Work out radiochemical dating instantly with clear inputs, formula shown and shareable results.
Radioactive decay is first order, so the age follows from the surviving fraction: t = ln(1/f) divided by the decay constant, and the decay constant is ln2 over the half-life. A carbon-14 sample at 25% of its original activity has run two half-lives, so it is about 11,460 years old.
Decay constant
lambda = ln2 / half-life
Age
t = ln(N0/N) / lambda = half-life x log2(N0/N)
One quarter. Each half-life halves what remains, so 100 g becomes 50 g then 25 g.
After roughly nine half-lives under 0.2% remains, which is too little to measure reliably against background.