FIB-4 Liver Fibrosis Index Calculator
The FIB-4 index is a non-invasive estimate of liver fibrosis built from four routine values: age, AST, ALT and platelet count. It helps triage who may have advanced fibrosis and need further evaluation, reducing the need for biopsy. The formula multiplies age by AST and divides by the platelet count times the square root of ALT. A low result suggests a low probability of advanced fibrosis and a high result suggests a high probability. Enter age in years, AST and ALT in U/L, and platelets in 10^9 per liter. This tool is educational, not a diagnosis.
FIB-4 formula
FIB-4 = (Age * AST) / (Platelets * sqrt(ALT))
Age in years; AST and ALT in U/L
Platelets in 10^9 per liter
Index is unitless
The square root of ALT in the denominator dampens the effect of large ALT values. Lower platelet counts, common in advanced fibrosis, raise the index.
FIB-4 interpretation
- Below 1.45: low probability of advanced fibrosis.
- 1.45 to 3.25: indeterminate, consider further testing.
- Above 3.25: high probability of advanced fibrosis.
- Cutoffs derived mainly in viral hepatitis populations.
- Older age can raise FIB-4 without advanced fibrosis.
FIB-4 index: frequently asked questions
How is the FIB-4 index calculated?
FIB-4 equals age in years times AST (U/L), divided by the platelet count (in 10^9 per liter) times the square root of ALT (U/L). It combines four routine values into a single non-invasive fibrosis estimate.
What do FIB-4 cutoffs mean?
In chronic liver disease a FIB-4 below 1.45 suggests a low probability of advanced fibrosis, while a value above 3.25 suggests a high probability. Values in between are indeterminate and may need further testing such as elastography.
What units do the inputs use?
Age is in years, AST and ALT in units per liter (U/L), and platelet count in 10^9 per liter (the same number as platelets per nL, for example 250 for 250,000 per microliter).
Are the cutoffs the same for everyone?
No. The 1.45 and 3.25 thresholds were validated mainly in hepatitis C and are widely applied, but performance differs by age and disease. Older adults can have higher FIB-4 without advanced fibrosis. Interpretation belongs with a clinician.
Is FIB-4 a diagnosis?
No. It is a non-invasive screening estimate, not a biopsy or a diagnosis. Abnormal values warrant clinical assessment. This calculator is for education only.
Official sources
- National Institute of Diabetes and Digestive and Kidney Diseases: Liver disease information.
- National Library of Medicine (NCBI): FIB-4 index validation literature.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.