HealthScorer

BMI Calculator

Free BMI calculator with WHO classification. Switch between kg/cm and lb/in. Sources cited (WHO, CDC, NHS), calculation runs in your browser.

Last updated: Sources verified:

How BMI is calculated

BMI is your weight in kilograms divided by your height in metres squared:

BMI = weight (kg) / [height (m)] ²

In imperial units the formula is BMI = 703 × weight (lb) / [height (in)] ², which mathematically converts to the same value. This calculator handles the conversion automatically when you toggle units.

WHO classification

BMIClassification
Below 18.5Underweight
18.5 – 24.9Normal weight
25.0 – 29.9Overweight
30.0 – 34.9Obesity (Class I)
35.0 – 39.9Obesity (Class II)
40.0 and aboveObesity (Class III)

When BMI is most useful — and when it isn’t

BMI is most useful as a screening signal at the population level, where small individual variations average out. For an individual, treat it as one input among several:

  • Pair it with waist-to-height ratio. A waist-to-height ratio above 0.5 is associated with elevated cardiometabolic risk regardless of BMI.
  • Account for body composition. Resistance-trained adults may have BMI ≥25 with low body fat; older adults may have BMI <25 with sarcopenia.
  • Account for ethnicity. Several Asian populations use thresholds 2 BMI units lower because risk rises earlier.
  • Don’t use it during pregnancy. Use ACOG pregnancy weight-gain guidance instead.

If you want a more complete metabolic snapshot, pair this with our BMR calculator. It estimates daily caloric expenditure using the Mifflin-St Jeor equation, which is the formula most clinical nutritionists prefer over older approaches. For mood and stress screening, the PHQ-9 covers depression severity in nine standardized items.

Privacy

All calculation happens in your browser. We never see, log, or store the values you enter. Anonymous, aggregate events (e.g. which severity bucket your result fell into) are sent to a privacy-first analytics service to help us improve the tool.

Frequently asked questions

What is a healthy BMI?
For adults, the WHO defines healthy BMI as 18.5–24.9. Below 18.5 is underweight and 25 or above is overweight; 30 or above is obesity. BMI does not account for muscle mass, body composition, age, ethnicity or pregnancy, and clinicians use it as a starting point alongside waist circumference and other markers.
What does a BMI of 27 mean?
A BMI of 27 sits in the middle of the overweight band (25.0–29.9). Berrington de Gonzalez 2010 (NEJM, n ≈ 1.46 million) showed cardiovascular risk rises above BMI 25, but the slope between 22 and 27 is gentler than between 27 and 32. An athlete with low body fat at BMI 27 often carries no excess metabolic risk. Waist circumference matters more here: above 40 inches in men or 35 in women is the firmer signal.
What does a BMI of 30 mean?
A BMI of 30 is the WHO threshold for Class I obesity. Statistically it ties to a 2–3× higher risk of type 2 diabetes and hypertension compared with BMI 22 (CDC adult obesity data). It is the point at which most US primary-care guidelines (USPSTF 2018) recommend offering intensive behavioral counseling. Treat it as a threshold for action, not a verdict.
How accurate is BMI?
BMI works well as a population-level screening tool but has known limitations for individuals. Romero-Corral 2008 (PMID 18283284) showed BMI misclassifies around 50% of people when compared with DXA-measured body fat. It can overestimate adiposity in athletes with high muscle mass and underestimate it in older adults with reduced lean mass. Pair it with waist-to-height ratio or body-fat estimates if you need a fuller picture.
At what BMI does obesity start?
Obesity starts at a BMI of 30.0 by WHO criteria. Class I covers 30.0–34.9, Class II 35.0–39.9, and Class III (formerly "morbid") is 40 or higher. In Asian populations the WHO 2004 expert consultation (PMID 14726171) suggested a public-health action point starting at BMI 27.5. The NIH and CDC use the standard 30.0 threshold.
What should I do if my BMI is 32?
BMI 32 is Class I obesity, and three evidence-based paths run in parallel (USPSTF 2018, AHA 2022). Diet plus at least 150 minutes of activity per week is a realistic foundation, with a 5–10% loss over six months as a clinical target. Behavioral support or a registered dietitian doubles that effect. Talk to your primary-care provider about GLP-1 medication (semaglutide, tirzepatide) or bariatric surgery if comorbidities are present. Start with the doctor visit so any associated conditions get screened.
Is BMI accurate for athletes?
Often not. Muscle is roughly 18% denser than fat by volume, so a powerlifter or rugby player at BMI 28 may have 12% body fat and the lowest cardiometabolic risk band. Romero-Corral 2008 (PMID 18283284) is the standard reference for this gap. If you strength-train more than twice a week, body-fat percentage and waist-to-hip ratio give a fairer read.
What is the healthiest BMI?
For adults under 65, the lowest point on mortality curves sits around BMI 22–24 (Aune 2016, BMJ meta-analysis, n ≈ 10.6 million). Above 65 the curve shifts: Winter 2014 (PMID 24898241) found the lowest mortality between BMI 25.0 and 27.9. There is no single "healthiest BMI" for everyone, and age, body composition, and existing conditions all move the optimum.
Does BMI use the same cutoffs everywhere?
The WHO 18.5/25/30 cutoffs are global, but several Asian populations use lower thresholds (overweight ≥23, obesity ≥27.5) because cardiometabolic risk rises at lower BMI in those groups (WHO 2004, Lancet vol. 363). Speak to your local clinician about region-specific guidance.
Can BMI be used in pregnancy?
Not for classification during pregnancy, but yes for the period before. Pre-pregnancy BMI sets the recommended gestational weight gain per the Institute of Medicine 2009 guidance: 25–35 lb at BMI 18.5–24.9, 15–25 lb at BMI 25–29.9, and 11–20 lb at BMI ≥30. Post-partum BMI typically takes about 12 months to settle.
Can children use this calculator?
No. Pediatric BMI uses age- and sex-specific percentiles (CDC growth charts in the US, WHO 2007 internationally). This calculator is intended for adults aged 18 and older.
How often should I measure my BMI?
Every 6–12 months is enough for a healthy adult. More frequent measurement (every 4–8 weeks) makes sense during a weight-loss program, where the trend matters more than any single reading. Daily weighing rarely helps because normal fluid shifts move weight ±1–2 kg over 24 hours.
How can I lower my BMI healthily?
Aim for a 0.5–1 kg (1–2 lb) weekly deficit through a 500–750 kcal-per-day shortfall, with two strength sessions to protect lean mass (NIH guidelines). A 5–10% loss within six months meaningfully lowers blood pressure, blood sugar, and triglycerides — even if it doesn't change your BMI band. Crash diets below 1,200 kcal usually rebound.
Does my data leave my device?
No. The calculation runs entirely in your browser. We do not transmit, store, or log the values you enter. Anonymous usage events (e.g. classification bucket) are sent to a privacy-first analytics service to help us improve the tool. No personally identifiable information is collected.

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Sources

  1. Obesity: preventing and managing the global epidemic (Technical Report Series 894) — World Health Organization (guideline, retrieved 2026-04-26)
  2. Adult BMI calculator and classification — Centers for Disease Control and Prevention (gov health, retrieved 2026-04-26)
  3. Body mass index (BMI) and waist circumference — NHS (gov health, retrieved 2026-04-26)