What this calculator does
The Osteoporosis Self-Assessment Tool uses only two pieces of information: age and body weight. Lower weight and higher age both raise risk, and subtracting one from the other captures most of what a more elaborate score would.
It is deliberately crude. It is designed to identify who should be referred for a bone density scan, not to say anything about whether a person has osteoporosis, which only that scan can establish.
The formula
Subtract age in years from weight in kilograms and divide by 5. The resulting index is read against separate risk bands for women and men.
| Term | Meaning |
|---|---|
| OST index | Weight less age, divided by five. |
| Risk band | Low, moderate or high, with different cut-offs by sex. |
| DXA | Dual-energy X-ray absorptiometry, the diagnostic bone density scan this screens for. |
The inputs explained
| Field | What to enter |
|---|---|
| Weight (kg) | Body weight in kilograms. |
| Age (years) | Age in years. |
| Sex | Sex, which selects the risk band thresholds. They differ substantially. |
When to use it
Deciding who to refer for a scan
The tool exists to triage referrals rather than to assess an individual in detail.
Understanding why weight matters
Lower body weight is one of the stronger predictors of low bone density.
Population screening
Two inputs make it usable at scale where a fuller assessment is not practical.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does body weight change the band?
The same age at three body weights.
Questions
Does a low-risk score mean my bones are fine?
No. The tool is a crude screen using two variables and it misses people. It cannot account for fracture history, steroid use, family history, smoking or any of the other recognised risk factors.
Why are the thresholds different for men and women?
Because baseline risk differs substantially, so the same index value corresponds to different probabilities. The tool was originally developed and validated in postmenopausal women and extended to men afterwards.
How does this compare to FRAX?
FRAX uses many more inputs and estimates an actual ten-year fracture probability, which OST does not attempt. OST trades that detail for being computable from two numbers anyone already knows.
What should I do with the result?
Take it to a doctor rather than acting on it. A screening score is a prompt for a conversation about whether bone density testing is appropriate for you, and that decision involves far more than these two figures.
For weight-for-height, see the BMI calculator. For a reference weight from height, see the ideal weight calculator.