Drug Half-Life Calculator

Enter Calculation

Initial Amount / Dose

Half-Life (hours)

Time Elapsed (hours)

Results

Amount Remaining

25

after the time elapsed

Percent remaining	25 %
Amount eliminated	75
Percent eliminated	75 %
Half-lives elapsed	2

What Is a Drug Half-Life Calculator?

A drug's half-life (t½) is the time it takes for the amount of the substance in the body to fall to half its original value. This calculator uses exponential decay to estimate how much of a dose remains after any amount of time has elapsed. It is a general pharmacokinetic model intended for education and rough estimation — it is not medical advice, and individual metabolism, organ function, and drug interactions can change real-world results.

How to Use It

Enter the initial amount or dose (in any consistent unit such as mg), the drug's half-life in hours, and the time elapsed in hours. The calculator returns the amount remaining, the percent remaining and eliminated, and how many half-lives have passed.

The Formula Explained

The model is simple exponential decay: $$\text{Remaining} = \text{Initial} \times \left(\frac{1}{2}\right)^{\frac{t}{t\frac{1}{2}}}$$ The exponent \(t / t\tfrac{1}{2}\) counts how many half-lives have elapsed. Each whole half-life halves the amount, so after 1 half-life you have 50%, after 2 you have 25%, after 3 you have 12.5%, and so on. The equivalent natural-exponential form, $$C = C_0 \cdot e^{-0.693 \cdot t / t\frac{1}{2}}$$ uses the elimination rate constant \(k = 0.693 / t\tfrac{1}{2}\) (since \(\ln 2 \approx 0.693\)).

Each half-life period reduces the remaining drug amount by 50%.

Worked Example

Suppose you take a 100 mg dose of a drug with a 4-hour half-life and want to know how much remains after 8 hours. Half-lives elapsed = \(8 / 4 = 2\). $$\text{Remaining} = 100 \times \left(\frac{1}{2}\right)^{2} = 100 \times 0.25 = 25 \text{ mg}$$ That means 75 mg (75%) has been eliminated.

Remaining amount after 1, 2, 3 and 4 half-lives: 50%, 25%, 12.5%, 6.25%.

FAQ

How many half-lives until a drug is "gone"? Clinically, a drug is considered effectively cleared after about 4–5 half-lives, when roughly 94–97% has been eliminated.

Does this account for repeated dosing? No — it models a single dose. With regular dosing, drug levels accumulate toward a steady state over about 4–5 half-lives.

What units should I use? Any consistent units work for the amount (mg, µg, etc.). Half-life and elapsed time must both be in hours.