Aluminum bats are measured in inches for length and ounces for weight.
The negative number on an aluminum bat represents its length-to-weight ratio. For instance, a 30-inch bat weighing 26 ounces would have a -4 designation.
A lower negative number translates into greater bat speed.
Aluminum bats are available with negative ratings as low as -13. These bats, which are extremely light and long, are only available for the youngest youth leagues. And due to the technology required to make such lightweight bats, they also tend to be more expensive.
High school and college baseball players must use bats with a length-to-weight ratio no lower than -3. Swinging bats with lower negative numbers would generate too much power and increase the risk of injury.