BMI Calculator · Practical guide
How to Calculate BMI with Pounds and Inches
One formula — 703 × lb ÷ in² — turns US measurements into a BMI with no unit conversion.
At a glance
The Imperial BMI Formula
To calculate BMI with pounds and inches, use BMI = 703 × weight (lb) ÷ height² (in²). The 703 factor converts pounds and inches into the same result the metric formula (kg ÷ m²) would produce, so no separate unit conversion is needed.
Interactive tool
BMI Calculator
Enter your own values, then compare the result with the examples below.
Step-by-Step Example: 154 lb at 5'9"
- Convert height to inches: 5 × 12 + 9 = 69 in
- Square it: 69 × 69 = 4,761
- Multiply weight by 703: 703 × 154 = 108,262
- Divide: 108,262 ÷ 4,761 ≈ BMI 22.7, normal weight
Second Example: 180 lb at 5'10"
5'10" is 70 inches, and 70 × 70 = 4,900. Then 703 × 180 ÷ 4,900 = 126,540 ÷ 4,900 ≈ 25.8, which lands just inside the overweight category that begins at 25.
Frequent Pitfalls
- Forgetting to convert feet: squaring 5.9 instead of 69 inflates the result wildly — always use total inches.
- Squaring the weight: only height is squared in the formula.
- Skipping 703: without it, the result is off by a factor of 703 and meaningless.
The calculator above accepts feet, inches, and pounds directly and shows your category, healthy range, BMI Prime, and Ponderal Index alongside the BMI itself.
Frequently Asked Questions
Why is the number 703 in the BMI formula?
It is the conversion factor between imperial and metric units (about 703.07 exactly). Multiplying by 703 makes pounds-and-inches produce the same BMI as kilograms-and-meters.
What is a normal BMI result?
A BMI between 18.5 and 24.9 is the normal range for adults. Below 18.5 is underweight, 25-29.9 is overweight, and 30 or above is obese.
Can I use this formula for children?
The math is the same, but children's results must be interpreted against CDC age- and sex-specific percentile charts rather than the fixed adult cutoffs.
How accurate is the imperial formula versus metric?
They are mathematically equivalent. Any tiny difference comes from rounding height or weight during measurement, not the formula.