How to Calculate BMI (And What Your Number Actually Means)
A plain-language walkthrough of the BMI formula, worked examples in metric and imperial units, and the limitations you should know about.
Body Mass Index, or BMI, is one of the most widely used — and widely misunderstood — health numbers. It's quick to calculate, which is exactly why it shows up everywhere from doctor's offices to insurance forms. Here's how the number is actually calculated, and how to read it sensibly.
The formula
In metric units, BMI is:
BMI = weight in kilograms ÷ (height in meters)²
So someone who weighs 68 kg and is 1.70 m tall has a BMI of 68 ÷ (1.70 × 1.70) = 23.5.
In imperial units, the formula includes a conversion constant:
BMI = (weight in pounds ÷ (height in inches)²) × 703
Both formulas describe the same relationship — weight relative to the square of height — just expressed in different units. If you're working from a mix of units, our unit converter can translate pounds to kilograms or inches to centimeters in one step.
Worked example
Take a person who is 5'7" (67 inches) and weighs 150 lb:
- Square the height: 67 × 67 = 4,489
- Divide weight by that: 150 ÷ 4,489 = 0.0334
- Multiply by 703: 0.0334 × 703 ≈ 23.5
That lands in the "healthy weight" category under standard adult BMI ranges.
The standard categories
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
Where BMI falls short
BMI was designed as a population-level screening tool, not an individual diagnosis. It doesn't know the difference between muscle and fat, so it can overstate the "risk" for muscular athletes and understate it for people with low muscle mass. It also doesn't account for where fat is distributed on the body, age, or sex differences in body composition.
If your BMI falls outside the "healthy" range, that's a reasonable prompt to talk with a healthcare provider — not a verdict on its own. Waist circumference, body composition scans, and a broader look at diet, activity, and family history all tell you more than one ratio can.
Try it yourself
Skip the manual math and plug your numbers into our BMI calculator — it supports both metric and imperial units and shows your category instantly. If you also want to track how your numbers change over time, our age calculator is handy for logging exact ages at each check-in.