Bioavailability Calculator (Absolute & Relative)

How it works

F = (AUC test × dose reference) ÷ (AUC reference × dose test) · absolute: test = oral (or other route), reference = IV · relative: test and reference are two products by the same route

Bioavailability is the fraction of a dose that reaches the systemic circulation unchanged. Because AUC is proportional to the amount absorbed (for drugs with linear kinetics and the same clearance in both studies), comparing dose-normalised AUCs gives it directly. Absolute bioavailability compares an extravascular route with an IV dose, which is 100% by definition; relative bioavailability compares two formulations, such as a generic tablet and the innovator product. For bioequivalence, regulators (FDA, EMA) require the 90% confidence interval of the test/reference ratio of AUC and Cmax to lie within 80–125% — a single ratio inside that range is not enough on its own.

Worked example

An oral dose of 100 mg gives an AUC of 45 mg·h/L and an IV dose of 50 mg gives 60 mg·h/L. Dose-normalised, that is 0.45 versus 1.2 h/L, so F = (45 × 50) ÷ (60 × 100) = 0.375 — an absolute oral bioavailability of 37.50%.

Sources: FDA — Bioavailability Studies Submitted in NDAs or INDs, General Considerations (2022) · StatPearls — Drug Bioavailability

Frequently asked questions