How to calculate BMI: metric and pounds-and-inches examples

BMI uses only two measurements: body weight and standing height. Write each number with its unit before you touch the calculator. A scale reading of 75 kg and a height of 170 cm are enough for the arithmetic. Age and sex do not change the BMI formula itself. They matter later when you choose an interpretation, especially for a child or teen. The result is a ratio of weight to height squared, conventionally reported in kilograms per square metre (kg/m²). It is not a percentage of body fat.

Use a recent weight reading and a measured standing height when possible. Check that the scale shows kilograms or pounds and that the height record shows centimetres, metres, feet, or inches. A copied height can be wrong if it came from a different unit or an old record. The CDC emphasizes accurate height and weight measurements for a correct BMI calculation. The arithmetic can be exact for the entered numbers while the inputs remain uncertain. Save the raw values so you can find an entry error later.

Convert height from centimetres to metres first by dividing by 100. In the hypothetical example, 170 cm becomes 1.70 m. Square that height: 1.70 m × 1.70 m = 2.89 m². Then divide 75 kg by 2.89 m². The result is about 25.9516 kg/m². Rounding once, at the end, gives 26.0 kg/m² to one decimal place. The extra digits in the unrounded division do not mean that height, weight, or body composition was measured to that precision.

The table is a calculation worksheet, not a comparison of two measurements from one person. Its rows use different hypothetical inputs. The first row shows every step needed when height is entered in centimetres but the formula uses metres. If you prefer to keep centimetres, calculate 75 ÷ 170 ÷ 170 × 10,000. That produces the same result before rounding. The factor 10,000 corrects for the two conversions from centimetres to metres; leaving it out makes the answer far too small.

Read the table from left to right when checking your own work. Confirm the input units, perform the stated operation, keep the unrounded value long enough to check it, and then choose a display precision. Do not round 1.70 m to 2 m before squaring it. A BMI calculator can remove arithmetic effort, but it cannot determine whether a typed height is in the unit shown on the scale or record.

For U.S. customary units, convert feet to inches and add any remaining inches. Five feet seven inches is 5 × 12 + 7 = 67 inches. Square 67 to get 4,489 in². Divide 165 lb by 4,489 in², then multiply by 703. The result is about 25.8398, displayed as 25.8 kg/m² to one decimal place. The multiplier is part of the pounds-and-inches formula. It is not an activity factor, a body-fat adjustment, or a number to apply again to a metric result.

Keep the order visible: pounds ÷ (inches × inches) × 703. Writing the squared height in parentheses prevents a common mistake in which only part of the height is squared. If a scale shows pounds and ounces, convert ounces to a fraction of a pound first; CDC notes that 16 ounces equals one pound. For example, 8 ounces is 0.5 lb. When using a calculator with centimetre and kilogram fields, convert both inputs or select the matching unit mode. Entering 165 as kilograms would make the result a different calculation.

For adults aged 20 or older, CDC places BMI below 18.5 in the underweight category; 18.5 to below 25 in the healthy-weight category; 25 to below 30 in the overweight category; and 30 or above in the obesity category. The hypothetical 26.0 metric result falls in the 25-to-below-30 screening category. That label describes the BMI range only. It does not diagnose a condition, measure body fat, or decide what weight is appropriate for a person. A clinician can consider it with medical history and other findings.

For children and teens aged 2 through 19, the same weight-and-height calculation gives a BMI number, but an adult category is the wrong next step. CDC compares that number with a BMI-for-age reference using age and sex. BodyNumeral’s BMI calculator has separate adult and child or teen choices. The child path asks for completed years and additional months, then shows a CDC BMI-for-age percentile. The selected chart method stays with its output. A percentile is a comparison with a reference population, not a body-fat percentage or a direct statement of health.

BodyNumeral’s child reference starts at age 2 and ends before age 20. For children younger than 2, CDC recommends WHO growth standards and does not recommend BMI-for-age charts. That age group needs a different growth assessment and professional interpretation. A child close to the boundary of a completed month also needs an accurate age entry because the reference changes with age. Use the pediatric path only for the supported age range, and bring questions about growth patterns to a qualified health professional. Do not interpret a single child BMI value with the adult table.

BMI treats the same height and weight as the same number even when two people have different proportions of muscle, fat, bone, or body water. It cannot show where fat is carried. CDC notes that a person with substantial muscle can have a BMI in a higher adult category even though BMI cannot separate muscle from fat. The reverse problem also matters: the formula cannot reveal low muscle mass from the final number alone. A BMI of 26.0 is therefore 26.0 kg/m², not 26% body fat.

The arithmetic also depends on the two entered measurements. A height typed as 170 metres instead of 170 centimetres produces a tiny, invalid result. A height typed as 67 centimetres instead of 67 inches produces a very different result. Even with correct units, a rounded or estimated height changes the denominator after it is squared. Recheck the inputs before interpreting a surprising result. Do not use more decimal places as a substitute for a better measurement.

During pregnancy, CDC bases weight-gain guidance on BMI before pregnancy. A BMI calculated from current pregnancy weight is not a substitute for that assessment. The adult category table is a screening reference, not a personal treatment plan. If a result affects a medical or nutrition decision, ask a qualified health professional to interpret it with the relevant context. The guide’s worked examples only demonstrate the calculation; they do not suggest a target weight or a diet.

In BodyNumeral’s BMI calculator, choose the adult option for age 20 or older, or the child and teen option for ages 2 through 19. Select Metric for centimetres and kilograms, or US for inches and pounds. In US mode, enter total inches: 5 ft 7 in is 67 in. The calculator converts those inputs internally. The adult path reports BMI to one decimal place and shows a CDC screening category. The child path also needs its age and sex reference inputs. Check the unit mode before entering a number; the tool does not infer units from its size.

Save the date, original height and weight, units, chosen adult or child path, and the displayed BMI. For a child or teen, also keep the reference age, sex category, and selected chart method. When comparing later values, start with the raw inputs and the same method. A change in BMI can result from changed weight, changed recorded height, or both. The companion guide on comparing measurements explains how to keep method details visible. If the question is about body-fat percentage or waist size, use the corresponding guide instead of treating BMI as that measurement.