What the ratio means
The femur to tibia ratio is your thigh bone length divided by your shin bone length. Many people quote the inverse, the tibia to femur ratio (TFR): shin ÷ thigh. Anthropologists write TFR × 100 and call it the crural index. A femur to tibia ratio of 1.28 is a TFR of 0.78 and a crural index of 78.
What is normal?
A 2021 study measured 1,077 adult legs on standing full-length X-rays. The average femur was 50.0 cm and the average tibia 39.0 cm, a ratio of 1.28 (TFR 0.78). Two standard deviations either side span 1.20 to 1.36 (TFR 0.74 to 0.83), which covers about 95% of people (MDPI Anatomia).
Populations differ. A study of Urhobo adults in Nigeria found an average crural index near 85, that is, relatively longer shins (Ebeye et al.). Neither pattern is healthier.
How lengthening changes it
Starting from the average of 50 cm femur and 39 cm tibia:
| Plan | Femur / tibia | TFR | Height change |
|---|---|---|---|
| No change | 1.28 | 0.78 | 0 cm |
| Femur +5 cm | 1.41 | 0.71 | +5 cm |
| Tibia +5 cm | 1.14 | 0.88 | +5 cm |
| Both +5 cm | 1.25 | 0.80 | +10 cm |
Femur lengthening raises the ratio (lowers TFR) and keeps your knees where they are. Tibia lengthening lowers it (raises TFR) and raises your knees. Doing both in stages keeps the ratio close to where it started. See the effect on your own body in the limb lengthening simulator.
How to measure your femur to tibia ratio
Only an X-ray, ideally a standing full-length leg X-ray, measures bone lengths properly. Without one, a tape measure between bony points you can feel under the skin gives a rough estimate:
- Thigh: from the bony bump on the outside of the hip (greater trochanter) to the joint line on the outside of the knee.
- Shin: from the joint line on the inside of the knee to the tip of the inner ankle bone.
- Divide thigh by shin for the femur to tibia ratio, or shin by thigh for TFR.
These points sit inside the ends of the bones, so tape results will not match X-ray values exactly. The simulator measures joint-to-joint distances on a photo (hip joint to knee, knee to ankle), which usually gives a lower ratio than X-ray bone lengths, so compare photo ratios with photo ratios and X-ray ratios with X-ray values.