Pharmacy

Drug Half-Life Calculator

Calculate time to reach steady state, time to full elimination, and remaining drug concentration at any time point. Includes a reference table of common drug half-lives and an elimination decay chart.

3 calculation modes Elimination decay chart PDF export

Half-Life Calculator

Common drugs:

Drug Half-Life

hours

Enter a valid half-life.

Result

Results:

Elimination over time:

Half-lives	Time	Remaining	Eliminated

Common Drug Half-Lives

Drug	Half-Life	Steady State	Elimination	Notes
Aspirin	15-20 min	~1.5h	~2h	Salicylate metabolite: 6-20h at high doses
Ibuprofen	2h	~10h	~12h	Extended release: 3-4h t½
Paracetamol	1.5-3h	~12h	~15h	Prolonged in hepatic impairment
Amoxicillin	1-1.5h	~7h	~9h	Renal dose adjustment needed
Gentamicin	2-3h	~15h	~18h	Prolonged in renal impairment
Vancomycin	4-6h (normal renal)	~30h	~36h	Monitor AUC. Extends greatly with CKD.
Metformin	6.5h (plasma)	~33h	~40h	Blood t½ longer (~17h)
Atorvastatin	14h	~70h	~84h	Active metabolites add to effect duration
Warfarin	36-42h	~8 days	~10 days	S-warfarin shorter (29h); R longer (45h)
Digoxin	36-48h	~8 days	~10 days	Prolonged in renal failure. Narrow TI.
Diazepam	20-100h	~2-3 weeks	~3 weeks	Active metabolite desmethyldiazepam: 36-200h
Amiodarone	40-55 days	~8 months	~1 year	Longest t½ of commonly used drugs
Amlodipine	30-50h	~7 days	~9 days	OD dosing appropriate due to long t½
Lisinopril	12h (effective)	~3 days	~3.5 days	Terminal t½ 30h due to tissue binding

Drug half-life and clinical pharmacokinetics

The half-life (t½) is the time for the drug concentration to fall by half. It determines dosing frequency, time to steady state, and how long a drug stays in the body after stopping it.

Steady state is reached after 4-5 half-lives. At 4 half-lives you're at 94% of steady state; at 5 half-lives, 97%. This is why amiodarone (t½ 40-55 days) takes months to reach therapeutic levels and months to wash out after stopping.

Key equations

What you want	Formula
Time to steady state	~4-5 × t½
Remaining concentration at time t	C = C₀ × (0.5)^(t/t½)
Elimination constant (ke)	ke = 0.693 / t½
Time to eliminate	~5 × t½ (97% eliminated)

Renal and hepatic impairment

Most drugs that are renally cleared have significantly prolonged half-lives in kidney disease. Gentamicin's t½ goes from 2-3 hours in normal renal function to 50-70 hours with severe CKD. The Cockcroft-Gault equation predicts creatinine clearance, which guides dosing adjustments. For drugs with hepatic elimination, Child-Pugh score guides dose modification. See Creatinine Clearance Calculator.

Related tools

For drug dosing by weight, see Weight-Based Dose Calculator. For renal dose adjustment, see Creatinine Clearance. For IV infusion rates, see IV Drip Rate Calculator.

Note: Half-life values in clinical literature show wide inter-individual variability. The values in this reference table are approximate population averages. Age, organ function, genetic polymorphisms, and drug interactions all affect individual t½ significantly.