Calculator inputs
Example data table
| Scenario | Model | Dose (mg) | F (%) | Weight (kg) | Vd (L/kg) | Time (h) | Half-life (h) | Vmax (mg/L/h) | Km (mg/L) |
|---|---|---|---|---|---|---|---|---|---|
| Immediate-release tablet | First-order | 500 | 80 | 70 | 0.70 | 8 | 6 | — | — |
| Enzyme-limited elimination | Michaelis-Menten | 1000 | 90 | 80 | 0.80 | 4 | — | 2.2 | 5.5 |
| Low-dose screening run | First-order | 250 | 65 | 60 | 0.55 | 12 | 4.5 | — | — |
Formula used
1) Core pharmacokinetic setup
Total distribution volume: Vd,total = Body Weight × Vd(L/kg)
Absorbed dose: Doseabsorbed = Dose × Bioavailability Fraction
Initial concentration: C0 = Doseabsorbed / Vd,total
2) First-order metabolism
Elimination constant: k = ln(2) / t1/2
Concentration over time: C(t) = C0 × e-kt
Metabolism rate by concentration: Rate = k × C(t)
Metabolism rate by mass: Ratemass = k × C(t) × Vd,total
Clearance: CL = k × Vd,total
3) Michaelis-Menten metabolism
Instantaneous concentration decline: dC/dt = -Vmax × C / (Km + C)
Instantaneous metabolism rate: Rate = Vmax × C / (Km + C)
Apparent first-order constant: kapp = Vmax / (Km + C)
Apparent clearance: CLapp = kapp × Vd,total
4) Numerical simulation
The saturable model uses stepwise numerical integration across the selected simulation window. The area under the concentration curve is estimated by the trapezoidal rule.
How to use this calculator
- Choose a kinetic model. Use first-order for proportional elimination and Michaelis-Menten for enzyme saturation.
- Enter dose, bioavailability, body weight, and volume of distribution.
- Enter the time point where you want the concentration and metabolism rate reported.
- Provide half-life for first-order calculations, or Vmax and Km for saturable kinetics.
- Set the simulation horizon and number of points for a smoother graph.
- Press Calculate metabolism to show results above the form.
- Review the summary table, graph, and simulated profile.
- Use the CSV and PDF buttons to export the summary and profile data.
Frequently asked questions
1) What does this calculator estimate?
It estimates drug concentration decline, metabolism rate, amount metabolized, apparent clearance, and exposure over time from either proportional or saturable elimination assumptions.
2) When should I use first-order kinetics?
Use first-order mode when elimination is approximately proportional to concentration and the drug is not near enzyme saturation across the studied range.
3) When is Michaelis-Menten more appropriate?
Use it when metabolism becomes capacity limited, especially at higher concentrations where enzyme systems approach saturation and clearance no longer stays constant.
4) What units should I enter?
Dose is in milligrams, weight in kilograms, distribution volume in liters per kilogram, time in hours, concentration in milligrams per liter, and Vmax in milligrams per liter per hour.
5) Does the calculator account for absorption phases?
No. It starts from an estimated initial concentration based on absorbed dose and distribution volume. It is most suitable for simplified post-absorption evaluations.
6) Why does apparent half-life change in saturable kinetics?
Because the effective elimination constant depends on concentration. As concentration changes, the apparent speed of elimination changes too.
7) What does AUC mean here?
AUC is the area under the concentration-time curve. It summarizes total systemic exposure over the chosen simulation window.
8) Can I use this for clinical dosing decisions?
It is best for education, screening, formulation work, and exploratory analysis. Clinical decisions require validated pharmacokinetic models and professional review.