Pharmacokinetics
Bioavailability Calculator
Works out bioavailability (F) — the percentage of a dose that reaches the circulation — from AUC data, or estimates it from a typical value for the route.
Bioavailability (F)
50.0%
Good bioavailability – suitable for oral administration
Inputs
Labels the result and highlights the route in the comparison chart.
Test formulation
Dose of the formulation being tested (e.g. oral).
AUC measured after the test dose.
Reference (IV or standard)
Dose of the IV or standard formulation.
AUC measured after the reference dose.
Pharmacokinetic parameters (optional)
With Vd, gives the half-life.
t½ = 0.693 × Vd / CL.
Try an example drug (fills the AUC method)
Working
Route comparison
Typical bioavailability by route (%). The Oral bar shows your result when the route is oral.
Typical bioavailability
F (%) = (AUCₜ / Dₜ) ÷ (AUCᵣ / Dᵣ) × 100
Estimate: effective dose = Dₜ × typical F; F = effective dose ÷ Dᵣ × 100
t½ = 0.693 × Vd / CL
Dividing each AUC by its own dose puts both formulations on a per-milligram footing, so the doses do not need to be equal. With an IV reference the result is absolute bioavailability; with an oral reference it is relative bioavailability.
The dose estimate does not use your AUCs at all: it applies a textbook F for the route (oral 80%, IV 100%, IM 95%, SC 90%, transdermal 85%) and writes illustrative AUCs of half the effective dose, which you can copy into the AUC method.
Worked example: Absolute bioavailability with unequal doses
In a crossover study, volunteers receive Drug X as a 200 mg oral tablet and, on another occasion, as a 100 mg IV dose. The AUC₀–∞ is 36 mg·h/L after the tablet and 30 mg·h/L after the IV dose. Find the absolute oral bioavailability.
- Route
- Oral
- Test dose Dₜ (oral)
- 200 mg
- Test AUCₜ
- 36 mg·h/L
- Reference dose Dᵣ (IV)
- 100 mg
- Reference AUCᵣ
- 30 mg·h/L
- Method
- From AUC data
Formula
F = (AUCₜ ÷ Dₜ) ÷ (AUCᵣ ÷ Dᵣ) × 100
Exposure per mg, oral
36 mg·h/L ÷ 200 mg = 0.18 h/L per mg
Exposure per mg, IV
30 mg·h/L ÷ 100 mg = 0.30 h/L per mg
Ratio
F = 0.18 ÷ 0.30 × 100 = 60.0 %
F = 60.0 % (0.60 as a fraction) — absolute oral bioavailability.
Only 60 % of each oral milligram reaches the systemic circulation, through incomplete absorption, first-pass metabolism or both. Notice that the oral AUC is larger than the IV AUC here, yet F is below 100 % — because twice the dose was given. Always compare exposure per milligram. This is an educational calculation on illustrative study data, not a dose conversion for a patient.
Common mistakes
Assumptions and limitations
Keep learning
References
- Ducharme MP, Shargel L, editors. Shargel and Yu's Applied Biopharmaceutics & Pharmacokinetics. 8th ed. New York: McGraw Hill; 2022.
- Rowland and Tozer's Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. 5th ed. Philadelphia: Wolters Kluwer; 2019.
Common questions
Absolute bioavailability compares a dose with an IV dose, which by definition reaches the circulation completely (F = 100%). Relative bioavailability compares two non-IV formulations, for example a generic tablet with the brand tablet.
Incomplete dissolution or absorption in the gut, and first-pass metabolism in the gut wall and liver before the drug reaches the systemic circulation. Propranolol loses most of each dose this way.
For true absolute bioavailability, no. A result above 100% usually means clearance differed between the two studies, the AUCs were not extrapolated to infinity, or — in relative studies — the test formulation is genuinely better absorbed than the reference.
Oral dose = IV dose ÷ F (as a fraction). A drug with F = 0.5 needs twice the IV dose by mouth to give the same exposure.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy