What caliber is 7.62x39 cartrige?

The 7.62 x 39mm round is commonly used in the AK-47 series of rifles, which includes many, many variants.

It is roughly .30 caliber, or 30-hundredths of an inch in diameter. The 39mm means that the brass case is 39mm from base to the rim.

For comparison, the newer AK-74 rifle fires a 5.45x39mm round, which has a diameter slightly less than a .22!