BMI calculator
Enter your height and weight to get your body mass index, the category it falls into, and the healthy weight range for someone your height.
Fill in your height and weight and your result appears here.
BMI is a screening number, not a diagnosis. It is calculated from height and weight alone, so it cannot separate muscle from fat or tell you where fat sits in your body.
The basics
What is BMI?
Body mass index is a single number that compares your weight to your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was describing populations rather than patients, and it is still the most widely used way to sort adults into weight categories.
The appeal is simplicity. Two measurements anyone can take produce a number that correlates reasonably well, across large groups, with the amount of body fat a person carries. That makes BMI useful for public health statistics and for a first look at an individual.
The limitation is the same simplicity. Height and weight cannot tell a doctor whether extra kilos are muscle or fat, whether fat sits under the skin or around the organs, or what is happening in your blood. BMI is a starting point for a conversation, not the conclusion of one.
How to calculate your BMI
The calculator above does the arithmetic, but the formula is short enough to do by hand.
BMI = weight in kg divided by height in metres, squared
Someone who is 1.75 m and weighs 70 kg: 70 divided by (1.75 x 1.75) = 22.9, which falls in the healthy weight range.
BMI categories for adults
These are the World Health Organization cut-offs for adults aged 20 and over. They are the same for men and women, which is one of the reasons the number needs context.
| Category | BMI range | What it means |
|---|---|---|
| Underweight | Below 18.5 | Your weight is low for your height. That can reflect a naturally slight build, but it can also point to low muscle mass, nutrient gaps or an underlying condition worth discussing with a doctor. |
| Healthy weight | 18.5 to 24.9 | Your weight sits in the range associated with the lowest average health risk at population level. It says nothing on its own about your body composition, your blood values or your fitness. |
| Overweight | 25.0 to 29.9 | Your weight is above the healthy range for your height. For many people this is the stage where blood pressure, blood sugar and blood lipids start to drift, often years before symptoms appear. |
| Obesity class I | 30.0 to 34.9 | Your result falls in the first obesity class. Population studies link this range to a higher risk of type 2 diabetes, cardiovascular disease and joint problems. Where your fat sits matters as much as the number. |
| Obesity class II | 35.0 to 39.9 | Your result falls in the second obesity class, a range associated with a substantially higher cardiovascular and metabolic risk. A medical assessment is the sensible next step. |
| Obesity class III | 40.0 and above | Your result falls in the third obesity class. This range carries the highest associated risk in population data, and it is worth taking to a doctor rather than acting on alone. |
Different cut-offs apply to children and teenagers, who are assessed against age and sex percentile charts rather than fixed ranges.
Read the number carefully
What BMI does not tell you
BMI is a good screening tool and a poor diagnostic one. Four things it cannot see are worth knowing before you act on your result.
It cannot tell muscle from fat
Muscle is denser than fat, so a well-trained rugby player and someone carrying the same weight as fat can land on identical BMIs. The same effect works in reverse: an older adult who has lost muscle can sit in the healthy range while carrying a high proportion of body fat, sometimes called normal weight obesity.
It says nothing about where fat sits
Fat stored around the abdominal organs, known as visceral fat, behaves differently from fat under the skin and is more closely linked in the research to insulin resistance and cardiovascular risk. Two people with the same BMI can have very different amounts of it.
The same cut-offs do not fit every population
The World Health Organization notes that the risk associated with a given BMI varies between populations, and several health authorities apply lower thresholds for people of South Asian, Chinese and other Asian ancestry, where metabolic risk tends to appear at a lower BMI.
It does not apply to everyone
Adult BMI categories are not meant for children, pregnant people, competitive athletes, or people with conditions that change body water or muscle mass. In those cases the number is best interpreted with a clinician rather than on its own.
A second measurement that adds a lot
Because BMI is blind to fat distribution, many clinicians pair it with a waist measurement. Waist to height ratio is the simplest version: measure your waist at the midpoint between the lowest rib and the top of the hip bone, then divide by your height in the same unit.
Keep your waist under half your height
A ratio below 0.5 is the rule of thumb used in several national guidelines, including the UK guidance on assessing weight-related risk. It takes ten seconds with a tape measure and captures something BMI structurally cannot.
Beyond the number
What tracks metabolic health more closely than BMI
If your BMI has prompted you to look further, these are the measurements that carry more information about what is actually happening in your body.
Visceral fat volume
Quantified from an MRI scan rather than estimated, visceral fat is the fat around your organs. It is the fat compartment most consistently associated with insulin resistance and cardiovascular risk in the literature.
Muscle mass and quality
Muscle is where most glucose is cleared from the blood, and losing it quietly with age is one of the ways a healthy BMI can hide a worsening metabolic picture. Imaging measures the amount and the fat infiltration in it.
Blood biomarkers
Fasting glucose, HbA1c, insulin, triglycerides, HDL and ApoB describe how your metabolism is coping. They move before weight does, and they move back with the right changes.
Change over time
A single reading is a snapshot. The direction of travel across repeated measurements is the part that tells you whether what you are doing is working.
Ahead Health membership
Your BMI is one number. A membership tracks the ones behind it.
An Ahead Health membership is a year of structured prevention: repeat blood panels, a personal report you can actually read, and a prevention team that goes through it with you. It is built for the question BMI raises but cannot answer, which is where your health is heading.
Cancel before renewal. Your data stays yours and is never sold.
- Up to four blood check-ups a year, 80 plus biomarkers each
- A personal report reviewed by Swiss physicians
- A prevention team you can message between check-ups
- Body composition and visceral fat from MRI, at member rates
- Your results tracked side by side, year over year
BMI questions, answered
Other calculators
Free tools that ask a different question about the same body.
This calculator provides general health information and is not a medical diagnosis. It does not replace an assessment by a doctor. If you have concerns about your weight or your health, speak to a physician.