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Half-Life Calculator

Calculate radioactive decay, remaining quantity, elapsed time, or required half-life — with a decay curve, step-by-step table, and isotope reference.

N(t) = N₀ × (½)^(t / t½) · λ = ln(2) / t½

Choose what you want to calculate

Initial Amount (N₀)

Unit

Remaining Amount (N)

— or — Remaining %

Half-Life (t½)

Half-Life Unit

Elapsed Time (t)

Elapsed Time Unit

Auto-fill half-life from a common isotope

Isotope

Common half-life problems

Summary

Remaining Amount

—

Initial (N₀)—

Half-Life (t½)—

Elapsed Time—

Half-Lives Elapsed—

Remaining—

Decayed—

Decay constant λ—

Remaining %

—of original

Key Formulas

RemainingN₀ × (½)^(t/t½)
Decay const.ln(2) / t½
Time elapsedt½ × log₂(N₀/N)
Half-lifet × ln2 / ln(N₀/N)