How do you calculate half-life of an isotope?

What is the formula for calculating half-life?

What is the ratio of parent isotopes to daughter isotopes?

The ratio of parent to daughter atoms after two half-lives is therefore 1:3 (one-quarter to three-quarters). Successive half-lives reduce the original parent to one-eighth, one-sixteenth, one-thirty-second, and so on. The ratios of parent to daughter isotopes for these are 1:7, 1:15, 1:31.

What is the ratio of parent to daughter isotopes after 3 half lives?

At the end of a third half-life, you would have, one-eighth, or 2.5 g of the original isotope remaining. At that point, three-fourths of the sample would now be the daughter isotope. By comparing the amounts of parent and daughter isotopes in a rock sample, scientists can determine the age of the sample.