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Method 1: knee height
Knee height changes very little with age, unlike standing height, which is why it's used for older adults.1
How to measure: sit or lie with the knee and ankle both bent at right angles. Measure from the bottom of the heel to the top of the thigh just above the kneecap, keeping the measuring tool parallel with the shin.2
The Chumlea equations (height and knee height in cm):
- Men: height = 64.19 − (0.04 × age) + (2.02 × knee height)
- Women: height = 84.88 − (0.24 × age) + (1.83 × knee height)
They were developed for adults aged 60 to 90.1 For younger adults, the estimate is rougher.
Method 2: arm span
With arms stretched straight out to the sides at shoulder height, measure from the tip of one middle finger to the other. In most adults, arm span is close to height, within a few centimeters. It's less reliable for people with curvature of the spine or conditions affecting the arms.
Demi-span (from the middle of the breastbone notch to the tip of the middle finger on one side) can be used when only one arm can be stretched out. Roughly doubled, it approximates arm span; clinical tools use published conversion tables for a more accurate figure.2
Method 3: ulna length
The Malnutrition Universal Screening Tool (MUST) from BAPEN includes a table that converts the length of the forearm bone (ulna), from the point of the elbow to the bony bump on the wrist, into height by age and sex.2 It's widely used in UK hospitals. See the BAPEN source linked below for the table.
Then calculate BMI
Use the estimated height in the normal formula: weight (kg) ÷ height (m)². For the example above with a weight of 60 kg: 60 ÷ (1.591 × 1.591) = 23.7. The calculator above does both steps.
If someone has had an amputation, the weight also needs adjusting: BMI for amputees.
Frequently asked questions
Can I use a remembered height instead?
A recent, reliably measured height is fine. Older remembered heights are often too tall, especially for older adults who have lost height.
How accurate are these estimates?
Typically within a few centimeters. Even a 2 to 3 cm error changes BMI by less than one point for most adults.
Sources
This page is for education and is not medical advice. Height estimates have a margin of error; a dietitian or nurse can measure and interpret them.
