Half-Life Calculator

Calculate remaining amount after radioactive or chemical decay over time.

By Konstantin Iakovlev · Updated September 2026 · Source: U.S. NRC Regulatory Guide 8.39, Rev. 1 (2020): physical and effective half-lives of I-131

Solve For

Remaining

12.5000

Time

15.00

Decay Details

Remaining12.5000
Decayed87.5000
% Remaining12.50%
Half-lives elapsed3.00

Use the Half-Life Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only. Results are estimates based on the information you provide and the assumptions described on this page.

How It Works

How much of a substance is left after a given stretch of time depends on its decay, and that question shows up across nuclear medicine, environmental science, and even the shelf-life of certain products. The half-life of carbon-14 underpins archaeological dating, while iodine-131 decay drives decisions in medical treatment and waste handling.

Behind the result is the half-life formula N(t) = N₀ * (1/2)^(t/T), in which N(t) is the amount remaining after time t, N₀ is the starting amount, and T is the half-life of the substance. This exponential model tracks how the quantity falls with each successive half-life, and the t/T exponent simply counts how many of those cycles have elapsed.

Time units have to match throughout: a half-life quoted in days requires the elapsed time in days as well. The model also assumes a constant decay rate, which holds reliably for radioactive isotopes but can shift with environmental conditions in chemical decay. Watch the distinction between the initial amount and the amount that has decayed, since mixing up the two is an easy way to answer the wrong question.

Example: Radioactive Iodine-131 in a Medical Setting

  1. 1 A patient receives 100 MBq (megabecquerels) of iodine-131 for thyroid treatment. Its physical half-life is about 8.02 days. How much of that activity is left, from radioactive decay alone, after 24.06 days?
  2. 2 Enter initial amount 100, half-life 8.02 and time elapsed 24.06. Half-lives elapsed: 24.06 / 8.02 = 3, so N(t) = 100 MBq × (1/2)^3 = 100 / 8.
  3. 3 The calculator shows 12.5000 MBq remaining, 87.5000 MBq decayed and 12.50% remaining after 3.00 half-lives.
  4. 4 That is physical decay only. The body also excretes iodine, so the activity actually left in the patient is lower: NRC Regulatory Guide 8.39 models I-131 with effective half-lives of about 0.32 days for iodine outside the thyroid and 5.2 to 7.3 days for iodine taken up by it. The calculator's figure applies directly to activity that is not being excreted, such as a stored vial or a waste container.

Source: U.S. NRC Regulatory Guide 8.39, Rev. 1 (2020): physical and effective half-lives of I-131 · Last updated: September 2026

Frequently Asked Questions

How do you calculate remaining amount after half-lives?
Remaining amount = initial amount x (1/2)^(time/half-life). After 3 half-lives, 12.5% remains (1/2³ = 1/8). After 10 half-lives, less than 0.1% remains.
What does half-life mean in simple terms?
Half-life is the time it takes for exactly half of a substance to decay or break down. After one half-life, 50% remains. After two half-lives, 25% remains. After three, 12.5% remains, and so on.
How is half-life used in medicine?
Drug half-life determines dosing frequency. A drug with a 4-hour half-life may need to be taken every 4-8 hours to maintain therapeutic levels. After 5 half-lives (about 97% eliminated), the drug is essentially cleared from your system.

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