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BMI calculator

The BMI calculator computes your Body Mass Index — your weight divided by your height squared, expressed in kg/m². Type your weight and height in either metric or imperial units, and the page returns a single number plus the WHO category label (underweight, normal, overweight, obese). BMI is a population-level screening metric, useful as a starting reference and not as a personal health verdict.

Formula​

BMI = weight (kg) / height (m)²

Imperial inputs are converted before computing:

  • 1 lb = 0.453592 kg
  • 1 in = 2.54 cm = 0.0254 m

So the imperial-equivalent formula is BMI = (lb × 0.453592) / (in × 0.0254)² or, more commonly written, BMI = (lb × 703) / in².

Categories (WHO)​

CategoryBMI range
Underweight< 18.5
Normal weight18.5 – 24.9
Overweight25.0 – 29.9
Obesity class I30.0 – 34.9
Obesity class II35.0 – 39.9
Obesity class III≥ 40

Source: World Health Organization, "BMI Classification" — the same cutoffs used worldwide for general public-health screening.

Inputs​

FieldRangeNotes
Weight20–500 kg (or 44–1100 lb)Validation rejects values outside the range
Height100–250 cm (or 39–98 in)Same
UnitsMetric / imperial togglePer-tool setting, not global

Worked example​

A 70-kg, 175-cm adult:

height_m = 175 / 100 = 1.75
BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 ≈ 22.86

22.86 falls in the Normal range (18.5–24.9).

In imperial: a 154 lb (≈ 70 kg), 5'9" (69 in) person:

BMI = (154 × 703) / (69 × 69) = 108,262 / 4761 ≈ 22.74

A small rounding difference comes from the conversion factors. Both land in Normal.

What BMI tells you​

  • Population-scale: BMI is well-correlated with average body-fat percentage across large groups.
  • Trend at the individual level: Your own BMI tracked over time can be a useful proxy for weight trajectory if your height isn't changing.
  • Quick risk screen: At population scale, sustained BMI > 30 is associated with higher cardiometabolic risk, per multiple large cohort studies (CDC, WHO, NHANES).

What BMI does not tell you​

The CDC explicitly lists BMI's limitations on its own patient pages:

  • Doesn't distinguish muscle from fat. Athletes with high muscle mass routinely register as "overweight" or "obese" by BMI.
  • Doesn't reflect fat distribution. Visceral (belly) fat carries different risk than subcutaneous fat; BMI ignores that.
  • Less accurate at extremes of height and for older adults (where muscle loss skews the relationship to body fat).
  • Doesn't account for ethnicity-specific risk thresholds. Some health bodies (NICE in the UK, several Asia-Pacific authorities) recommend lower BMI cutoffs for South Asian and East Asian populations because cardiometabolic risk rises at lower BMIs in those groups.
  • Doesn't capture body composition changes that matter for someone training — fat loss + muscle gain can leave BMI unchanged while body composition improves significantly.

Use BMI as one input among several, not as the metric.

The /health/bmi page in the health module also logs your BMI over time if you want to track the trend — same formula, with persistence.

Frequently asked​

Why does BMI ignore body composition? BMI is intentionally a two-input metric (weight + height) because it was designed for large-population screening. More accurate body-composition assessment requires DEXA scans, hydrostatic weighing, or air-displacement plethysmography — none of which fit in a free web calculator.

Is BMI useful for pregnancy? Not in the standard form. Pregnancy weight gain is expected and healthy; pre-pregnancy BMI is what informs the Institute of Medicine's gestational-weight-gain ranges. Track pregnancy weight directly during pregnancy, not BMI.

What about children? Pediatric BMI uses age-and-sex percentile charts (CDC growth charts), not the adult cutoffs. LifeWell does not provide pediatric BMI here. The child growth page in the baby module handles infant/toddler growth via WHO charts.

Does LifeWell save my BMI when I use this calculator? No. The calculator is stateless — close the page, the inputs and result are gone. To log BMI over time, use /health/bmi (the health-module surface) instead.

The result says "Overweight" but I'm an athlete. The "Overweight" label reflects BMI alone. If you carry significant muscle, BMI overestimates your fat mass. Body-fat percentage (DEXA, skinfolds, bioimpedance) is the better signal in that case.

Last updated: 2026-05-11 Author: Ahsan Mahmood

:::note Health information — not medical advice LifeWell and this documentation are provided for general wellness and educational purposes only. They are not a substitute for professional medical advice, diagnosis, or treatment, and nothing here is intended to diagnose, treat, cure, or prevent any condition. Always consult a qualified healthcare professional with questions about your health, and never disregard or delay professional advice because of something you read here. In an emergency, contact your local emergency services. :::