Skip to main content
Toollyz

Search tools

Search for a command to run...

BMI Calculator

Calculate your Body Mass Index from weight and height in kg/cm or lb/ft+in. See WHO categories on a colour-coded scale, your ideal-weight range, BMI prime and ponderal index. Free, private, with explicit caveats about what BMI doesn't measure.

What is the BMI Calculator?

Body Mass Index (BMI) is weight ÷ height squared — a quick proxy for whether an adult's body weight is in the WHO healthy range. Toollyz BMI Calculator implements the standard formula BMI = kg ÷ m² entirely in your browser and supports both metric (kg + cm) and imperial (lb + ft + in) inputs. Imperial values are converted with the exact constants 1 lb = 0.45359237 kg and 1 in = 0.0254 m. The hero shows your BMI in big animated digits and a category chip (Severe / Moderate / Mild thinness, Healthy range, Overweight, Obese class I/II/III) coloured to match the WHO scale, plus a banded progress bar from 12 to 40+ with a marker for your position. Three secondary stats are computed: your ideal-weight range (BMI 18.5–24.9 × height² — expressed in your chosen unit), BMI prime (BMI ÷ 25 — 1.0 is the upper end of healthy) and ponderal index (weight ÷ height³ — less sensitive to height than BMI). The Caveats panel is unusually prominent: BMI is a population statistic, not a diagnostic; it doesn't measure body fat, muscle mass or fat distribution; athletes often land "overweight" despite low body fat; pregnant people, growing children and adolescents need specialised charts. Toollyz has no backend; inputs save to localStorage on this device only.

How to use it

  1. Pick Metric (kg, cm) or Imperial (lb, ft+in) at the top of the form.
  2. Type your weight and height — the BMI updates live as you type.
  3. Read your category, ideal-weight range, BMI prime and ponderal index in the cards below.
  4. Read the "What BMI isn't" panel before drawing conclusions about health.

Benefits

  • Metric (kg + cm) and imperial (lb + ft + in) inputs with exact unit conversions.
  • WHO categories from Severe thinness to Obese class III on a colour-coded scale.
  • Ideal-weight range for your height in your chosen unit.
  • BMI prime (BMI ÷ 25 — 1.0 is the upper end of healthy).
  • Ponderal index (weight ÷ height³) — a less height-sensitive alternative.
  • Live banded progress bar (sky / emerald / amber / rose) with a marker for your value.
  • Honest caveats panel about what BMI doesn't measure.
  • 100% private — Toollyz has no backend, your weight, height and unit choice save to localStorage on this device only.

Frequently asked questions

What's the BMI formula?

BMI = weight (kg) ÷ height (m)². Imperial input is converted first — 1 lb is exactly 0.45359237 kg and 1 in is exactly 0.0254 m.

What's a "healthy" BMI?

The WHO classifies BMI 18.5–24.9 as the healthy range for adults. Anything lower is some degree of thinness; anything higher is overweight or obese. Toollyz uses the WHO bands exactly.

Does BMI work for athletes?

Often not. Muscle is denser than fat, so well-trained athletes routinely land in the "overweight" or even "obese" band despite very low body-fat percentages. Body composition (DEXA scan, skinfold caliper, bioelectric impedance) gives a much better picture.

What about children and teenagers?

Adult BMI categories don't apply. For ages 2–19, paediatricians use BMI-for-age percentiles relative to a sex-specific growth chart. Use a specialised paediatric BMI calculator instead.

What's BMI prime?

BMI prime = BMI ÷ 25. A value of 1.00 sits at the upper end of the healthy range; 1.20 means you're 20% above that boundary. It's a handy way to express how far above/below healthy you are.

What's the ponderal index?

Weight (kg) ÷ height (m)³. It corrects for the fact that BMI tends to slightly under-classify tall people and over-classify short people. Less common in clinical use but useful when comparing very tall or very short individuals.

Does it work for pregnant or breastfeeding people?

No — pregnancy and breastfeeding change the math fundamentally. Use a pregnancy weight-tracking tool with your doctor instead.

How accurate is the calculation?

The math is double-precision floating-point — accurate to ~15 significant digits, far beyond any health relevance. The displayed BMI rounds to one decimal place.

Are my measurements uploaded?

No. Toollyz has no backend — the math runs in your browser. Your weight, height and unit choice save to localStorage on this device only.

Is this BMI Calculator free?

Completely free with no signup and no limits. Calculate as many BMIs as you like — privately in your browser.

Related tools

See all calculators tools