weight is healthy. The BMI calculation divides an adult's weight in kilograms by their height in metres squared — the standard definition used by the World Health Organization since the 1980s. Healthcare providers use BMI as a quick screening tool because it costs nothing, needs no equipment beyond a scale and a tape measure, and correlates reasonably well with body fatness across large populations.

The BMI Formula

BMI = weight (kg) ÷ height² (m²)

For example, a person weighing 70 kg who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — comfortably inside the normal weight range.

In imperial units (pounds and inches), the same formula carries a conversion factor of 703:

BMI = 703 × weight (lb) ÷ height² (in²)

A person weighing 155 lb at 5'10" (70 inches) has a BMI of 703 × 155 ÷ 4,900 = 22.2. Both formulas produce identical results for the same body — only the units change.

BMI Categories (WHO)

BMI Range Category
Below 18.5 Underweight
18.5 – 24.9 Normal weight
25.0 – 29.9 Overweight
30.0 and above Obesity

For clinical purposes obesity is sometimes subdivided: Class I (30–34.9), Class II (35–39.9), and Class III (40+). Asian populations often use lower thresholds (23 for overweight, 27.5 for obesity) because health risks rise at lower BMIs in those populations — several Asian health authorities publish these adjusted cut-offs.

Healthy Weight Range

The calculator above also shows the healthy weight range for your height — the weight band where BMI stays between 18.5 and 24.9. The range is surprisingly wide: for a height of 175 cm (5'9"), any weight between about 56.7 kg and 76.3 kg (125–168 lb) counts as normal weight. That width matters: losing even a few kilos from the overweight band delivers measurable health benefits, and there is no single perfect weight to chase.

BMI Limitations: What BMI Does Not Tell You

BMI is a screening tool, not a diagnosis. It measures total weight relative to height but cannot distinguish muscle from fat, bone density, or fat distribution. Consequently:

  • Muscular people (athletes, bodybuilders) often register as overweight despite low body fat, because muscle is denser than fat.
  • Older adults may carry more body fat at the same BMI as in their younger years, since muscle mass declines with age.
  • Pregnant women should not use BMI as a health measure during pregnancy.
  • Fat distribution matters: waist circumference adds independent risk information — a waist over 94 cm (37") in men or 80 cm (31.5") in women signals increased health risk even inside the normal BMI band.

If your BMI surprises you, pair it with a waist measurement and, where available, body-composition testing before drawing conclusions.

BMI for Children and Teens

Children and teenagers aged 2–19 use BMI-for-age percentiles rather than the adult categories, because body composition changes as they grow. A child's BMI is compared against growth charts for their age and sex: a BMI-for-age at or above the 95th percentile counts as obesity, the 85th–94th percentile as overweight, and below the 5th percentile as underweight. Pediatricians interpret these percentiles — parents should not apply the adult table to children.

FAQ

How do I calculate my BMI?

Enter your weight and height in the calculator at the top of this page. The underlying formula divides weight in kilograms by height in metres squared (or use 703 × pounds ÷ inches² in imperial units).

What is a good BMI?

For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. Values above 25 indicate overweight, and values below 18.5 indicate underweight.

Is BMI accurate for athletes?

Not reliably. Athletes carry more muscle mass, which weighs more than fat, so their BMI frequently classifies them as overweight despite low body fat. Waist measurement or body-composition scans suit athletic populations better.

How much should I weigh for my height?

The calculator shows the healthy weight range for your height — the band where BMI stays between 18.5 and 24.9. For 175 cm that band runs from about 56.7 to 76.3 kg.

Does BMI differ for men and women?

The BMI formula and categories are identical for adult men and women. Body composition differs on average (women naturally carry more fat at the same BMI), but the WHO thresholds apply to both sexes.

Related Tools