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Steady-State Concentration Calculator

Enter the infusion rate and clearance to find the steady-state concentration.

How it is calculated

The tool applies the basic steady-state relationship for a continuous infusion: Css = R ÷ CL. R is the infusion rate in milligrams per hour and CL is the drug's clearance in litres per hour, so Css comes out in mg/L. At steady state, the drug going in each hour equals the drug the body clears each hour. The rate cannot be negative and the clearance must be above zero. The result is linear: doubling the rate doubles Css, and doubling the clearance halves it. It assumes the whole infused dose reaches the blood (no bioavailability factor), that clearance is constant and that the level has actually reached steady state. It does not give a time to steady state and is not medical advice.

Worked example

A drug is infused at 50 mg per hour and its clearance is 4.2 L per hour. Css = 50 ÷ 4.2 = 11.905 mg/L. If the infusion rate were doubled to 100 mg/h, Css would double to 23.810 mg/L; if clearance fell to 2.1 L/h with the original rate, Css would also double to 23.810 mg/L.

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Common mistakes

  • Mixing units: the tool expects mg per hour and litres per hour. A rate in mg per minute or mcg/kg/min, or clearance in mL/min, must be converted first.
  • Entering a loading dose as the rate. The formula describes the maintenance infusion only.
  • Entering a clearance per kilogram (L/h/kg) without multiplying by the patient's weight, which makes Css far too high.
  • Using the result for an oral drug without accounting for bioavailability, because the tool assumes all the dose reaches the blood.
  • Reading the value as the level right now. Css is only reached after several half-lives; before that the actual level is lower.

About this calculator

For a drug given as a continuous intravenous infusion, the plasma concentration rises and then levels off once the rate of drug entering the body equals the rate the body clears it — this plateau is the steady-state concentration (Css), and it depends only on the infusion rate and the drug’s clearance: Css = Infusion rate (R0) / Clearance (CL).

This relationship is basic pharmacokinetics, and it’s why Css is reached (practically) after about four to five drug half-lives regardless of the infusion rate — only the level it settles at changes with rate, not how long it takes to get there. Clinicians and pharmacists use this formula when dosing continuous infusions such as vasopressors, sedatives, or aminoglycoside antibiotics, to predict the concentration a given infusion rate will produce, or to work backward and pick an infusion rate that targets a desired concentration. This calculator computes Css from the infusion rate and clearance you enter — it is not medical advice and does not replace clinical dosing judgment.

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