What is steady-state concentration?

Steady state is reached when the rate of drug input equals the rate of drug elimination over a dosing interval. The average plasma concentration no longer changes between doses.

How is clearance related to half-life?

CL = 0.693 × Vd / t½, where Vd is the volume of distribution. If you know half-life and Vd you can derive CL without measuring it directly.

How many doses does it take to reach steady state?

It takes approximately 3.32 half-lives to reach 90% of steady state and 6.64 half-lives to reach 99%, regardless of dose or dosing interval.

Is this tool suitable for patient dosing decisions?

No. This tool is for educational and reference purposes only. Clinical dosing requires patient-specific data, therapeutic drug monitoring, and physician oversight.

What does the bioavailability fraction mean?

Bioavailability (F) is the fraction of the administered dose that reaches systemic circulation. An oral drug with 70% bioavailability has F = 0.70. Intravenous drugs have F = 1.0.

What units does the calculator use?

Dose is entered in milligrams, clearance in L/hr, volume of distribution in liters, and time in hours. The resulting Css is in mg/L (equivalent to mcg/mL).

Steady-State Drug Concentration Calculator

Calculate the average steady-state plasma drug concentration (Css) using bioavailability, dose, clearance, and dosing interval.

Inputs

All fields required. Use consistent units (mg, L, hr).

Dose (mg)

Dosing Interval τ (hr)

Bioavailability F (0 – 1)

Clearance Input Mode

Clearance CL (L/hr)

Results

Enter values and click Calculate.

Formula Reference

C_ss = (F × Dose) ÷ (CL × τ)

CL = 0.693 × Vd ÷ t½

C_min = C_max × e^{(-k × τ)}

C_max = C_ss × (k × τ) ÷ (1 − e^{(-k × τ)})

For educational use only. Not for clinical decision-making.

Summary

This calculator estimates the average steady-state plasma concentration (Css) of a drug using the standard pharmacokinetic formula: Css = (F × Dose) / (CL × τ). You can supply clearance directly or derive it from volume of distribution and half-life. The tool also shows how long it takes to reach 90% and 99% of steady state. For educational pharmacology reference only — not for clinical decision-making.

How it works

Enter the dose (mg) and dosing interval (hours).
Set the bioavailability fraction F (0–1; use 1.0 for IV routes).
Enter clearance (L/hr) directly, or switch to half-life mode and provide half-life and volume of distribution so CL is derived automatically.
Click Calculate to see Css in mg/L along with minimum and maximum concentrations.
Review the time-to-steady-state estimates (3.32 × t½ for 90%, 6.64 × t½ for 99%).
Use the results for academic or instructional pharmacokinetics work only.

Use cases

Pharmacy students practicing pharmacokinetic calculations.
Educators demonstrating steady-state principles in lectures.
Researchers verifying manual PK computations.
Comparing how dosing intervals shift the steady-state level.
Understanding the impact of bioavailability on therapeutic exposure.
Estimating time to reach steady state for various drug half-lives.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu