12.5 mg remaining
Half-Life Calculator
Use this free half-life calculator to find remaining amount, elapsed time, or half-life from initial and final amounts with formulas, percentages, steps, copy, and history.
Remaining amount Elapsed time Solve half-life Decay steps
100 mg for 18 hours12.5 mg remaining
12.5% remains and 87.5% has decayed.
- Half-lives passed
- 3
- Decayed amount
- 87.5 mg
- Percent remaining
- 12.5%
Steps
- Use the decay formula: remaining amount = initial amount x (1/2)^(elapsed time / half-life).
- 18 hours / 6 hours = 3 half-lives.
- 100 mg x (1/2)^3 = 12.5 mg.
How to use the half-life calculator
- Choose whether you want remaining amount, elapsed time, or half-life.
- Enter the known initial amount and the other values requested for that mode.
- Use matching time units for elapsed time and half-life, such as hours with hours.
- Press Calculate half-life to see the answer, percentages, formula steps, and recent history.
Common uses
Estimate how much of a substance remains after a number of half-lives.
Find elapsed time when you know the initial amount, final amount, and half-life.
Solve for half-life when you know initial amount, final amount, and elapsed time.
Check chemistry, physics, biology, environmental science, and study decay examples.
Examples
36 hours
5 days
Frequently asked questions
Quick answers about decay formulas, units, elapsed time, solving half-life, safety limits, and privacy.
What formula does the Half-Life Calculator use?
For remaining amount, it uses remaining amount = initial amount x (1/2)^(elapsed time / half-life). It also rearranges that formula to solve for elapsed time or half-life.
What is a half-life?
A half-life is the time it takes for a quantity to decrease to half of its starting amount under exponential decay.
Can I solve for elapsed time?
Yes. Choose Elapsed time, enter the initial amount, final amount, and known half-life, then calculate. The final amount must be greater than zero and not greater than the initial amount.
Can I solve for the half-life itself?
Yes. Choose Half-life, enter the initial amount, final amount, and elapsed time. The final amount must be less than the initial amount so the decay rate can be calculated.
Do the units matter?
Yes. Keep elapsed time and half-life in the same time unit, such as hours with hours or years with years. The amount unit is only a label and should match between initial and final amounts.
Can I use this for medicine dosing or radiation safety decisions?
No. This tool is for general math, study, and planning examples. Do not use it as medical advice, dosing advice, or radiation safety guidance.
Is my half-life calculation history private?
Yes. Recent half-life answers stay only in the current browser tab while you use the page. They are not sent to a server.
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