BMI in pounds is calculated by multiplying 703 by your weight in pounds (lb) and dividing the result by your height in inches squared (in²): BMI = 703 × lb / in². The 703 factor is a unit-conversion constant that turns a pound-per-square-inch ratio into the standard kilogram-per-square-metre value used by the World Health Organization to classify adults as underweight, normal, overweight, or obese. It is the same Quetelet index — weight divided by height squared — that Belgian mathematician and astronomer Adolphe Quetelet devised in the 1830s to describe the physical characteristics of human populations, simply rescaled so anyone with a bathroom scale and a tape measure can work with their own units. The result is a single number with no unit attached, and that number can be compared directly against the WHO adult cut-offs: below 18.5 is underweight, 18.5–24.9 is normal weight, 25.0–29.9 is overweight, and 30.0 or higher is obese. Because the calculation is so quick once you know the formula, a free BMI Calculator is the easiest way to handle the arithmetic and read your WHO category at the same time.
What the Imperial BMI Formula Means
The metric definition of BMI is weight in kilograms divided by height in metres squared, written as kg/m². In the United States and a handful of other countries, weight is measured in pounds and height in feet and inches, which means a raw lb/in² calculation lands in the wrong units and the resulting number comes out roughly 703 times too small to compare against the WHO bands. Multiplying by 703 rescales lb/in² back into kg/m² so a BMI computed in pounds lands on exactly the same scale as one computed in kilograms. The constant comes from converting pounds to kilograms (1 lb ≈ 0.4536 kg) and inches to metres (1 in = 0.0254 m), then squaring the height conversion: 0.4536 ÷ (0.0254)² ≈ 703. You do not need to memorise the derivation — only remember that 703 is the bridge that makes the imperial formula produce the same BMI value as the metric one for the same person. To check the agreement, take a person who weighs 154 lb and stands 67 in tall. The metric formula gives about 24.1 kg/m², and the imperial formula with the 703 multiplier gives the same 24.1. The two formulas are mathematically identical; only the inputs and the conversion factor differ.
How to Calculate BMI in Pounds Step by Step
Follow these three steps to work out your BMI by hand using pounds and inches.
- Measure your weight in pounds. Use a bathroom scale in the morning, after using the bathroom and before eating, for the most consistent reading. Round to the nearest pound and write the number down.
- Measure your height in inches. Stand against a wall without shoes, mark the top of your head, then measure from the floor to the mark. If your height is in feet and inches, convert it to a single number in inches by multiplying the feet by 12 and adding the remaining inches — for example, 5 ft 8 in = 5 × 12 + 8 = 68 in.
- Plug the numbers into the formula. Square your height in inches, multiply your weight in pounds by 703, then divide the second result by the first. The arithmetic is BMI = 703 × weight (lb) ÷ height (in)². Read the final number against the WHO adult categories to see where you fall.
Worked Example: A Person Who Weighs 170 lb at 5 ft 8 in
To make the formula concrete, take an adult who weighs 170 lb and stands 5 ft 8 in tall. Step 1 — convert the height to a single unit. 5 ft × 12 in/ft = 60 in, plus 8 in gives 68 in total. Step 2 — square the height. 68 × 68 = 4,624 in². Step 3 — multiply the weight by the conversion factor. 703 × 170 = 119,510. Step 4 — divide by the squared height. 119,510 ÷ 4,624 = 25.85, which rounds to a BMI of about 25.8. Compare that to the WHO bands and 25.8 falls inside the 25.0–29.9 window, so the category is overweight. The same person measured in metric (about 77.1 kg at 1.727 m) gives 77.1 ÷ (1.727 × 1.727) = 77.1 ÷ 2.983 ≈ 25.8 kg/m² — the identical number, confirming that the imperial and metric formulas really do agree when the 703 conversion factor is applied correctly. For an interactive version of this calculation, the BMI Calculator with the Imperial toggle selected produces the same value the moment you finish typing.
WHO Weight Categories at a Glance
Once you have a BMI number, the next step is reading it against the World Health Organization's adult weight categories. These bands are the same whether your BMI was computed in pounds or kilograms, because the 703 factor has already done the unit conversion for you. The cut-offs are applied as "below the lower number", "below 25", "below 30", and "30 or higher", so the ranges are continuous and every BMI value falls into exactly one band.
| BMI (kg/m²) | WHO category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight (healthy range) |
| 25.0 to 29.9 | Overweight |
| 30.0 or higher | Obese |
These bands were designed as a quick, low-cost population-level screening tool, not as a personal verdict. A BMI of 27 in a sedentary office worker and a BMI of 27 in a muscular rugby player carry very different health implications, even though the number is identical.
Metric vs Imperial: Same Result, Different Math
Both formulas arrive at the same BMI on the same scale; only the inputs and the arithmetic differ. The metric version is the simpler one to write down because it does not need a conversion constant — you divide weight by height squared and you are done. The imperial version has to carry the 703 multiplier to convert the lb/in² ratio into kg/m². For most people who already know their height in feet and inches and their weight from a bathroom scale, the imperial version is the more practical one to do by hand. For everyone else, an online tool removes the conversion step entirely and handles the squaring, the multiplication, and the WHO classification in a single keystroke.
| Feature | Metric formula | Imperial formula (pounds) |
|---|---|---|
| BMI formula | weight (kg) ÷ height (m)² | 703 × weight (lb) ÷ height (in)² |
| Weight unit | Kilograms | Pounds |
| Height unit | Metres (or cm ÷ 100) | Inches (feet × 12 + in) |
| Conversion constant | None | 703 (lb/in² → kg/m²) |
| Output scale | kg/m² | kg/m² (same scale) |
| Best for | Most of the world, scientific use | United States, UK informal, any imperial scale |
What BMI Doesn't Tell You
BMI is convenient because it uses only two numbers most adults already know, but that simplicity is also its biggest limitation. The formula cannot tell muscle from fat, so a very fit person with low body fat and a sedentary person with the same height and weight receive the same BMI, even though their health profiles are very different. The index also ignores where fat is stored on the body — abdominal fat is a stronger risk factor for cardiovascular disease and type 2 diabetes than fat carried on the hips or thighs — and it does not adjust for bone density, age, sex, pregnancy, or ethnicity, all of which shift what a given number means for a given individual. For these reasons, public-health bodies and clinicians treat BMI as a screening signal — the first quick look that flags whether a fuller assessment is worth doing — rather than a diagnosis on its own. If your BMI lands in a band that surprises you, especially if you are very muscular, very young, very old, pregnant, or from an ethnic background where lower cut-offs apply, the right next step is a conversation with a qualified healthcare professional, not another round of self-measurement.
Use the BMI Calculator for Pounds and Inches
Doing the arithmetic by hand is a useful exercise, but for everyday use the fastest way to get an accurate BMI in pounds is the BMI Calculator with the Imperial toggle selected. Choose Imperial, type your weight in pounds and your height in inches, and the BMI value plus your WHO category appear the moment you finish typing — no need to remember the 703 constant or convert feet to inches first. Everything is computed in your browser, so nothing is uploaded, and switching back to Metric gives the same number for the same person expressed in kilograms and metres. For background on how the Quetelet index was developed, the Wikipedia entry on body mass index covers the historical derivation, and the World Health Organization's fact sheet on obesity and overweight explains the category thresholds in detail. Treat the result as an educational estimate, not a medical verdict, and speak to a qualified healthcare professional for anything that affects your health decisions.