Only a Trillion is a collection of ten science essays and three scientific spoof articles by Isaac Asimov.

A paperback edition published by Ace Books in 1976 included updates of outdated material (re-issued in 1980).

The title refers to the number of atoms of astatine-215 in the top 10 miles of the Earth's crust of the North and South American continents – only a trillion.

Floyd C. Gale stated that Asimov "[a]lmost certainly had enormous fun writing this collection of articles.

[1] Following the publication of Only a Trillion, Asimov compiled several more science essays into the manuscript for another, similar book titled Only a Light-Year, but that book was never published, because Only a Trillion had not sold enough copies.