What time of day should I weigh myself?

First thing in the morning, after using the bathroom and before eating or drinking, gives the most consistent reading — body weight naturally fluctuates by a kilogram or two across a single day.

Does clothing affect BMI results?

It can shift the weight input enough to matter, especially heavier clothing or shoes. Light, consistent clothing (or none) at each measurement gives the most accurate and comparable result.

How often should I re-measure height?

Adult height rarely changes, so measuring it once and reusing that value is normal — no need to remeasure before every BMI calculation. Weight is the value worth checking regularly.

Why does my measured height seem to change during the day?

Spinal discs compress slightly under the weight of standing and sitting throughout the day, so height measured in the evening is typically a little shorter than in the morning. This is normal and not a measurement error.

How to Measure Height and Weight Accurately

BMI is only as accurate as the numbers you put into it. A few common measurement habits — inconsistent timing, clothing, or footwear — can shift the result enough to change which category someone falls into.

Measuring height

Stand against a flat wall, no shoes. Stand with your back against a wall on a hard floor, heels together, without shoes or thick socks. Carpet and shoes both add measurable height.
Look straight ahead. Keep your head level, looking straight ahead rather than tilted up or down — chin position changes the reading by more than people expect.
Mark and measure. Place a flat, rigid object (a book or ruler) level on top of your head, mark the wall, then measure from the floor to the mark with a tape measure.
Measure at the same time of day if tracking over time. Height varies by up to about a centimetre across the day due to spinal compression — measuring first thing in the morning gives the most consistent readings.

Measuring weight

Use the same scale each time. Different scales can disagree by a kilogram or more. Consistency matters far more than any single scale being perfectly calibrated.
Weigh at a consistent time. Body weight fluctuates by a kilogram or two across a single day from food, fluid and digestion. First thing in the morning, before eating or drinking, gives the most comparable readings.
Place the scale on a hard, flat surface. Carpet and uneven flooring both distort a mechanical or digital scale's reading. A hard, level floor is required for an accurate number.
Wear consistent (or no) clothing. Clothing can add a noticeable amount of weight. Weighing with minimal, consistent clothing removes that variable when tracking change over time.

Why consistency matters more than precision

A single BMI reading matters less than the trend over time. If you measure the same way each time — same scale, same time of day, similar clothing — small systematic errors cancel out when you're comparing this month to last month, even if no individual reading is perfectly exact.

Ready to calculate? Use the BMI calculator with your measurements.

Common questions

What time of day should I weigh myself?: First thing in the morning, after using the bathroom and before eating or drinking, gives the most consistent reading — body weight naturally fluctuates by a kilogram or two across a single day.
Does clothing affect BMI results?: It can shift the weight input enough to matter, especially heavier clothing or shoes. Light, consistent clothing (or none) at each measurement gives the most accurate and comparable result.
How often should I re-measure height?: Adult height rarely changes, so measuring it once and reusing that value is normal — no need to remeasure before every BMI calculation. Weight is the value worth checking regularly.
Why does my measured height seem to change during the day?: Spinal discs compress slightly under the weight of standing and sitting throughout the day, so height measured in the evening is typically a little shorter than in the morning. This is normal and not a measurement error.