Major League Baseball bats must be made of wood. Rule 3.02(a) of the 2026 Official Baseball Rules says the bat must be one piece of solid wood, no more than 2.61 inches in diameter at its thickest part and no more than 42 inches long. Aluminum, composite, and other metal bats are not allowed, and laminated or experimental wood bats can be used only after Major League Baseball approves the manufacturer’s design and methods.
In practice, most big-league bats are hard (sugar) maple. The Society for American Baseball Research reports that about 75 percent of major-league hitters used maple at the start of the 2017 season, with ash a distant second at roughly 10 to 15 percent and yellow birch a smaller but growing choice. Ash was the standard wood until maple caught on in the late 1990s and early 2000s.
Source: Major League Baseball, 2026 Official Baseball Rules, Rule 3.02. Additional source: Society for American Baseball Research, “The Bats … They Keep Changing!”
Image: Custom wood bat by Wikideas1, CC0, via Wikimedia Commons.