How to Calculate BMI Manually (and What the Number Actually Means)
Body Mass Index (BMI) is a simple ratio of weight to height, used as a population-level screening tool -- not a diagnosis. Here's exactly how it's calculated, in both unit systems, and what the result actually tells you.
Key Takeaway
The real BMI formula in both metric and imperial units, plus an honest look at what the number does and doesn't tell you.
The Formula
Metric: BMI = Weight (kg) ÷ Height (m)²
Imperial: BMI = Weight (lb) × 703 ÷ Height (in)²
Worked Example (Imperial)
- 1
Note your weight and height
170 lb, 5'8" (68 inches total).
- 2
Square the height in inches
68 × 68 = 4,624.
- 3
Multiply weight by 703
170 × 703 = 119,510.
- 4
Divide by the squared height
119,510 ÷ 4,624 = 25.8 BMI.
The Standard Category Ranges
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
What BMI doesn't measure
BMI can't tell the difference between muscle and fat -- a muscular athlete can land in the "overweight" range on BMI alone while having very low body fat. It also doesn't account for bone density, body frame, age, or where fat is distributed on the body. It's a quick population screening number, not an individual diagnosis -- treat an unusual result as a reason to ask a doctor, not a verdict on its own.
Frequently Asked Questions
Is BMI accurate for children?
No -- children and teens need age- and sex-specific BMI percentile charts, not the flat adult category ranges above, since healthy body composition changes significantly throughout growth. Pediatric BMI should always be assessed by a doctor using the correct growth charts.
Why do some athletes have a "high" BMI despite being lean?
Because BMI only uses weight and height -- it has no way to separate muscle mass from fat mass. A bodybuilder or strength athlete can have very low body fat but a BMI in the "overweight" range simply because muscle is denser than fat for the same volume.
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