What is it?
The Half-Life Calculator shows how much of a substance is left after a given time, if you know its half-life. It also shows how many half-lives have passed and how long it takes to fall to 10% and 1%.
Who is it for?
Physics and chemistry students, and anyone learning radioactive decay or carbon dating.
How to use it
Enter the Starting amount (grams, atoms, % or any unit), the Half-life, and the Time passed (same unit as the half-life). The answer appears as you type; there is no Calculate button. Tap Copy result to copy it, or Share on WhatsApp to send it.
The formula
N = N₀ × (½)^(t ÷ half-life). Carbon-14’s half-life is 5,730 years. After 1 half-life 50% is left, after 2: 25%, after 3: 12.5%, after 10: about 0.1%.
Uses
- Physics and chemistry homework on radioactive decay.
- Carbon dating: if 25% of the carbon-14 remains, about 11,460 years have passed.
- Medicines: drugs are mostly gone after 4–5 half-lives. Doctors use this for dosing; don’t change your medicine schedule on your own.
Units
Keep time units the same: if the half-life is in hours, enter time in hours.
Worked example
100 mg of a substance with a 6-hour half-life after 18 hours: 18 ÷ 6 = 3 half-lives, so 100 × ½ × ½ × ½ = 12.5 mg left.
Errors and how to fix them
- “Enter the half-life (more than 0)”
Open the tool: Half-Life Calculator
Still stuck?
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Questions, or an error not listed here?
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