Body mass index
What this test measures
BMI is weight in kilograms divided by height in metres squared. It is a rough population measure of body fatness that does not distinguish muscle from fat.
Why doctors request it
Used across the NHS as a quick screen; many doctors read it alongside waist measurement.
What commonly pushes it up
- Increased body fat
- A high muscle mass, which BMI misreads as excess weight
- Fluid retention
What commonly pushes it down
- Weight loss from any cause
- Low muscle mass
Everyday reasons for a value outside the range
- Muscular people score high without excess fat
- Thresholds are set lower for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean ancestry
- It is a calculation from two measurements and inherits their errors
About the sample
Calculated from measured weight and height.
Upload a photo or PDF and see every line against the range your lab printed, explained in plain English. First report free.
Read my report freeThis is an educational summary of results you uploaded. It is not medical advice, a diagnosis or a treatment plan, and it does not replace the doctor who requested the test. If you are worried, or a result is marked outside range, discuss it with your doctor. Written and reviewed by a UK consultant physician. Reference ranges vary between laboratories.