BMI Calculator

What BMI Actually Measures

Body Mass Index is a single ratio: weight divided by the square of height. In metric units it is BMI = weight (kg) / height (m)². The imperial version is BMI = 703 × weight (lb) / height (in)²; the 703 is purely a conversion factor that makes the two systems agree. Nothing else goes into the number.

That simplicity is both the strength and the weakness of BMI. It needs only two inputs, so it works as a fast screening figure across large groups. But it treats every kilogram as equal, whether that kilogram is muscle, bone, fat, or water, which is why it can misread individuals.

How to Use the Calculator

1. Pick Metric or Imperial with the toggle at the top. The height and weight labels switch between cm/kg and in/lb, and the placeholder values update to match.
2. Enter your height and weight. Use the numbers from a recent measurement rather than an estimate.
3. Enter your age and select male or female. These feed the BMR and calorie section, not the BMI value.
4. Press Calculate BMI. The result, category, gauge, ideal weight range, and calorie table all appear together.

The results panel stays hidden until every field is a positive number, and it does not update live as you type. If the button appears to do nothing, check that height, weight, and age are all filled in.

Reading the Result: Categories and Worked Examples

The score is matched to an adult category, colour-coded on a gauge that runs from roughly 15 to 40:

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obese Class I
35.0 – 39.9	Obese Class II
40.0 and above	Obese Class III

The ideal-weight figure is simply the weight band at your height that maps to the normal range. At 170 cm it is about 53.5 kg to 72.0 kg; at 67 inches it is about 118.2 lb to 159.0 lb.

Metric: 65 kg at 170 cm. Height in metres is 1.70, squared is 2.89, so 65 / 2.89 = 22.49, displayed as 22.5 — normal weight.

Imperial: 143 lb at 67 inches. Square the height (4,489), then 703 × 143 / 4,489 = 22.4 — the same person, expressed the other way. Rounding the intermediate steps is the usual reason a hand calculation drifts from the tool.

BMR and Daily Calories: What the Extras Mean

When you supply age and sex, the page also estimates Basal Metabolic Rate with the Mifflin-St Jeor equation: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 for males, or − 161 for females. The example above (65 kg, 170 cm, age 30, male) gives 650 + 1062.5 − 150 + 5 = 1567.5 calories per day at rest.

Daily needs are that BMR multiplied by an activity factor: 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 active, and 1.9 very active. For the same person that spans roughly 1,881 to 2,978 calories. These are estimates of average energy expenditure, not prescriptions.

Measuring Well: A Short Checklist

BMI is only as accurate as the inputs, so a little care in measuring pays off. Weigh yourself at the same time of day, ideally in the morning after using the bathroom and before eating or drinking, because body weight commonly swings one to two kilograms across a day with meals, fluids, and hydration. Use a hard, level floor rather than carpet, and read the scale to the nearest 0.1 kg or 0.2 lb.

For height, stand without shoes, heels together, back straight, and look straight ahead. Slouching or measuring against a soft surface can shave a centimetre or more off your height, and because the formula squares height, that error moves the result by several tenths. Record both values to one decimal place rather than rounding first.

Finally, treat a single reading as a snapshot and the trend over weeks as the signal. The BMI bands are several points wide, so ordinary daily fluctuation rarely changes a category, but a two- or three-kilogram move near a boundary can. If you are tracking a change, compare measurements taken the same way, at the same time of day, and log them alongside your weight rather than trying to remember the last number.

Limitations and Scope

This calculator is an educational tool, not medical advice, and it is not a diagnosis. BMI was designed for adult populations and has well-known blind spots: trained athletes, older adults who have lost muscle, people with a large frame, and anyone retaining fluid can all be misclassified. It also says nothing about where fat sits, and abdominal fat carries more risk than the same weight distributed elsewhere.

For a fuller picture, pair BMI with a waist-to-height ratio or a body-fat estimate, and interpret the result with a qualified clinician who knows your history. Do not start, stop, or change any treatment based on a number from this page.

Related Tools

• Body Fat Calculator — Estimate body-fat percentage with the US Navy tape method.
• Ideal Weight Calculator — Compare several ideal-weight formulas for your height.
• Calorie Calculator — Work out daily calorie targets from activity level.
• Macro Calculator — Split those calories into protein, carbohydrate, and fat.