Loading dose and maintenance dose calculator (pharmacokinetics)
Enter your values. The answer and the full worked solution update as you type.
Result
One-compartment model: the maintenance dose keeps the average steady-state concentration at the target. Peaks and troughs of intermittent doses are not shown. For exam practice only. This is a study aid, never a clinical decision: always follow the prescription, local protocols and the product information.
Calculate a loading dose and a maintenance dose from basic pharmacokinetic parameters, as in pharmacology and clinical pharmacy exams. Enter the target concentration, the volume of distribution (in L or L/kg), the clearance, the salt factor S, the bioavailability F and the dosing interval. The calculator gives the loading dose, the maintenance dose rate and the dose per interval, and the elimination half-life that follows from Vd and CL.
Step-by-step solution
Vd in L and CL in L/h
Loading dose
Maintenance dose rate
Maintenance dose per interval
Elimination rate constant and half-life
Key formulas
- Rate constant and half-life
- k = ln 2/t½ = 0.693/t½
- Volume and clearance
- Vd = dose/C₀, CL = k·Vd
- Loading dose
- LD = Vd·C/(S·F)
- Maintenance rate
- R = CL·Css/(S·F)
- Average steady state
- Css,avg = S·F·D/(CL·τ)
How to calculate a loading and maintenance dose
- Convert Vd to litres (L/kg × weight) and the clearance to L/h (mL/min × 60 ÷ 1000).
- Loading dose = Vd × target concentration ÷ (S × F). If some drug is already in the body, use the target minus the current concentration.
- Maintenance dose rate = CL × target steady-state concentration ÷ (S × F), in mg/h.
- Dose per interval = maintenance rate × dosing interval τ, for example mg/h × 12 h for twice-daily dosing.
- The elimination rate constant is k = CL/Vd and the half-life is t½ = 0.693/k. Without a loading dose, steady state takes about 4 to 5 half-lives.
Questions and answers
What is the salt factor S?
The fraction of the dose that is the active drug. A salt weighs more than the drug itself: aminophylline is about 80% theophylline, so S = 0.8. Use S = 1 when the dose is stated as the drug itself.
What is the bioavailability F?
The fraction of an oral or other non-IV dose that reaches the circulation unchanged. For IV doses F = 1. A lower F means a larger dose is needed for the same concentration.
Why does the loading dose depend on Vd and the maintenance dose on clearance?
A loading dose fills the volume the drug distributes into, so it depends on Vd. A maintenance dose replaces what is eliminated per unit of time, so it depends on clearance.
Is mg/L the same as mcg/mL?
Yes. 1 mg/L = 1 mcg/mL, and 1 mcg/L = 1 ng/mL. Drug levels are reported in either unit.
What model does this use?
A one-compartment model with first-order elimination. The maintenance dose gives the average steady-state concentration; peak and trough levels of intermittent doses need further equations.
Practise it yourself
Practise Clinical pharmacokinetics
A calculator gives the answer. In the exam you need the method: practise it with new problems, hints and worked solutions.