How to Calculate BMI Correctly

BMI math itself is almost insultingly simple — divide weight by height squared. So when people get an unexpected or seemingly wrong BMI result, the formula is rarely the problem. The measurements that go into it usually are.

Step 1: Measure height correctly

Height should be measured standing fully upright, without shoes, heels together, looking straight ahead — not tilted up or down. Measuring first thing in the morning gives a very slightly taller reading than later in the day, since minor spinal compression accumulates over hours of being upright; the difference is small (a few millimetres) but worth being consistent about if you're tracking BMI over time.

Step 2: Measure weight correctly

Weigh at a consistent time of day — first thing in the morning, after using the bathroom, before eating or drinking, is the most stable and comparable reading. Weight can swing by a kilogram or more across a single day due to food, water, and sodium intake, so a single measurement at a random time isn't a reliable input for tracking purposes, even though it's perfectly fine for a one-off BMI check.

Step 3: Do the calculation

BMI = weight (kg) ÷ height (m)²

For example, someone weighing 70kg at a height of 1.75m: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. If you're working in pounds and inches, multiply weight in pounds by 703, then divide by height in inches squared — both versions of the formula produce the same result, just scaled for different units. The BMI Calculator on this site handles the unit conversion automatically either way.

Common mistakes that throw off the result

Rounding height too aggressively (172cm entered as 170cm) shifts the result more than people expect, since height is squared in the formula — small height errors compound. Weighing with shoes or heavy clothing on adds a kilogram or more depending on what you're wearing. And comparing a same-day "after a big meal" weight against a "first thing in the morning" weight from a previous check creates an apples-to-oranges comparison that looks like real change when it's just normal fluctuation.

Once you have the number

A single accurate BMI calculation is a useful data point, not a verdict. Reading the result in context — alongside how your clothes fit, your energy levels, and ideally a second measure like waist-to-height ratio — gives a far more complete picture than the BMI number on its own.