Drug half-life calculator
Estimate how much of a medication may remain after a given amount of time. This calculator uses a first-order elimination model, which is the standard simple model for many drugs but not a rule for every medicine or every person.
This is a planning and learning aid only. Do not use it to decide whether to skip, repeat, stop, or restart a medication.
Estimate medication remaining from half-life
Enter the starting amount, half-life, and elapsed time. This estimates the amount left for a single dose under first-order elimination.
Estimated amount remaining
How this estimate works: The calculator converts the time units, then uses remaining amount = starting amount x 0.5^(elapsed time / half-life). It assumes a single dose and first-order elimination.
How to use the calculator
Enter the amount you want to start from, the drug's half-life, and the elapsed time. Choose the unit for the half-life and the unit for the elapsed time; the calculator converts them before estimating the fraction remaining. The amount unit can be milligrams, micrograms, units, or doses.
Use a half-life from a prescription label, FDA label, DailyMed label, pharmacist, or prescriber. Do not guess a half-life from how long the medication feels active. Duration of effect and elimination half-life are related but not the same thing.
Formula
For first-order elimination, the estimate is:
| Step | Formula |
|---|---|
| Half-lives elapsed | elapsed time / half-life |
| Fraction remaining | 0.5 ^ half-lives elapsed |
| Amount remaining | starting amount x fraction remaining |
| Percent eliminated | 1 - fraction remaining |
After one half-life, about 50% remains. After two half-lives, about 25% remains. After five half-lives, about 3% remains.
Example
If a medication has a half-life of 6 hours and you start from 100 mg, then after 24 hours four half-lives have passed. The estimate is 100 x 0.5^4, or about 6.25 mg remaining.
That does not mean the medicine has no effect, is safe to combine with another medicine, or is safe to restart. Clinical decisions depend on the drug, dose, active metabolites, medical conditions, and other medications.
Limits and safety notes
This calculator assumes a single starting amount and first-order elimination. It does not model repeated dosing, extended-release products, loading doses, active metabolites, organ impairment, pregnancy, age-related changes, drug interactions, overdose, or zero-order elimination.
Ask a pharmacist or prescriber if you are using this estimate because of a missed dose, side effects, a possible overdose, a drug test, surgery timing, or a plan to combine medicines. Seek urgent help for severe symptoms, overdose concern, trouble breathing, chest pain, fainting, or confusion.
Related tools
If you are estimating half-life because your routine is hard to track, build a weekly plan with the medication schedule builder or estimate your next refill with the refill calculator.
Common questions
Is drug half-life the same as how long a medicine works?
No. Half-life estimates how quickly the amount in the body decreases. The effect can be shorter or longer depending on receptor activity, active metabolites, dose, and the condition being treated.
How many half-lives until a drug is gone?
Many first-order estimates treat four to five half-lives as a practical point where most of the drug has been eliminated. Small amounts can remain longer, and some drugs do not follow this simple pattern.
Can I use this after repeated doses?
Use caution. Repeated dosing can cause accumulation, and this simple calculator starts from one amount at one time. Ask a pharmacist or prescriber for medication-specific guidance.
Can this tell me when it is safe to take another medicine?
No. Safety depends on the medicines, dose, health conditions, interactions, and active metabolites. Ask a pharmacist or prescriber before combining or changing medicines.