Time to Steady State Calculator
For drugs following first-order kinetics, the fraction of steady state achieved after n doses is 1 - (0.5)^n half-lives elapsed. In practice, 5 half-lives corresponds to approximately 97% of steady-state concentration. This calculator shows the time required to reach 90%, 95%, and 97% (5 half-lives) of steady state for any drug given its elimination half-life. Knowing the time to steady state helps clinicians decide when to draw trough levels for therapeutic drug monitoring and when plasma concentrations will be stable enough for reliable interpretation.
Steady state accumulation formula
% steady state = (1 - 0.5^(t / t1/2)) x 100
Time to 97% SS = 5 x t1/2
To solve for the time to reach a target percentage p of steady state: t = -t1/2 x log2(1 - p/100). At 90%: 3.32 x t1/2; at 95%: 4.32 x t1/2; at 97%: approximately 5 x t1/2.
Clinical significance of steady state timing
- Therapeutic drug monitoring levels drawn before steady state is reached will underestimate the eventual steady-state concentration.
- Adjusting doses before steady state has been reached can lead to over-correction and toxicity.
- Long half-life drugs (e.g., amiodarone t1/2 40 to 55 days, warfarin t1/2 20 to 60 hours) take substantially longer to reach steady state.
- Short half-life drugs (e.g., aspirin t1/2 15 to 20 min) reach steady state within hours of first dose.
- Eliminating half-lives also governs drug washout: 5 half-lives after stopping a drug, more than 97% is eliminated.
Time to steady state: frequently asked questions
What is steady state in pharmacokinetics?
Steady state is the condition where the rate of drug administration equals the rate of elimination, resulting in stable plasma concentrations that fluctuate between a consistent peak and trough with each dosing cycle. It is the pharmacokinetic equilibrium of a multiple-dose regimen.
Why does steady state take 5 half-lives?
With each half-life that passes, the cumulative drug level rises by half of the remaining gap to steady state. After 1 half-life: 50%; after 2: 75%; after 3: 87.5%; after 4: 93.75%; after 5: 96.9%. Five half-lives is the standard clinical approximation for reaching more than 97% of steady state.
Does dosing interval affect time to steady state?
No. Time to steady state depends only on the drug's half-life, not on the dosing interval or dose size. A larger dose or more frequent dosing raises the steady-state concentration level, but the time to reach it remains approximately 5 half-lives.
What happens if a loading dose is given?
A loading dose rapidly brings plasma levels close to the target steady-state concentration before multiple doses accumulate. It effectively bypasses the 5-half-life wait, achieving therapeutic levels within one or two doses.
Can this calculator be used for clinical decisions?
This tool is for educational and informational purposes only. Clinical decisions about when a drug has reached steady state must be made by qualified healthcare professionals using therapeutic drug monitoring and patient-specific data.
Official sources
- U.S. Food and Drug Administration: FDA Pharmacokinetics Guidance.
- NIH National Library of Medicine: Pharmacokinetics (StatPearls).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.