Bioavailability Calculator
Absolute bioavailability (F) quantifies what fraction of an oral dose reaches systemic circulation compared to intravenous administration. The formula corrects for any difference in doses between the two arms of the comparison study: F = (AUC oral / Dose oral) / (AUC IV / Dose IV). If equal doses were used in both arms, F simplifies to AUC oral / AUC IV. Enter AUC values (same units, e.g., mg/L x hr) and doses for each route to calculate F as a percentage.
Bioavailability formula
F = (AUC oral x Dose IV) / (AUC IV x Dose oral)
When oral and IV doses are equal, this simplifies to F = AUC oral / AUC IV. F is expressed as a fraction (0 to 1) or a percentage (0% to 100%). The IV reference has F = 1 by definition.
Typical bioavailability values for common drugs
- Aspirin: approximately 50 to 68% (significant first-pass hydrolysis to salicylate).
- Morphine: approximately 20 to 40% (high hepatic first-pass effect).
- Warfarin: approximately 100% (near-complete oral absorption, minimal first-pass).
- Metformin: approximately 50 to 60% (incomplete absorption, no first-pass).
- Amoxicillin: approximately 80 to 90% (good oral absorption).
Bioavailability calculator: frequently asked questions
What is bioavailability?
Absolute bioavailability (F) is the fraction of an administered oral (or non-IV) dose that reaches the systemic circulation unchanged, compared to an IV dose. IV administration is the reference because F is 1 (100%) by definition. F ranges from 0 to 1.
What is AUC in pharmacokinetics?
AUC (Area Under the Curve) is the integral of the plasma concentration-time curve. It represents total drug exposure. AUC is typically measured in units of mg/L x hr (or mcg/mL x hr) using the trapezoidal rule from plasma concentration samples.
Why does absolute bioavailability matter?
Bioavailability determines the effective dose that reaches systemic circulation after oral administration. A drug with F = 0.20 (20%) requires 5 times the oral dose to match the systemic exposure of an IV dose. It is essential for oral-to-IV dose conversion.
What factors reduce bioavailability?
First-pass hepatic metabolism is a major factor for high-extraction drugs (e.g., morphine, lidocaine). Other factors include intestinal wall metabolism, incomplete GI absorption, drug efflux transporters (P-glycoprotein), and formulation characteristics.
Is this the same as relative bioavailability?
No. Absolute bioavailability compares a route to IV. Relative bioavailability compares two non-IV routes or formulations (e.g., tablet vs. oral solution), using the same formula but with a reference non-IV AUC instead of IV AUC.
Official sources
- U.S. Food and Drug Administration: FDA Bioavailability and Bioequivalence Studies Guidance.
- NIH National Library of Medicine: Pharmacokinetics (StatPearls).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.