Maintenance Dose Calculator
The maintenance dose is the amount of drug given at each dosing interval to sustain a target steady-state plasma concentration (Css). The standard pharmacokinetic formula is: MD = CL x Css x tau / F, where CL is total body clearance (L/hr), Css is the desired average steady-state concentration (mg/L), tau is the dosing interval (hr), and F is bioavailability. At steady state, the rate in (dose/tau x F) equals the rate out (CL x Css), so the formula simply rearranges this balance. Renal or hepatic impairment reduces CL, requiring a lower maintenance dose.
Maintenance dose formula
MD = CL x Css x tau / F
CL = total body clearance (L/hr); Css = target average steady-state concentration (mg/L); tau = dosing interval (hr); F = bioavailability (fraction). Daily dose = MD x (24 / tau).
Factors affecting maintenance dose
- Renal clearance reduction (e.g., in CKD) directly reduces total CL and requires a lower MD or longer tau.
- Hepatic disease can reduce clearance of high-extraction-ratio drugs (e.g., lidocaine, morphine).
- Drug interactions can inhibit or induce CYP enzymes, altering CL and required MD.
- Age-related changes in renal function, hepatic mass, and body composition affect CL in elderly patients.
- Protein binding changes (e.g., in hypoalbuminemia) alter the free drug concentration and apparent Css.
Maintenance dose calculator: frequently asked questions
What is a maintenance dose?
A maintenance dose is the regular dose given at each dosing interval to maintain the target steady-state plasma concentration (Css). It replaces the drug eliminated during each interval, balancing input against output.
What is clearance (CL) in pharmacokinetics?
Clearance (CL) is the volume of plasma completely cleared of drug per unit time (units: L/hr). It is the primary determinant of steady-state concentration. Total body clearance includes renal and hepatic clearance components.
What is the dosing interval (tau)?
Tau is the time between doses. Common intervals are q4h (4 hours), q8h, q12h, q24h. A shorter interval maintains more stable plasma levels; a longer interval may produce larger peak-to-trough swings.
How does bioavailability affect maintenance dose?
Bioavailability (F) is the fraction of dose reaching systemic circulation. For oral drugs with F = 0.5 (50%), the maintenance dose must be doubled compared to the IV dose to achieve the same steady-state concentration.
Can this calculator be used for clinical dosing decisions?
This tool is for educational and informational purposes only. Clinical dosing requires patient-specific assessment by a licensed healthcare professional, with therapeutic drug monitoring and institutional protocols.
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.