Which is a bigger caliber a 40 or a 45?

Answer 1

Assuming the question is in regard to firearms and ammunition, you can read the "caliber" of a round as a decimal how wide the bullet is in inches. So a .40 caliber round is .4 inches wide, or about 10.16 millimeters wide. A .45 caliber round would be .45", so a little bit fatter than the .40 caliber round.

The caliber doesn't tell the whole story of a round though, it doesn't say how long the bullet is, how heavy, how big the casing behind the round is, how much kinetic energy is hits with, etc. The .40 S&W round has an average of 425 ft/lbs of energy right at the muzzle, while the .45 ACP, a "bigger" round, has about 400 ft/lbs.