Body mass index (BMI) is one number from height and weight. Public-health charts then place that number in an adult band: underweight, normal weight, overweight, or obesity. This guide explains those WHO ranges, the metric and imperial formulas, and the cases where the chart does not apply. To run the same math in your browser, use the BMI Calculator. For click-by-click steps, see How to Use BMI Calculator.
Medical disclaimer
This article and the calculator are informational only. They are not medical advice, a diagnosis, or a treatment plan. BMI does not measure body fat, muscle, or overall health. Talk to a clinician before using BMI to make health decisions. Do not use these adult ranges for children, teens, or pregnancy.
Quick answer
For adults, WHO groups BMI as underweight (under 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9), and obesity (30.0 or higher). Metric: kg divided by height in meters squared. Imperial: 703 x lb / inches squared. The number is a screening band, not a verdict.
The four adult WHO ranges
These cutoffs match the BMI Calculator on ToolsMinify. The tool rounds BMI to one decimal, then picks a category - the same display most consumer charts use.
- Underweight: BMI under 18.5
- Normal weight: BMI 18.5 to 24.9
- Overweight: BMI 25.0 to 29.9
- Obesity: BMI 30.0 or higher
Some clinical charts split obesity further into class I (30.0 to 34.9), class II (35.0 to 39.9), and class III (40.0 or higher). The ToolsMinify calculator keeps a single obesity band at 30.0+, which matches the four-band adult table used on many public-health pages.
Why 24.9 and 25.0 both appear
Normal weight is written as 18.5 to 24.9 and overweight starts at 25.0. After rounding to one decimal, 24.9 stays in the normal band and 25.0 starts overweight. There is no leftover 24.95 on a one-decimal chart.
How BMI is calculated
BMI is weight relative to height squared. Units must match. Switch the calculator to metric or imperial so you do not mix kilograms with inches.
Metric formula
BMI = kg / m^2. Height is usually entered in centimeters, so convert first: meters = cm / 100. Example: 70 kg at 175 cm is 70 / (1.75 x 1.75) = 22.857..., which rounds to 22.9 (normal weight).
Imperial formula
BMI = 703 x lb / in^2. Convert feet plus leftover inches to total inches first. Example: 150 lb at 5 ft 5 in is 65 in: 703 x 150 / (65 x 65) = 24.96..., which rounds to 25.0 (overweight). The 703 factor is the conventional adult conversion so imperial results sit on the same scale as kg/m^2.
Worked examples
- 70 kg at 175 cm → BMI 22.9, normal weight (18.5 to 24.9).
- 150 lb at 5 ft 5 in (65 in) → BMI 25.0, overweight (25.0 to 29.9).
- 50 kg at 170 cm → BMI 17.3, underweight (under 18.5).
- 110 kg at 170 cm → BMI 38.1, obesity (30.0 or higher).
Paste the same numbers into the BMI Calculator to see the matching category and a healthy-weight span for that height (the weights that produce BMI 18.5 and 24.9).
What the ranges do not tell you
BMI is a screening shortcut. It is silent on several things that matter for health:
- Muscle vs fat: two people can share a BMI and have very different body composition.
- Where fat sits: waist size, visceral fat, and fitness are not in the formula.
- Age and sex: the adult table is not tailored to every group.
- Ethnicity: some public-health guidance uses extra action points (for example 23.0 and 27.5) for some Asian populations. The calculator uses the standard adult WHO four-band table.
- Short-term change: water, food, and clothing shift weight. They do not rewrite health overnight.
Who should not use these adult bands
- Children and teens: use BMI-for-age percentiles from a pediatric growth chart, not adult cutoffs.
- Pregnancy: gestational weight-gain charts start from pre-pregnancy BMI. Do not read a pregnant measurement against non-pregnant adult bands.
- Anyone whose clinician has already given a different screening method or target.
Healthy weight range at a given height
If you fix height, you can back-solve the weight that produces BMI 18.5 and the weight that produces BMI 24.9. That span is the healthy weight range shown on the tool. It is still only the adult WHO normal band expressed as kilograms or pounds - not a personal target and not medical advice.
Frequently asked questions
Is BMI a diagnosis?
No. A category label describes where a number sits on a chart. It does not diagnose a disease, predict a specific outcome, or replace a clinical exam.
Is a BMI of 25 overweight?
On the adult WHO table used here, 25.0 is the start of overweight. 24.9 is still in the normal-weight band. Categories are not a treatment plan.
Why does the calculator say obesity at 30 instead of listing classes?
The four-band adult table groups every BMI of 30.0 or higher as obesity. Class I / II / III splits are extra detail some clinics add. They are not required to read the basic chart, and they still are not a diagnosis.
Can athletes have a high BMI and still be healthy?
Often the number is high because muscle raises weight. That is one reason BMI is a weak stand-alone health test. It still does not measure fitness. Ask a clinician if you need a personal reading.
Does my height or weight leave the device?
No. The BMI Calculator runs entirely in your browser. Measurements are not uploaded or stored on a server.
How do I calculate BMI step by step?
Use the formulas above, or follow How to Use BMI Calculator for the on-page fields, unit switch, and copy button.
Check adult BMI in your browser
Pick metric or imperial, enter height and weight, and read BMI plus the matching adult WHO category. Nothing is uploaded:
- BMI Calculator - metric and imperial BMI with adult WHO categories.
- How to Use BMI Calculator - step-by-step guide for the same tool.
- Age Calculator - years, months, and days from a date of birth.
- Percentage Calculator - everyday percent-of, change, and what-percent math.
- All calculator tools - the full category index.
Calculate BMI - free
Open the BMI Calculator, choose metric or imperial, and read BMI plus the matching adult WHO category. Informational only - not medical advice. Everything runs in your browser.