Little League bats are not legal in Big League. All bats must be "Big Barrel" bats with a barrel diameter of 2-5/8 to 2-3/4 inches. This rule applies to all Big league age groups.
In addition, there are weight limits on bats for each age level. Newer bats are marked with a "minus" rating. This rating can be calculated by subtracting the length of the bat in inches form the weight of the bat in ounces. For example a 32 inch bat which weighs 23 ounces is a "minus 9" or -9 bat. The higher the "minus" number, the lighter the bat
For 13-year-olds, the bat can be no lighter than a "-9". A "-8.5" or a "-7" bat is legal, but a "-9.5" or "-10" bat would not be legal.
For 14-year-olds, the bat can be no lighter than a "-5". A "-3" bat is legal, but a "-7" or "-9" bat would not be legal.
For 15-year-olds, the bat can be no lighter than a "-3". To the relief of parents everywhere, a -3 bat is considered an adult bat and is legal for the rest of their amateur career. A "-5" bat, "-7" bat or "-9" bat would not be legal.
Bats which do not meet the regulations above or lack clear markings are not allowed. There are severe penalties if a player uses an illegal bat in a game.