That's what I get trying to balance rough numbers in my head. I was using roughly the number of atoms, not grams. Here's the math laid bare: Frac Remain In

That's what I get trying to balance rough numbers in my head. I was using roughly the number of atoms, not grams. Here's the math laid bare: Frac Remain In 1 Day = 0.5 ^ ((1/365)/4,500,000,000 years) Frac Remain In 1 Day = 0.999999999999578 Frac Decay In 1 Day = 1 - Frac Remain In 1 Day Frac Decay In 1 Day = 4.22 * 10^-13 Have 4 Decays In 1 Day = Total U238 * Frac Decay In 1 Day 4 = Total U238 * 4.22 * 10^-13 4 / (4.22 * 10^-13) = Total U238 Total U238 = 9.48 * 10 ^ 12 atoms I stand corrected, thanks for catching that.