Reference Guides

Calculating Half-Life


Half-life (t1/2) is the amount of time needed to reduce the concentration of a substance by half. Understanding how half-life works is important when trying to determine the excretion rate of a specific chemical. All chemicals have different half-lives; however, they all follow the same concept: once a half-life has passed, 50% of the chemical amount is eliminated from the body. The symbol for the half-life is T½.

How to calculate half-life

For example, 200mg of a chemical with a 1-day half-life is administered, the following can be estimated:

  • • 1 day after administration, 100mg remains.
  • • 2 days after administration, 50mg remains.
  • • 3 days after administration, 25mg remains.
  • • 4 days after administration, 12.5mg remains.
  • • 5 days after administration, 6.25mg remains.

In theory, after 5 days we can see that almost 97% of the chemical has been eliminated. Chemicals are considered to have a negligible effect after about 4-5 half-lives. However, even if there is a negligible effect after such time, the chemical can still be detected.


Calculating Half-life is important for calculating the excretion rate of a chemical. This helps us to better undetand how safe a chemical is to handle.

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