# Maintenance Dose Calculator (MD = Css · CL · τ / F)

Calculate the maintenance dose that holds the average steady-state concentration at a target value: MD = Css,avg · CL · τ / F, where Css,avg is the target average concentration, CL is clearance, τ is the dosing interval, and F is bioavailability (1 for IV, <1 for oral). Derived from steady-state mass balance (rate in = rate out): the dose per interval equals the amount cleared over one interval, corrected for bioavailability. Targets the AVERAGE Css,avg — the peak Cmax,ss and trough Cmin,ss fluctuate around it depending on t½ and τ. Valid only for linear pharmacokinetics with fixed dose and interval — NOT for Michaelis-Menten drugs (phenytoin). Optional body-weight input converts the result to mg/kg. Not medical advice.

> Canonical page: https://elysiatools.com/en/tools/maintenance-dose-calculator

- **Category:** Health

- **Keywords:** maintenance dose, MD, Css, clearance, dosing interval, bioavailability, F, steady state, mass balance, pharmacokinetics, pk, drug, medicine

## Overview

The Maintenance Dose Calculator estimates the dose per interval needed to maintain a target average steady-state concentration using MD = Css,avg × CL × τ / F. Enter the target concentration, clearance, dosing interval, and bioavailability; optionally add body weight for an mg/kg result. This calculator is for linear pharmacokinetics with a fixed dose and interval and is not medical advice.

## Inputs

- **Target Css,avg (mg/L)** (number): Target average steady-state concentration. Must be positive.
- **Clearance CL (L/h)** (number): Drug clearance in L/h. Must be positive.
- **Dosing interval τ (h)** (number): Time between consecutive doses in hours. Must be positive.
- **Bioavailability F** (number): Fraction of dose reaching systemic circulation. IV=1, oral<1. Range (0, 1\].
- **Body weight (kg, optional)** (number): Optional body weight to report the maintenance dose in mg/kg.
- **Decimal Places** (number)

## When to use

- Estimate a maintenance dose from a target average steady-state concentration, clearance, dosing interval, and bioavailability.
- Compare maintenance doses for IV administration and oral dosing with a specified bioavailability.
- Convert the calculated maintenance dose to mg/kg when body weight is available.

## How it works

- Enter the target average steady-state concentration Css,avg in mg/L.
- Enter clearance in L/h, the dosing interval τ in hours, and bioavailability F between greater than 0 and 1.
- The calculator applies MD = Css,avg × CL × τ / F to calculate the maintenance dose in mg.
- Optionally enter body weight to report the dose in mg/kg and choose the number of decimal places.

## Use cases

- Pharmacokinetic teaching: demonstrate how clearance, dosing interval, and bioavailability affect maintenance dose.
- Route comparison: compare an IV maintenance dose with an oral dose when oral bioavailability is below 1.
- Weight-based reporting: add body weight to express the calculated dose in both mg and mg/kg.

## Frequently asked questions

### What formula does the calculator use?

It uses MD = Css,avg × CL × τ / F, where Css,avg is the target average concentration, CL is clearance, τ is the dosing interval, and F is bioavailability.

### What bioavailability should I enter for IV dosing?

Enter F = 1 for IV dosing. For oral dosing, enter the applicable fraction between greater than 0 and 1.

### Does the result represent the peak or trough concentration?

No. The calculation targets the average steady-state concentration. Peak and trough concentrations can fluctuate around that average.

### Can I calculate a dose per kilogram?

Yes. Enter body weight in kilograms to receive the maintenance dose in mg/kg in addition to the total dose in mg.

### When should this calculator not be used?

It is not intended for nonlinear or Michaelis-Menten pharmacokinetics, such as the dose-concentration relationship associated with phenytoin. It is not medical advice.

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