BMI Calculator: Metric and Imperial Units
Body Mass Index remains one of the most widely used screening numbers in everyday health checks. This BMI calculator converts height and weight into a single index and maps it to standard WHO categories so you can see where a result sits at a glance. It works in both metric and imperial units and is designed for adults. Remember that BMI does not distinguish muscle from fat or account for bone density, so it is a starting conversation point rather than a diagnosis.
Please enter valid, positive height and weight values.
How to Use the BMI Calculator
- Choose metric (cm, kg) or imperial (in, lb).
- Enter height and weight accurately.
- Select Calculate to see your BMI value and the matching category.
- Read the short notes on limitations if the number will influence decisions.
Choose metric (centimetres and kilograms) or imperial (inches and pounds) first. Enter your height and weight in those units. Imperial height goes in inches only, so 5 ft 9 in is 69. The calculator shows your BMI to one decimal place and the matching category. BMI is a screening number for adults, so use it as a starting point, not as a diagnosis.
BMI Calculator Formula Explained
Weight in kilograms is divided by the square of height in metres. Imperial inputs are converted first (1 inch = 0.0254 m, 1 lb ≈ 0.4536 kg) so the same equation is used. Categories follow WHO adult cut-offs: under 18.5 underweight, 18.5–24.9 normal range, 25–29.9 overweight, 30 and above obesity classes. For children and teens different percentile charts are required; this tool is intended for adults.
The calculator divides weight by height squared. For imperial units it uses weight (lb) ÷ height (in)² × 703, which matches the metric result to within rounding. Below 18.5 is Underweight, 18.5 to 24.9 is Normal weight, 25 to 29.9 is Overweight and 30 or more is Obese. BMI does not measure body fat or muscle, and these adult ranges do not apply to children or teens. WHO publishes the adult overweight (25 and above) and obesity (30 and above) cut-offs in its WHO obesity and overweight fact sheet.
BMI Calculator Example Calculation
A person who is 175 cm tall and weighs 70 kg has a BMI of about 22.9, which sits in the WHO normal range. Someone 180 cm and 95 kg lands near 29.3 (overweight band). Always interpret BMI with context: a rugby player and an office worker can share a similar BMI with very different body composition.
Worked example: choose metric and enter 175 cm and 70 kg. BMI is 70 ÷ 1.75² = 22.9, which is Normal weight. Enter 180 cm and 95 kg, and BMI is 29.3, which is Overweight. In imperial, 69 inches and 160 lb give a BMI of 23.6.
When to use this tool
Use BMI as a quick screening number for adult health check-ins, workplace wellness programmes, or preparing for a doctor visit. Pair it with waist measurement and clinical advice rather than treating the index as a diagnosis.
Key Features & Benefits of This BMI Calculator
- Metric or imperial — centimetres and kilograms, or inches and pounds; imperial height goes in inches only, so 5 ft 9 in is 69.
- WHO adult categories — Underweight below 18.5, Normal weight 18.5 to 24.9, Overweight 25 to 29.9 and Obese 30 or more.
- One-decimal result — for example 175 cm and 70 kg gives 22.9, which is Normal weight.
- Screening number, not a diagnosis — BMI does not measure body fat or muscle, and the adult ranges do not apply to children or teens.
- Instant, free and private — runs entirely in your browser, no sign-up, nothing stored.
BMI Calculator — FAQs
Does a high BMI always mean high body fat?
No. Athletes and people with high muscle mass can score in the overweight or obese range while carrying relatively little fat. Context matters.
Is BMI different for men and women?
The mathematical formula is the same. Interpretation can differ in clinical settings, and body-composition tools often provide more detail by sex.
Should I use BMI alone to judge health?
No. Combine it with waist measurement, blood markers, fitness level and professional advice rather than treating the number as a final verdict.
Is BMI accurate for children?
No. BMI-for-age percentile charts are used for children and teens instead of the fixed adult categories this tool shows.
Sources and review
Sources last checked: . These sources give the background for this calculator. They are general information and do not replace advice from a doctor, midwife or registered dietitian.
- WHO: Obesity and overweight: adult BMI cut-offs of 25 for overweight and 30 for obesity.
- NHS: Calculate your body mass index (BMI): the NHS BMI tool, with separate calculators for adults and for children and teenagers.