Solve remaining quantity, elapsed time or half-life for any first-order decay.
Answer
How to use this tool
Remaining quantity, elapsed time, or half-life from two measurements.
Only the fields for your mode are used.
The big number is the solved value.
The half-lives elapsed tells you how many halvings the result represents.
Formula & worked example
100 units with a 10 hour half-life leave 12.5 units after 30 hours (three half-lives).
Limitations — read this before you rely on it
Assumes pure first-order decay with a constant half-life; real systems with changing conditions, multiple compartments or measurement error will deviate.
Frequently asked questions
The time for a quantity to halve under first-order decay. It is constant, independent of the starting amount.
The maths is identical, but drug calculators add dosing, absorption and body-weight models; this tool is the general decay law.
Each half-life halves what is left, so the curve approaches zero without ever touching it.
Lambda = ln(2) / half-life. It gives the fractional decay rate per time unit.
No: growth uses a doubling time and a positive exponent. Use an exponential growth tool for that.