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How to measure your height
- Take off shoes and hair accessories, and undo hairstyles that sit on top of your head.
- Stand on a hard floor against a flat wall, with no skirting board pushing your heels forward. Heels together and touching the wall.
- Stand tall and look straight ahead. Your line of sight should be level with the floor, not tilted up or down.
- Lower a flat object onto the top of your head. A hardcover book or a box works. Keep it level, at a right angle to the wall.
- Mark the wall where the bottom of the object touches it, then step away and measure from the floor to the mark with a metal tape measure.
- Write it down to the nearest 1/8 inch or 0.1 cm. Measure twice; if the results differ by more than 1/4 inch (0.5 cm), measure again.1
Height is slightly greater in the morning, because the discs in your spine compress during the day. The difference is usually around a centimeter.
How to weigh yourself
- Use a digital scale on a hard, level floor. Carpet can make some scales read low.
- Weigh at the same time each time, ideally in the morning after using the bathroom and before eating or drinking.
- Wear the same light clothing, or none.
- Stand still in the center of the scale with your weight spread evenly.
- Check the scale now and then with something of known weight, such as a bag of flour, or compare with a doctor's scale.
How much do errors change BMI?
The tool at the top shows your BMI with small errors in each direction. In general:
- Height errors matter most, because height is squared. Being off by 1 inch changes an average adult's BMI by about 0.6 to 0.7.
- Weight errors matter less. Two pounds changes BMI by about 0.3 for an average-height adult.
- Errors near a boundary can change your category, for example between 24.9 and 25.0. If you're close to a boundary, measure carefully twice.
Measuring children
Children should be measured the same way, without shoes and against a wall.1 For children under 2, doctors measure length lying down, and BMI isn't used; see BMI for kids and teens.
Frequently asked questions
What if I can't stand up straight to be measured?
Height can be estimated from knee height or arm span. See BMI without a height measurement.
How often should I weigh myself?
For tracking trends, once a week at the same time of day is enough. Daily weight can vary by a few pounds with water and food.
Sources
Kept up to dateLast updated October 9, 2026
Private by designYour numbers never leave your device
This page is for education and is not medical advice. Talk with a healthcare professional about your weight and health.
