How Is a FRAX Score Calculated? The Risk Factors Explained

What goes into a FRAX score?

A short, fixed list of facts about you — and nothing outside that list.

FRAX is not a questionnaire that builds a general picture of your health. It is a statistical model with defined inputs, each one chosen because large population studies showed it independently shifts fracture risk. Put the same inputs in, and you get the same number out.

The authoritative UK list comes from the National Osteoporosis Guideline Group, whose 2024 clinical guideline is what UK clinicians actually work from.

The FRAX risk factors in full

NOGG lists the variables entered into FRAX as: NOGG, 2024

Input	Detail as the guideline defines it
Age	—
Sex	—
Body mass index	"calculated from weight and height in kg/m2"
Previous fragility fracture	"including morphometric vertebral fracture"
Parental history of hip fracture	A parent, specifically, and a hip fracture specifically
Current glucocorticoid treatment	"any dose, by mouth for 3 months or more"
Current smoking	Current, not past
Alcohol intake	"3 or more units daily"
Rheumatoid arthritis	Named on its own, separate from other conditions
Secondary causes of osteoporosis	A defined list — see below
Femoral neck BMD	Optional

The secondary causes are not left to interpretation. NOGG specifies them as: type 1 diabetes, long-standing untreated hyperthyroidism, untreated hypogonadism or premature menopause, chronic malnutrition or malabsorption, chronic liver disease, and non-dialysis chronic renal failure (CKD 3a–5). NOGG, 2024

Why these factors and not others?

Because each one carries fracture risk that isn't already captured by bone density.

That is the design principle, and it explains the list's slightly odd shape. FRAX is not trying to describe your skeleton — a bone density scan does that. It is trying to capture risk that a density reading would miss. A parental hip fracture tells you something about your bone quality and architecture that your own T-score does not. Long-term steroid use weakens bone in ways that show up in fractures before they show up in density.

There is one wrinkle worth knowing about the secondary causes. NOGG notes that "The listed secondary causes are conservatively assumed to be mediated through low BMD." NOGG, 2024 In plain terms: the model assumes those conditions raise fracture risk by lowering bone density, so if you enter an actual density measurement, their separate contribution is treated as already accounted for. It is a deliberately cautious assumption rather than a claim about biology.

Does FRAX need a bone density scan?

No — and that is the point of its design.

NOGG is explicit that "Femoral neck BMD is an optional input." NOGG, 2024 FRAX was built to be usable in a ten-minute GP appointment with no equipment at all, anywhere in the world — which is why simplified paper versions exist, "based on the number of risk factors". FRAX / FRAXplus

But optional is not the same as irrelevant. FRAX is described by its own authors as integrating "the risks associated with clinical risk factors as well as bone mineral density (BMD) at the femoral neck" — the density value is part of the design. FRAX / FRAXplus And the Royal Osteoporosis Society notes that as part of an assessment, "If you haven't already, you may also be referred for a bone density scan." Royal Osteoporosis Society

The practical difference:

• Without a scan — FRAX estimates your risk from your circumstances.
• With a scan — FRAX estimates your risk from your circumstances and the measured state of your bone.

Which one you get changes how the result is then read. We cover that in FRAX calculator UK: how to use it and what it asks, and what the density number itself means in DEXA scan results and T-scores explained.

What does FRAX actually calculate?

Two 10-year probabilities. NOGG puts it as: "The FRAX tool computes the 10-year probability of hip fracture and/or of major osteoporotic fracture. A major osteoporotic fracture is a clinical spine, hip, forearm or humerus fracture." NOGG, 2024

(The humerus is the upper arm bone — the FRAX tool's own materials describe the same set as clinical spine, forearm, hip or shoulder. FRAX / FRAXplus)

Both come back as percentages over ten years. What those percentages then mean — and the point at which UK guidance says to act — is a separate question, covered in FRAX score interpretation.

Who should have a FRAX assessment?

The UK guideline gives a clear trigger: "A FRAX assessment should be performed in any postmenopausal woman, or man age ≥50 years, with a clinical risk factor for fragility fracture." NOGG, 2024

Note the shape of that sentence. It is not "anyone worried about their bones", nor "everyone over 50". It is a defined population plus at least one risk factor — and the risk factors are the ones listed above.

What FRAX doesn't ask about

This is the most useful thing to understand before you read your result.

FRAX does not ask about falls, physical activity, diet or calcium intake, vitamin D status, or how many fractures you've had rather than simply whether you've had one. None of those appear in the input list.

That is not an oversight — a statistical model includes only variables it was validated on. But it does mean a FRAX percentage is a floor built from specific evidence, not a complete picture of your risk. Someone who falls frequently carries risk that FRAX cannot see, because it never asked.

It is also why the tool sits alongside other things rather than replacing them: grip strength and fracture risk, falls prevention exercises, and an actual measurement of your bone.