How to calculate BMI with stones and pounds

The stones-and-pounds method

Start by converting all weight to pounds. Multiply the stone value by 14, then add the leftover pounds. Next, convert height to inches by multiplying feet by 12 and adding the remaining inches. Use BMI = pounds ÷ inches² × 703. ^[1]

The NHS adult BMI calculator also accepts stones and pounds as an input option. That is a convenience for data entry; it does not create a different definition of BMI. ^[2]

Worked example: 11 st 6 lb at 5 ft 8 in

First convert the weight: (11 × 14) + 6 = 160 lb. Then convert the height: (5 × 12) + 8 = 68 in. Square the height: 68 × 68 = 4,624.

Now calculate 160 ÷ 4,624 × 703. The result is approximately 24.3. Under the standard adult scale, that sits in the healthy-weight category of 18.5 to less than 25. ^[3]

Why 11 st 6 lb is not 11.6 stone

Stone and pounds use a mixed-unit notation, like feet and inches. Six pounds is 6/14 of a stone, or about 0.429 stone. So 11 st 6 lb is about 11.429 stone—not 11.6. Treating the notation as a decimal overstates the weight and changes the BMI.

The same trap appears with height. Five feet nine inches is 69 inches, not 5.9 feet. Keeping each mixed unit in its own field prevents both errors.

Do stones, kilograms, and pounds produce the same BMI?

Yes, apart from small rounding differences. BMI is a unit-independent ratio when the conversions are done correctly. A calculator may display a tenth-point difference if the starting measurements were rounded during conversion.

Massy accepts and displays the unit system you choose, then converts the measurements locally for its calculation and history. You can use stones, US units, or metric without manually translating mixed units.

Interpret the result as a screening reference

The formula is exact, but its meaning is limited. BMI does not directly measure body fat and cannot distinguish fat, muscle, and bone. Use the number and category as a general adult screening reference, not as a diagnosis or personal prescription. ^{[4] [1]}