A BMI calculator converts your height and weight into a single value, placing you on a standard scale used by health services worldwide. Enter both values below: the calculator will determine your body mass index and the corresponding category. The rest of the page explains the formula used, the meaning of each category, and—equally important—what body mass index does not reveal about you.
How the BMI Calculator Works
Body mass index is weight divided by height squared. It was devised by Adolphe Quetelet in the 1830s as a way to describe populations, and it survives today because it needs only two measurements that anyone can take.
In metric units the formula is BMI = kg ÷ m². Height goes into the calculator in centimetres, so 170 cm becomes 1.70 m before squaring. Someone weighing 65 kg at 170 cm has a BMI of 65 ÷ 2.89 = 22.5, which is the calculator’s default setting. The WHO’s own published example uses 70 kg at 1.75 m, giving 70 ÷ 3.0625 = 22.9.
On the imperial setting the formula multiplies by 703 to correct for the different units: BMI = (lb ÷ in²) × 703. Someone weighing 154 lb at 5’9″ (69 inches) has a BMI of (154 ÷ 4,761) × 703 = 22.7.
What You Need to Enter
- Height — in centimetres, or feet and inches on the imperial setting
- Weight — in kilograms, or pounds on the imperial setting
- Metric / Imperial toggle — switches both fields at once
That is the whole calculation. BMI uses no other input — no age, no sex, no measure of body composition — which is both its strength and its main limitation.
BMI Categories and Obesity Classes
The World Health Organization defines these ranges for adults. They are the same figures used by most national health services, and the same ones shown in the calculator’s results panel.
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
The calculator groups everything from 30 upward as a single band. The three classes below split that range further, because the health associations at BMI 31 and BMI 45 are quite different and one label would flatten the distinction.
These cut-offs apply to adults aged 20 and over. Children and teenagers are assessed against age and sex percentile charts instead, because body composition changes throughout growth. A WHO expert consultation has also identified additional public health action points at 23 and 27.5 for Asian populations, though the standard classifications remain the international standard.
Age matters at the other end too. A meta-analysis of 32 cohort studies covering almost 198,000 people aged 65 and over found mortality risk was lowest between BMI 24 and 31 — noticeably higher than the standard healthy range. Some clinical guidance therefore adjusts the classifications upward for older adults.
What BMI Does Not Measure
BMI is a screening number, not a diagnosis. It has known blind spots worth understanding before reading much into a result, and Queensland Health’s clinical guidance on using BMI sets them out in detail.
- It cannot tell muscle from fat — a rugby player and a sedentary person of the same height and weight get the same BMI.
- It ignores where weight sits — BMI treats all weight identically regardless of distribution.
- It was built from European population data — its cut-offs fit some populations better than others.
- It says nothing about an individual — BMI describes how a number compares to a distribution; it does not describe a person’s health.
The Waist to Height Ratio
One widely used complement is the waist to height ratio — waist measurement divided by height, in the same units. A ratio below 0.5 is the commonly cited guideline. Because it captures abdominal measurement, it picks up something BMI misses entirely, and several health bodies now suggest using the two together rather than either alone.
Waist circumference on its own is also used clinically. Risk of obesity-related chronic disease is considered increased at 94 cm for men and 80 cm for women, and substantially increased at 102 cm and 88 cm respectively. A tape measure captures something two numbers on a scale cannot.
Frequently Asked Questions
How do you calculate BMI?
Divide weight in kilograms by height in metres squared. For 65 kg at 170 cm: 65 ÷ (1.70 × 1.70) = 22.5. In pounds and inches, divide weight by height squared and multiply by 703.
What is a healthy BMI?
The WHO defines 18.5 to 24.9 as the normal range for adults. It is a population guideline rather than a personal target, and a doctor is better placed than a number to interpret it.
What are the obesity classes?
Class I is BMI 30.0 to 34.9, class II is 35.0 to 39.9, and class III is 40.0 and above. The classes distinguish between very different levels within a single category.
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so a muscular person can register as overweight on BMI while carrying little body fat. Body fat percentage is more informative in that case.
Does BMI work for children?
Not with adult cut-offs. Children are assessed against age and sex percentile charts, since normal body composition shifts throughout childhood and adolescence.
Is BMI the same for men and women?
The formula and thresholds are identical. Women typically carry more body fat than men at the same BMI, which is one reason BMI is treated as a screening tool rather than a measure of body composition.
Related Calculators
- Body Fat Calculator — estimates composition rather than total weight.
- Calorie TDEE Calculator — daily energy expenditure.
- Age Calculator — BMI interpretation is age-dependent.
- Percentage Calculator — for working with body fat percentages.
- Date Difference Calculator — track intervals between measurements.