To calculate BMI in pounds and inches, multiply 703 by your weight in pounds, then divide the result by your height in inches squared — written as BMI = 703 × lb ÷ in². That single number is your Body Mass Index, the same weight-for-height ratio used by clinicians and public-health bodies worldwide. The 703 constant scales the imperial units (pounds and inches) into the metric standard of kilograms per square metre so the two formulas give identical results. For a fast, error-free calculation, the BMI Calculator handles both metric and imperial inputs in your browser and labels your result with the matching WHO weight category as you type.
The imperial form of the formula is a unit-converted version of the original metric equation BMI = kg ÷ m², which was devised by the Belgian mathematician and astronomer Adolphe Quetelet in the 1830s. Because pounds and inches are not part of the metric system, a scaling factor of 703 is required to bring the imperial units into line with the metric reference. This makes the calculation equally accurate whether you weigh yourself in kilograms or step onto a bathroom scale that reads in pounds.
The Imperial BMI Formula in Plain English
The imperial BMI formula is short and has only three pieces: your weight in pounds, your height in inches, and the constant 703.
- Weight (lb): your body weight measured in pounds on a scale.
- Height (in): your height measured in inches. If you only know it in feet and inches, multiply the feet by 12 and add the remaining inches. A person who is 5 feet 8 inches tall is 68 inches (5 × 12 + 8 = 68).
- 703: the conversion factor that scales pounds-per-square-inch into kilograms-per-square-metre, so the result matches the metric formula.
Putting it together: BMI = (703 × weight in lb) ÷ (height in in)². The height has to be squared because BMI is a density measure — it relates your mass to the area of your body, and area scales with the square of any linear dimension.
For example, someone who weighs 154 lb and is 68 in tall (5′8″) has a BMI of 703 × 154 ÷ 68² = 108,262 ÷ 4,624 ≈ 23.4. That falls inside the WHO normal-weight range, which runs from 18.5 to 24.9.
How to Calculate BMI in Pounds and Inches
The fastest way to get a BMI value in imperial units is to let a calculator do the arithmetic for you. The BMI Calculator is built around three short steps that work entirely in your browser — nothing is uploaded.
- Switch the toggle to Imperial. At the top of the BMI Calculator, choose the Imperial option so the input boxes expect pounds and inches rather than kilograms and metres.
- Enter your weight and height. Type your weight in pounds into the lb field and your height in inches into the in field. If you only know your height in feet and inches, multiply the feet by 12 and add the inches first.
- Read your BMI and WHO category. The BMI value and your World Health Organization weight category appear immediately and update live as you edit either number or switch back to Metric.
Because the result is computed locally in your browser, you can try different values, switch units to cross-check, or repeat the calculation without leaving the page. This makes it useful when you want to compare weight scenarios, follow a child's growth trend, or simply re-check a number from a doctor's office.
WHO BMI Categories for Adults
The World Health Organization defines four adult weight categories based on BMI. These cut-offs are screening thresholds, not diagnoses — they tell you where a person falls in the broad population distribution, not whether a specific weight is healthy for that individual.
| BMI value (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 above | Obese |
The calculator applies the boundaries as less-than 18.5, less-than 25, less-than 30, and 30 or above, which keeps the four ranges continuous with no gaps or overlaps. Anything from 18.5 up to but not including 25 reads as normal weight, anything from 25 up to but not including 30 reads as overweight, and 30 or higher reads as obese.
These ranges are used by clinicians and public-health bodies as a quick, low-cost triage step — a starting point for a conversation, not a verdict on an individual's health.
Where the 703 Constant Comes From
The 703 in the imperial BMI formula is not arbitrary. It comes from converting pounds to kilograms and inches to metres inside the original formula BMI = kg ÷ m².
One pound equals roughly 0.45359237 kilograms, and one inch equals 0.0254 metres. Squaring the height conversion gives (0.0254)² = 0.00064516 m² per in². When you divide the pound-to-kilogram factor by the squared inch-to-metre factor — 0.45359237 ÷ 0.00064516 — you arrive at approximately 703.0696, which is rounded to 703 for everyday use. That single number lets the imperial formula produce exactly the same BMI value as the metric one for the same person.
This is why the BMI Calculator can offer a Metric/Imperial toggle and still produce a consistent result: the underlying ratio is identical, only the conversion factor changes.
What BMI Doesn't Tell You
BMI is a useful screening tool precisely because it is so simple, but that simplicity is also its biggest limitation. The formula uses only two numbers — weight and height — so it cannot distinguish muscle from fat, account for bone density, or read fat distribution.
A heavily muscled athlete and a sedentary person of the same height and weight receive the same BMI, even though their body composition and health risks are very different. The athlete may register as overweight or obese while carrying very little body fat, while the sedentary person may have a high body-fat percentage and still land in the normal range.
BMI also ignores age, sex, and ethnicity. These factors all change what a given BMI value means for a particular person — for instance, older adults naturally carry more body fat at the same BMI, and Asian populations often face elevated health risks at lower BMI thresholds than the standard WHO cut-offs suggest. For a more complete picture, BMI is best read alongside other indicators such as waist circumference, body-fat percentage, blood pressure, and bloodwork. The Body Fat Calculator is a natural complement when you want a number that reflects muscle-versus-fat balance.
Treat any BMI figure as a screening estimate rather than a diagnosis. For anything that affects your health decisions, speak to a qualified healthcare professional.