Skip to content A BMI calculator can do the arithmetic correctly and still leave important questions unanswered. Start with consistent height and weight measurements, then treat the result as one screening measure—not a full picture of health.
How BMI is calculated
For adults, BMI is weight relative to height. In metric units, divide weight in kilograms by height in metres squared: BMI = kg ÷ m². In US customary units, the equivalent formula is weight in pounds divided by height in inches squared, multiplied by 703. A calculator performs this arithmetic for you, so a manual calculation and a digital tool should be close when the same values and units are used.
Small differences can come from rounding, converting feet and inches, entering centimetres as metres, or recording a slightly different weight. Check units before interpreting a surprising result. For a general overview, see the CDC’s BMI formula and limitations and our tools and calculators.
How to improve measurement consistency
- Use a scale on a firm, level surface and follow its instructions.
- Measure height without shoes, standing upright, with the measuring method appropriate to your equipment.
- Enter the same unit shown by the calculator; do not mix pounds with kilograms or inches with centimetres.
- If tracking over time, use a consistent routine rather than comparing measurements taken under very different conditions.
These steps reduce avoidable input errors. They do not account for every factor that affects health, and day-to-day scale readings can vary. If a number seems inconsistent or you have a health concern, confirm measurements and interpretation with a health professional.
What an accurate BMI cannot tell you
The CDC describes BMI as a screening measure that should be considered alongside other information. BMI does not distinguish fat, muscle, or bone, and it does not show where body fat is carried. A calculator also does not evaluate medical history, blood pressure, laboratory findings, medications, fitness, or other individual circumstances.
Standard adult categories are intended for adults, not children and teens, whose BMI is interpreted using age- and sex-specific growth references. BMI can also be less informative for people with unusually high muscle mass or other body-composition differences. A calculated value should start a conversation, not replace one.
Short video: three measurement checks
Frequently asked questions
Can a BMI calculator be mathematically accurate but still not describe my health?
Yes. The formula can be calculated correctly while still missing important context. BMI is one screening measure, not a diagnosis or a direct body-composition test.
Why do two calculators show slightly different values?
They may round differently or handle age, height, and unit conversions differently. Check the measurements and units, then focus on appropriate interpretation rather than tiny differences.
Can I use adult BMI for a child?
No. Children and teens need age- and sex-specific BMI-for-age references. Ask a pediatric health professional to interpret individual results.
Sources
For general education only; not personalized medical advice. Consult a qualified health professional about individual concerns.
