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BMI Prime Calculator

Find your BMI Prime.

How it is calculated

First the tool computes BMI as BMI = weight / (height/100)², with weight in kilograms and height in centimetres (converted to metres inside the formula). BMI Prime is then BMI / 25, where 25 kg/m² is the upper limit of the normal range. A value below 1 means the BMI is under that limit, exactly 1 means at the limit, and above 1 means over it. BMI is shown with one decimal and BMI Prime with two. Height must be greater than zero. BMI Prime is a screening ratio only; it ignores muscle mass, age, sex and body composition.

Worked example

A person is 175 cm tall and weighs 70 kg. Height in metres: 1.75, squared 3.0625. BMI = 70 / 3.0625 = 22.9 (22.857 before rounding). BMI Prime = 22.857 / 25 = 0.91. Because the value is below 1, the BMI is under the 25 limit, at about 91% of it.

Try this example →

Common mistakes

  • Entering height in metres (1.75) when the field expects centimetres (175).
  • Entering pounds or inches. The formula uses kilograms and centimetres.
  • Reading a BMI Prime of 0.91 as 0.91 kg/m². It is a ratio and has no unit.
  • Treating a value above 1 as a diagnosis. Athletes with high muscle mass can exceed 1 without excess body fat.

Sources

About this calculator

This calculator finds your BMI Prime, a simple ratio of your BMI to 25 kg/m² — the upper boundary of the normal weight range. A BMI Prime of exactly 1.0 sits right at that boundary; below 1.0 means your BMI falls under the “normal” ceiling, and above 1.0 shows by how much it exceeds it (a value of 1.20, for example, means your BMI is 20% above that threshold).

It's used the same way BMI itself is used — by clinicians, researchers, and fitness trackers as a quick screening number — but it makes it easier to see at a glance how far someone's BMI sits from the normal-weight cutoff, in either direction.

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Comments (1)

  • Editorial team

    BMI Prime is BMI divided by 25, so 1.00 sits exactly at the top of the normal range. The lower edge of that range, BMI 18.5, corresponds to a prime of 0.74, which gives you a normal band of 0.74 to 1.00. Height goes in centimetres and weight in kilograms. Like BMI, it does not separate muscle from fat. Do you find the prime easier to read than plain BMI?

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