Bedridden Patient Height Calculator (estimate height from knee height)

Understanding the bedridden patient height calculator — concept and principle

This calculator estimates the height of a person who cannot stand, such as someone bed-bound, in a wheelchair or with a curved spine, from knee height and age, using equations developed for Korean adults. It shows the estimated height, the typical error and a likely range. Nurses, dietitians and carers use it when height is needed for nutrition or medication calculations.

How it is calculated

Hwang et al. (2009) equations from the Size Korea 2004 survey:

• Men: height (cm) = 74.63 − 0.09 × age + 1.95 × knee height
• Women before menopause: 66.13 − 0.07 × age + 1.99 × knee height
• Women after menopause: 70.87 − 0.14 × age + 1.96 × knee height
• Typical errors (RMSE): 3.32, 2.90 and 2.88 cm; likely range = estimate ± 2 × RMSE

The principle

The long bones of the leg hardly shrink with age, while the spine can lose height through disc compression and curvature. Knee height therefore reflects a person's adult stature better than a standing measurement in older or frail people. Knee height is measured on the left leg with the person lying on their back, knee and ankle each bent to 90°, using a sliding calliper from under the heel to the front of the thigh just above the kneecap. The equations were fitted in Korean adults aged 20–69 and checked in older and mobility-impaired people.

Worked example

Defaults: man, 78 years, knee height 50 cm.

1. Height = 74.63 − 0.09 × 78 + 1.95 × 50 = 74.63 − 7.02 + 97.5 = 165.1 cm.
2. Typical error = 3.32 cm.
3. Likely range = 165.1 ± 6.64: from 158 to 172 cm.

How to read the result

About 95% of people fall within the likely range, so the estimate is good for calculations such as body mass index or energy needs, but not precise. Use the same method each time when monitoring. Other populations may need their own equations.

Common mistakes

• Measuring knee height with the knee not at 90°.
• Using a tape measure instead of a calliper.
• Applying the Korean equations to other populations without checking.
• Using the estimate as an exact height for clinical dosing without considering the error.

Frequently asked questions

Why use knee height to estimate height?

Leg bones change little with age, unlike the spine.

How accurate is the estimate?

Typical error is about 3 cm; about 95% of people are within ± 6–7 cm.

Which leg is measured?

The left leg, by convention, with knee and ankle at 90°.

Sources: Hwang IC, Kim KK, Kang HC, Kang DR, "Validity of stature-predicted equations using knee height for elderly and mobility impaired persons in Koreans", Epidemiology and Health 2009;31:e2009004