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Pharmacokinetics Calculator

Bioavailability Calculator: Absolute & Relative F from AUC

Estimate absolute bioavailability from oral and IV AUC and dose, or compare relative bioavailability between test and reference formulations. Built for formulation development, Phase 1 PK studies, and bioequivalence planning — then verify against protocol statistics and regulatory guidance.

Quick Answer

Bioavailability (F) is the fraction of an administered dose reaching systemic circulation unchanged. Absolute F compares oral exposure to IV using F = (AUCpo/AUCiv) × (Doseiv/Dosepo) × 100; relative F compares test vs reference non-IV products with the same dose-normalization logic. Pharma teams use F in formulation development, Phase 1 PK studies, and bioequivalence planning — but regulatory BE conclusions require 90% confidence intervals on AUC and Cmax ratios (typically 80–125%), not a single point estimate from this calculator.

Absolute Bioavailability
F = (AUCpo / AUCiv) × (Doseiv / Dosepo) × 100
AUCpo = oral exposure; AUCiv = IV exposure. Dose units must match.
Relative Bioavailability
Frel = (AUCT / AUCR) × (DoseR / DoseT) × 100
T = test product or formulation; R = reference product or formulation.

Calculate Bioavailability

Estimate absolute F from oral and IV AUC and dose, or compare relative bioavailability between formulations.

Calculation mode
Absolute mode estimates the fraction of an oral dose reaching systemic circulation using IV exposure as the 100% reference.
AUC and dose values

Use the same AUC unit for both values, such as h·ng/mL or h·mg/L.

Use the same dose unit for both values, such as mg.

Absolute bioavailability

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Enter values to calculate F
Decimal F
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fraction of reference exposure
AUC ratio
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before dose adjustment
Dose adjustment factor
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Doseiv / Dosepo

Interpret this as a point estimate. Regulatory bioequivalence conclusions require validated study data and statistical analysis.

How to Use This Calculator

1
Select absolute mode for oral versus IV exposure, or relative mode for test versus reference formulations.
2
Enter both AUC values using the same exposure metric and unit. Do not mix AUC0-t, AUC0-inf, and partial AUC unless the protocol specifically supports it.
3
Enter both doses using the same dose unit. The calculator applies the dose-normalization factor automatically.
4
Review percent F and decimal F with formulation, food effect, assay, sampling, and study design context.

Absolute vs Relative Bioavailability

Absolute bioavailability compares extravascular exposure, often oral exposure, against an IV reference. It estimates how much drug reaches systemic circulation after absorption and first-pass processes. A 40% result means the dose-normalized oral exposure is 0.40 of the IV reference exposure.

Relative bioavailability compares two non-IV products or conditions, such as a new tablet formulation against a reference capsule, a fed state against a fasted state, or a reformulated product against a legacy product. It supports formulation and drug development decisions but is not by itself a full bioequivalence assessment.

Oral Exposure and First-Pass Metabolism

Oral exposure reflects drug dissolution, intestinal permeability, transporter activity, gut metabolism, hepatic extraction, and systemic clearance. Low absolute F can point to poor absorption, extensive first-pass metabolism, formulation limits, or instability before the drug reaches plasma.

Worked example

Oral AUC = 42.5 h·ng/mL, IV AUC = 65.0 h·ng/mL, oral dose = 100 mg, IV dose = 50 mg. F = (42.5 / 65.0) × (50 / 100) × 100 = 32.7%, or decimal F = 0.327.

Bioequivalence Caveats

Bioequivalence evaluates rate and extent of exposure using protocol-defined pharmacokinetic endpoints — typically AUC and Cmax — after log transformation and 90% confidence interval analysis. Under FDA and EMA guidance, the geometric mean ratio for test vs reference must fall within 80% to 125% for both AUC and Cmax. This calculator outputs a single point estimate only; it does not evaluate Cmax, Tmax, intra-subject variability, sequence or period effects, washout, fed or fasted conditions, or subject-level statistics.

Use the output as an educational and development-stage estimate. For regulatory submissions, apply the current FDA, EMA, or relevant agency guidance for the dosage form, route, analyte, and study design.

Pharma & clinical trial context

Bioavailability estimation is central to formulation development, biopharmaceutics classification, and clinical pharmacology study design. Sponsors use absolute F from oral-vs-IV crossover studies to quantify first-pass loss and absorption barriers during lead optimization. Relative F supports prototype comparison, food-effect assessment, and strength proportionality before committing to a pivotal bioequivalence trial.

Bioequivalence studies for generic and reformulated products follow FDA and EMA guidance: replicate crossover designs, fasted or fed conditions as specified, validated bioanalytical methods, and 90% confidence intervals on AUC and Cmax geometric mean ratios within 80–125%. This calculator supports early point estimates and educational review — not submission-ready BE statistics.

Discovery and lead optimization

Absolute F helps teams understand whether low exposure is driven by absorption barriers, metabolic extraction, or clearance before advancing a compound.

Formulation and BE studies

Relative F compares prototypes, excipient strategies, particle-size changes, modified-release designs, and food-effect conditions ahead of pivotal BE trials.

Clinical pharmacology

Dose-normalized AUC comparisons connect exposure to route selection, therapeutic window, and drug-drug interaction interpretation in Phase 1 protocols.

