# Drug Half-Life Calculator

URL: https://speedycalculator.com/calculators/health/drug-half-life/
Last updated: 2026-10-30

> The drug half-life calculator shows how much of a medicine remains in the body after a given time, how long it takes to be mostly eliminated (about 5 half-lives) and the time to reach steady state with regular dosing.

## Inputs

- Initial amount (mg)
- Half-life (h, min, days)
- Time since the dose (h, min, days)

## Results

- Amount remaining
- Percentage remaining
- Time to ~97% elimination (5 half-lives)
- Elimination rate constant
- Half-lives elapsed

## 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
```

## Explanation

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

FieldWhat to enterInitial amountIn mgHalf-lifeUnits: h, min, daysTime since the doseUnits: 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)503.13754.691006.251257.811509.3820012.5
*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.*

## 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.

## FAQ

**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.


## Sources

- [Hallare J, Gerriets V. Half life. StatPearls, 2023](https://www.ncbi.nlm.nih.gov/books/NBK554498/)