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Pharmacokinetics Calculator

Maintenance Dose Calculator: PK Css × CL / F

Estimate maintenance dose rate from target steady-state concentration, clearance, and bioavailability — then convert to dose per interval. Built for PK study design, protocol dose tables, and therapeutic drug monitoring after loading or at steady state.

Quick Answer

A maintenance dose replaces drug eliminated over time to sustain target steady-state concentration (Css). The core PK equation is MD rate = Css × CL / F, where CL is clearance and F is bioavailability; dose per interval = rate × τ. Pharma teams use this in Phase 1–3 protocol dose tables, renal or hepatic adjustment planning, and therapeutic drug monitoring — often after a loading dose for drugs with long half-lives.

Core Formula

Maintenance dose rate = Css,target × CL / F

Css,target = target steady-state concentration  |  CL = clearance  |  F = bioavailability fraction
Dose per interval = maintenance dose rate × τ

Calculate Maintenance Dose

Estimate dose rate from target steady-state concentration, clearance, bioavailability, and dosing interval.

Pharmacokinetic inputs

mg/L and mcg/mL are numerically equivalent.

mL/min values are converted to L/h before dosing math.

Dosing schedule

Optional. Adds mg/kg/day when weight is entered.

Maintenance Rate

-

mg/hour

Daily Dose

-

mg/day

Dose per Interval

-

mg per interval

Weight-Normalized Daily Dose

-

mg/kg/day

How the Calculation Works

Maintenance dosing is designed to replace the amount of drug cleared from the body over time. At steady state, the average rate of drug administration equals the average rate of elimination, so clearance is the key pharmacokinetic driver of the maintenance dose.

Worked example

Target Css 15 mg/L, clearance 4 L/h, bioavailability 100%, interval 12 hours:
Rate = 15 × 4 / 1 = 60 mg/hour
Daily dose = 60 × 24 = 1,440 mg/day
Dose every 12 hours = 60 × 12 = 720 mg every 12 hours

Maintenance Dose vs Loading Dose

A loading dose is used when the target concentration needs to be reached quickly, and it is primarily based on volume of distribution. A maintenance dose is the ongoing regimen that sustains exposure after therapy begins, and it is primarily based on clearance. For drugs with long half-lives, skipping a loading dose may delay therapeutic concentrations even if the maintenance dose is correct.

Clinical Caveats

Steady state

Maintenance dosing targets average steady-state exposure. Peak/trough fluctuation still depends on half-life, dosing interval, formulation, absorption, and distribution.

Renal impairment

Lower renal clearance can increase accumulation. Use drug-specific renal dose guidance and confirm whether labels use creatinine clearance, eGFR, dialysis status, or measured clearance.

Hepatic impairment

Reduced hepatic metabolism, altered protein binding, and portal-systemic shunting can change exposure. Child-Pugh class may guide some labels, but recommendations are drug specific.

Therapeutic drug monitoring

For narrow-therapeutic-index drugs, measured concentrations and clinical response should refine the calculated regimen.

Pharma & clinical trial context

Maintenance dose planning is central to Phase 1–3 pharmacokinetic study design, steady-state exposure targets, and protocol pharmacy manuals. Sponsors specify target Css or AUC-derived exposure, population clearance from prior PK data or literature, route-specific bioavailability, dosing interval, and PK sampling windows after consistent dosing — typically once ≥4 half-lives have elapsed at the maintenance regimen.

This calculator integrates with the NovaPharmaNews PK hub: pair with the Loading Dose Calculator when rapid Css is required, derive clearance with the Clearance Calculator, estimate steady-state timing with the Half-Life Calculator, and quantify exposure from concentration–time data with the AUC Calculator.

Trial protocols should document maintenance rationale, maximum administered dose, renal or hepatic adjustment rules, and TDM sampling relative to dose time at steady state. For crossover designs, confirm washout before interpreting maintenance-dose PK. Narrow therapeutic index drugs require institution-specific caps and monitoring thresholds beyond generic PK math.

