A half-life is the time required for exactly half of a substance to decay or be eliminated. It is constant for a given substance and independent of the initial amount.

Does half-life apply to medications?

Yes. In pharmacokinetics, drug half-life describes how long it takes the body to eliminate half of a drug dose. After 4–5 half-lives, approximately 94–97% of the drug has been cleared.

What is the difference between half-life and mean lifetime?

Half-life (t½) is when 50% remains. Mean lifetime (τ) is the average time a particle exists: τ = t½ / ln(2) ≈ 1.443 × t½.

Can this apply to population decay or investment loss?

Yes. Any process following exponential decay — including certain population models, capacitor discharge, and cooling — follows the same mathematical form.

What units should I use?

Any consistent time unit works (seconds, minutes, hours, days, years). The half-life and elapsed time must be in the same units.

Math

↵ to calculate

Half-Life Calculator - Radioactive Decay Time Online

Calculate radioactive decay, drug half-life, or any exponential decay. Solve for remaining amount, time elapsed, half-life, or initial amount.

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How to use this calculator

👉 Fill in the boxes below and your answer appears instantly — no maths needed, we do it all for you! 🎉

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In plain English — what does this do?

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📌 Direct Answer & Summary

The half-life calculator uses the exponential decay formula N(t) = N₀ × (1/2)^(t/t½) to solve for any unknown given the other values. It applies to radioactive decay, pharmacokinetics, and any process that decreases by a constant fraction over time.

Solve For

What is Half-Life Calculator - Radioactive Decay Time Online?

How to use it

1️⃣ Select what you want to solve for: remaining amount, time elapsed, half-life, or initial amount.
2️⃣ Enter the known values in the fields that appear.
3️⃣ Click Calculate to get your result.
4️⃣ Results include the percentage remaining and number of half-lives elapsed.
5️⃣ Switch modes to solve for different unknowns using the same data.

Formula

N(t) = N₀ × (1/2)^(t/t½). Rearranged: t = t½ × log₂(N₀/N(t)). t½ = t × log(1/2) / log(N(t)/N₀). N₀ = N(t) / (1/2)^(t/t½).

💡 See it in action — a real example

If a substance has a half-life of 5 days and starts at 100 g, after 15 days: N = 100 × (0.5)^(15/5) = 100 × 0.125 = 12.5 g remaining (3 half-lives elapsed, 12.5% remaining).

❓ Common questions

What is a half-life?: A half-life is the time required for exactly half of a substance to decay or be eliminated. It is constant for a given substance and independent of the initial amount.
Does half-life apply to medications?: Yes. In pharmacokinetics, drug half-life describes how long it takes the body to eliminate half of a drug dose. After 4–5 half-lives, approximately 94–97% of the drug has been cleared.
What is the difference between half-life and mean lifetime?: Half-life (t½) is when 50% remains. Mean lifetime (τ) is the average time a particle exists: τ = t½ / ln(2) ≈ 1.443 × t½.
Can this apply to population decay or investment loss?: Yes. Any process following exponential decay — including certain population models, capacitor discharge, and cooling — follows the same mathematical form.
What units should I use?: Any consistent time unit works (seconds, minutes, hours, days, years). The half-life and elapsed time must be in the same units.

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