The half-life is the time it takes for the concentration of a drug in the blood to fall by half. It determines how often a medicine is taken and how long effects or side effects last after stopping. Liver or kidney disease can lengthen it.
How to use the Drug Half-Life Calculator
- Initial amount
- Half-life
- Time since the dose
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the dose (or concentration), the drug’s half-life and the time since the dose.
Inputs explained
| Field | What to enter |
|---|---|
| Initial amount | In mg |
| Half-life | Units: h, min, days |
| Time since the dose | Units: h, min, days |
What each result means
- Amount remaining = [Initial amount]×0.5^([Time since the dose]÷[Half-life])
- Percentage remaining = 0.5^([Time since the dose]÷[Half-life])×100
- Time to ~97% elimination (5 half-lives) = 5×[Half-life]
- Elimination rate constant = ln(2)÷[Half-life]
- Half-lives elapsed = [Time since the dose]÷[Half-life]
Example values
How the main result changes when Initial amount changes and everything else stays at the example values above:
| Initial amount (mg) | Amount remaining (mg) |
|---|---|
| 50 | 3.13 |
| 75 | 4.69 |
| 100 | 6.25 |
| 125 | 7.81 |
| 150 | 9.38 |
| 200 | 12.5 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Remaining = initial amount × 0.5^(time ÷ half-life)
Elimination constant k = ln 2 ÷ half-life
About 97% eliminated after 5 half-lives; steady state reached after about 4–5 half-lives of regular dosing
Worked example
Inputs — Initial amount: 100 mg; Half-life: 6 h; Time since the dose: 24 h.
Results — Amount remaining = 6.25 mg; Percentage remaining = 6.25 %; Time to ~97% elimination (5 half-lives) = 30 h 0 min; Elimination rate constant = 0.1155 per hour; Half-lives elapsed = 4.
Key terms
- Steady state
- When drug intake and elimination are balanced with regular dosing.
Frequently asked questions
What does half-life mean?
The time for the amount of drug in the body to halve.
How long until a drug is out of my system?
Roughly 4–5 half-lives for most drugs.
Why does half-life matter for dosing?
Drugs with long half-lives can be taken less often but take longer to reach steady state.