Evidence & sources

Frequently Asked Questions

A maintenance dose is the ongoing dose needed to replace drug eliminated over time and keep average steady-state concentration (Css) near a target. For linear pharmacokinetics, the maintenance dose rate is driven by target concentration, clearance, and bioavailability — not volume of distribution.
Maintenance dose rate (mg/hour) = Css × CL / F and represents the average amount of drug that must enter systemic circulation each hour at steady state. Dose per interval = maintenance dose rate × τ (dosing interval in hours), which converts the continuous rate into a practical bolus or intermittent dose — for example, 60 mg/hour × 12 hours = 720 mg every 12 hours. This calculator outputs both.
For a continuous IV infusion with 100% bioavailability (F = 1), the infusion rate in mg/hour equals Css × CL. This is the same maintenance dose rate equation without an interval conversion step. Intermittent IV bolus or oral regimens require dividing by F and multiplying by τ to obtain dose per interval.
A loading dose rapidly reaches target Css and is driven by volume of distribution (LD = Css × Vd / F). A maintenance dose sustains Css after therapy begins and is driven by clearance (rate = Css × CL / F). Many regimens use both: plan the load with the Loading Dose Calculator, then transition to maintenance dosing at a defined interval.
With consistent maintenance dosing, steady state is approached after approximately 4–5 half-lives when input equals elimination. The maintenance dose sets average Css; half-life sets how quickly Css is reached. Without a loading dose, drugs with long half-lives may remain subtherapeutic for days or weeks — use the Half-Life Calculator to estimate timing before deciding whether to load first.
At steady state, the dosing rate must match the elimination rate. Higher clearance removes drug faster, requiring a higher maintenance dose rate for the same Css. Clearance links to half-life via CL = ke × Vd; estimate CL from dose/AUC with the Clearance Calculator or AUC Calculator when direct clearance is unknown.
Renal or hepatic impairment can reduce clearance and increase accumulation at the same maintenance dose. Adjust per product label, creatinine clearance or eGFR, Child-Pugh class when relevant, dialysis status, and FDA renal/hepatic PK guidance. Never extrapolate population clearance without patient-specific organ function and drug-specific adjustment rules.
TDM is most valuable for narrow therapeutic index drugs, high clearance variability, defined concentration-response targets, serious toxicity risk, or altered PK from renal or hepatic impairment, critical illness, obesity, pregnancy, or drug interactions. Measured trough or peak concentrations should refine calculated maintenance doses — especially after loading or dose changes.
Use the Loading Dose Calculator for initial Css targeting, Clearance Calculator to derive CL from dose/AUC or ke × Vd, Half-Life Calculator for steady-state and washout timing, and AUC Calculator to quantify exposure from concentration–time data. Together they support protocol dose tables from first dose through steady-state PK sampling.
Protocols should specify target Css or exposure rationale, population clearance assumptions, dosing interval, maximum dose caps, and sparse or rich PK sampling at steady state (typically after ≥4 half-lives of consistent dosing). Document pre-dose troughs and post-dose peaks relative to τ, align bioanalytical methods with ICH M10, and cross-reference the protocol pharmacy manual for formulation-specific constraints.
Bioavailability (F) appears in the denominator: lower F increases the administered maintenance dose needed for the same Css. IV dosing typically uses F = 100%. Oral or other extravascular routes require route-specific F from label or PK study data — the same F correction applies to both loading and maintenance equations.
No. Maintenance dosing must respect maximum doses, formulation limits, infusion rates, organ impairment caps, and TDM thresholds defined in prescribing information and trial protocols. This tool supports PK planning and education; confirm all regimens against the current label, institution pharmacy manual, and monitoring plan before clinical use.

Related Calculators

Loading Dose Calculator Clearance Calculator Half-Life Calculator AUC Calculator Dosage Calculator

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