By the Tooliday Team. Last updated 14 November 2026.
A BMI calculator is only as good as the numbers you enter. A few centimetres of error in height, or weighing yourself at different times of day, can change the result more than you would expect. This guide shows how to measure properly at home and convert units. You can then enter your numbers into our BMI Calculator.
Measuring height
- Take off your shoes and any hair accessories that add height.
- Stand with your back against a flat wall, with your heels, buttocks and shoulders touching it.
- Look straight ahead, so that your chin is level with the floor.
- Ask someone to lay a flat object, such as a book, on top of your head, pressed against the wall, and mark the spot.
- Measure from the floor to the mark with a tape measure, and write the number down in centimetres.
Height can be about a centimetre lower in the evening than in the morning, so try to measure at a similar time of day if you repeat it.
Measuring weight
- Use the same scale, on a hard, flat floor, not on carpet.
- Weigh yourself at a similar time each day, for example in the morning after using the toilet.
- Wear light clothes and no shoes, and empty your pockets.
- Stand still with your weight evenly on both feet.
Converting units
- Inches to centimetres: multiply by 2.54. A height of 5 feet 7 inches is 67 inches, and 67 × 2.54 = 170.18 cm.
- Pounds to kilograms: multiply by 0.4536. A weight of 154 pounds is about 69.9 kg.
How small errors change BMI
For a person who is 70 kg and 170 cm tall, BMI is 70 ÷ (1.70 × 1.70) = 24.2. Now see what happens if the height is entered a little wrong:
| Height entered | BMI result |
|---|---|
| 168 cm | 24.8 |
| 170 cm | 24.2 |
| 172 cm | 23.7 |
A 2 cm error moves BMI by about half a point. A 1 kg difference in weight moves it by about 0.35 at this height. That is why one reading matters less than measuring carefully and watching the trend over time.
Remember what BMI is
BMI is a simple screening number. It does not measure body fat or muscle, and it does not say whether you are healthy. For more on its limits, read How to Calculate BMI, and for the weight range at each height see Healthy Weight Range by Height.
This article is for general information and is not medical advice. Please talk to a doctor or registered dietitian about your health and weight goals.