(In). In whole numbers, without regarding the fractions. Thus we say the population of the British Isles is forty millions in round numbers, and that of London four millions (1895). The idea is that what is round is whole or perfect, and, of course, fractions, being broken numbers, cannot belong thereto.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Round Numbers from Infoplease:
24 X 7
Explore Grade 5 Math ,