This calculator integrates with the NovaPharmaNews PK hub: derive exposure with the AUC Calculator, relate elimination with the Clearance Calculator, plan loading with the Loading Dose Calculator, sustain exposure with the Maintenance Dose Calculator, and estimate washout timing with the Half-Life Calculator.

Evidence & sources

Frequently Asked Questions

Absolute bioavailability is the fraction of an administered dose that reaches systemic circulation compared with an intravenous reference dose. Because IV dosing is treated as 100% systemically available, absolute F is calculated from dose-normalized AUC values: F = (AUCpo / AUCiv) × (Doseiv / Dosepo) × 100. It quantifies how much oral (or other extravascular) exposure reaches the systemic circulation after absorption and first-pass processes.
Relative bioavailability compares exposure between two non-IV products, formulations, strengths, or dosing conditions — for example a new tablet vs a reference capsule, or fed vs fasted state. It uses Frel = (AUCT / AUCR) × (DoseR / DoseT) × 100. Absolute F always requires an IV reference arm; relative F compares two extravascular arms without IV data. Relative F supports formulation development but is not by itself a regulatory bioequivalence conclusion.
For absolute F: F = (AUCpo / AUCiv) × (Doseiv / Dosepo) × 100. For relative F: Frel = (AUCT / AUCR) × (DoseR / DoseT) × 100. The dose ratio corrects for different administered doses so exposure is compared on a per-milligram basis. AUC values must share the same unit (e.g. h·ng/mL) and doses must share the same unit (e.g. mg). Express the result as a percentage or decimal fraction.
Oral AUC = 42.5 h·ng/mL, IV AUC = 65.0 h·ng/mL, oral dose = 100 mg, IV dose = 50 mg. F = (42.5 / 65.0) × (50 / 100) × 100 = 32.7%, or decimal F = 0.327. This means dose-normalized oral exposure is about one-third of the IV reference — consistent with absorption limits, first-pass metabolism, or both.
Under FDA and EMA bioequivalence guidance, the 90% confidence intervals for the geometric mean ratios of AUC and Cmax for test vs reference products must fall within 80% to 125% (equivalently, 0.80–1.25 on a ratio scale). This applies to replicate crossover PK studies with log-transformed data and predefined statistical methods — not to a single point estimate from two mean AUC values. Narrower limits may apply for narrow therapeutic index drugs.
Use the same AUC metric for both arms in the comparison. AUC0-inf includes extrapolated terminal exposure and requires sufficient terminal sampling to justify the extrapolation (typically ≥3 points in the log-linear phase with acceptable %AUC extrapolated). AUC0-t uses observed data only to the last measurable time point. Mixing AUC0-t with AUC0-inf between arms invalidates the comparison. Follow the protocol-specified endpoint — most BE studies use AUC0-inf and AUC0-t together.
First-pass metabolism reduces oral F when drug is metabolized in the gut wall or liver before reaching systemic circulation. High hepatic extraction (e.g. propranolol, morphine) can yield low absolute F despite good absorption. First-pass loss is bypassed with IV dosing, which is why absolute F compares oral exposure against IV. Transporter effects, gut metabolism, and enterohepatic recirculation also modulate oral exposure beyond dissolution and permeability alone.
Oral bioavailability can be reduced by incomplete absorption, dissolution limits, gut-wall metabolism, transporter effects, chemical instability in the GI tract, food effects, and hepatic first-pass metabolism before the drug reaches systemic circulation. IV dosing bypasses these barriers, so absolute F captures the combined impact of absorption and pre-systemic elimination on systemic exposure.
No. Bioavailability describes the rate and extent of systemic exposure, usually reflected by AUC and Cmax. Bioequivalence is a regulatory conclusion based on predefined statistical comparisons — typically 90% confidence intervals for test/reference ratios of AUC and Cmax under a crossover study design with validated bioanalytical methods. A relative F point estimate near 100% does not prove bioequivalence without subject-level statistics.
Loading and maintenance dose equations divide by F for extravascular routes: LD = (Css × Vd) / F and maintenance dose rate = Css × CL / F. Lower bioavailability increases the administered dose needed for the same target exposure. Use the Loading Dose Calculator and Maintenance Dose Calculator with route-specific F from label or PK study data. Clearance and AUC tools help connect exposure back to elimination and study endpoints.
Both AUC values must use identical exposure units (e.g. h·ng/mL, h·mg/L, or µg·h/mL) and the same AUC definition (AUC0-t or AUC0-inf). Both doses must use identical mass units (e.g. mg). Do not mix routes, analytes (parent vs metabolite), or salt forms unless the protocol explicitly defines the comparison. Use the AUC Calculator to derive exposure from concentration–time data with consistent trapezoidal rules.
No. This calculator gives a point estimate from mean or single-pair AUC and dose values only. Bioequivalence evaluation requires an appropriate clinical pharmacokinetic study, replicate crossover design, validated LC-MS/MS assays per ICH M10, log-transformed subject-level data, 90% confidence intervals on AUC and Cmax ratios, and applicable FDA or EMA guidance for the dosage form and study conditions. It cannot assess Cmax, Tmax, variability, food effects, or sequence/period effects.

Related Calculators

AUC AUC Calculator LD Loading Dose Calculator MD Maintenance Dose Calculator CL Clearance Calculator t½ Half-Life Calculator

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