A walkthrough, end to end.
- 1
Enter the initial quantity (N₀), the half-life (t½), and the elapsed time (t).
- 2
The calculator returns remaining quantity, decayed amount, and the decay constant λ.
- 3
Use any time unit, as long as t and t½ share the same unit.
Half-life formula
First-order exponential decay halves the quantity every t½ time units. Decay constant λ relates to half-life by λ = ln(2) / t½.
What you can do with this.
Radioactive isotopes
Carbon-14: t½ ≈ 5,730 years. After 11,460 years, 25% of original ¹⁴C remains.
Drug elimination half-life
Caffeine ≈ 5h. After 10h, ~25% remains; after 25h, <5% remains.
RC discharge
Capacitor in an RC circuit decays exponentially with time constant τ. Half-life = τ·ln(2).
Carbon dating
Estimate age of organic samples from remaining ¹⁴C ratio.
Cooling and Newton's law
Temperature difference decays exponentially toward ambient.
Pharmacokinetics
Plan dosing intervals using elimination half-life.
First-order chemistry
Many chemical reactions follow first-order kinetics with constant half-life.
Half-life calculator 2026 — what's current
Standard textbook model; calculator wins for instant solving across the four variables.
Frequently asked.
Any — seconds, hours, years. Just make sure t and t½ use the same unit.
Yes; same exponential model applies to drug elimination.
Use t½ = t · ln(2) / ln(N₀ / N(t)). The basic form here uses N₀, t½, t.
No. Calculations run entirely in your browser.