Half-Life Calculator
Calculate remaining quantity, elapsed time, or half-life for radioactive decay problems.
Remaining quantity
25 units
N(t) = N0 × (1/2)^(t / half-life)
Enter the initial quantity, half-life, and elapsed time to find the remaining amount of a decaying substance — or solve for the elapsed time or the half-life itself. Uses the exponential decay formula and runs entirely in your browser.
How to use
- Select what you want to solve for: remaining quantity, time, or half-life.
- Fill in the known values.
- Read the calculated result and decay formula below.
Frequently asked questions
- What is the decay formula used?
- N(t) = N0 × (1/2)^(t / T), where N0 is the initial quantity, t is elapsed time, and T is the half-life.
- Can I use any time unit?
- Yes, as long as elapsed time and half-life use the same unit (e.g. both in years or both in days).
- What if remaining quantity exceeds the initial quantity?
- That's not physically possible for decay, so the tool won't return a result — check your inputs.