BMI Calculator
Check your adult body mass index using kilograms, centimetres, stones, pounds, feet or inches, then read the result with sensible safety notes.
Calculate your BMI
Enter your height and weight. This calculator is for adults aged 18 and over.
What is BMI?
BMI, or body mass index, is a simple calculation that compares your weight with your height. It is often used as a quick starting point to understand whether an adult may be under, within, or above a common healthy weight range.
It is useful because it is quick and easy, but it is limited. It does not directly measure muscle, bone, body fat, waist size or medical risk.
BMI formula
The metric formula is straightforward:
BMI = weight_kg ÷ (height_m × height_m)For UK imperial inputs, this calculator converts stones and pounds into kilograms, then converts feet and inches into metres before applying the same formula.
Adult BMI categories
These are broad adult screening categories, not a complete health assessment.
| BMI result | Common adult category | How to read it |
|---|---|---|
| Below 18.5 | Underweight range | You may benefit from speaking to a GP, dietitian or qualified professional, especially if weight loss was unplanned. |
| 18.5–24.9 | Healthy range | This is often used as a general healthy BMI range, but other measures still matter. |
| 25–29.9 | Overweight range | This may suggest excess weight for height, but body composition and waist measurement matter too. |
| 30 and above | Obesity range | This may suggest a higher risk category, but only a health professional can assess your full situation. |
When BMI can be misleading
BMI is less useful when your weight does not reflect typical body composition. A muscular person may have a high BMI without high body fat, while someone with a “normal” BMI may still carry more fat around the waist.
- Children and teenagers: use a child BMI calculator because age and sex matter.
- Pregnancy: BMI during pregnancy needs different interpretation.
- High muscle mass: athletes and strength-trained people can appear heavier for their height.
- Health conditions: fluid retention, limb differences, illness and medication can affect weight.
For a fuller picture, compare BMI with waist measurement, body fat percentage estimates and professional advice where needed.
Worked BMI examples
What to do with your result
If your BMI result worries you, do not use one calculator to make health decisions. A sensible next step is to look at patterns: waist size, activity, sleep, diet quality, energy levels and any symptoms or health conditions.
You can use the related calculators below to estimate daily calories, protein needs and maintenance calories, but these should still be treated as rough planning tools.
BMI calculator FAQs
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. For example, 70kg and 1.75m gives 70 ÷ 3.0625 = 22.9.
What is a healthy BMI?
For many adults, 18.5 to 24.9 is commonly described as the healthy BMI range. It is not a diagnosis and does not replace medical advice.
Is BMI accurate for everyone?
No. BMI can be misleading for children, pregnancy, people with high muscle mass, some ethnic groups, older adults and people with medical conditions affecting weight or height.
Should I use BMI or body fat percentage?
BMI is simpler, but body fat and waist measurements can add useful context. None of these are perfect on their own.
Sources and safety notes
This page uses the standard BMI formula and common adult screening categories. For official guidance, see the NHS adult BMI calculator, NICE overweight and obesity guidance, and the NHS waist-to-height ratio tool.
Calculatorz pages are for general information only and are not medical advice.