It has been calculated that there is at most 30 g of francium in the earth's crust at any time, due to its extremely short half-life of 22 minutes.
Telah dilakukan perhitungan bahwa terdapat sebanyak-banyaknya 30 g fransium dalam kerak bumi sepanjang masa, akibat waktu paruhnya yang sangat pendek, hanya sekitar 22 menit.