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Uranium-235 Dating Calculator

Estimate how old a sample is from the remaining ratio of uranium-235 (half-life about 704 million years), or from the ratio of leftover parent to daughter isotopes.

Input

Estimate how old a sample is from how much uranium-235 (half-life about 704 million years) remains. Enter the remaining ratio directly, or compute it from the amounts of the remaining parent isotope (uranium-235) and the daughter isotope produced by decay.

Input method
%

In parent/daughter mode the remaining ratio is taken as parent ÷ (parent + daughter), assuming all daughter atoms come from decay of the parent and that nothing enters or leaves the system. Real dating uses different isotope systems and corrections, so results will vary.

Result

Estimated elapsed age (approximate)

about 7.04 hundred million years

about 704,000,000 years

Elapsed age

about 704,000,000 years

Uranium-235 half-life

704,000,000 years

Remaining ratio N/N0

50.00 %

Half-lives elapsed

1.00 half-lives

Approximation using t = T × log₂(N0/N), obtained by solving the radioactive decay law N = N0 × (1/2)^(t/T) for t. The half-life T is taken as about 704 million years (7.04×10⁸ years).

Remaining ratio vs. elapsed age (reference)

Half-lives elapsedRemaining ratio N/N0Elapsed age
1 half-life (half-life × 1)50.00 %about 7.04 hundred million years
2 half-lives (half-life × 2)25.00 %about 14.08 hundred million years
3 half-lives (half-life × 3)12.50 %about 21.12 hundred million years
4 half-lives (half-life × 4)6.25 %about 28.16 hundred million years
5 half-lives (half-life × 5)3.13 %about 35.20 hundred million years

How it works

  • In radioactive decay an initial amount N0 falls over time t as N = N0 × (1/2)^(t/T), where T is the half-life. For uranium-235 we use about 704 million years (7.04×10⁸ years).
  • Solving this for the elapsed age t gives t = T × log₂(N0/N) = T × ln(N0/N) ÷ ln2, so the age can be found once the remaining ratio N/N0 is known. This tool calculates with that formula.
  • In remaining-ratio mode, the current amount ÷ initial amount (%) is used directly as N/N0. For example, a remaining ratio of 50% means about one half-life, roughly 704 million years; 25% means about 1.41 billion years.
  • In parent/daughter mode the remaining ratio is computed as parent ÷ (parent + daughter), assuming every daughter atom comes from decay of the parent (uranium-235) with no inflow or outflow. Real samples use other isotope systems and isotope-ratio corrections, so results differ.
  • Uranium-235 decays through the actinium series and ultimately to lead-207. Actual geological dating uses more rigorous methods, such as combining several isotope systems and correcting for initial isotope ratios.
  • The figures shown are approximations based on the established decay equation and a representative half-life value, and do not guarantee real dating results. Treat them as a rough guide and consult official agencies or academic sources for expert judgment.