Can I use M18 batteries in M12 tools?
No: M12 and M18 batteries are different shapes and voltages. Here's what does work, including chargers that charge both.
No. M12 and M18 are two separate battery systems. M12 batteries are 12-volt packs that slide up into the handle; M18 batteries are bigger 18-volt packs that slide onto the bottom of the tool. They don't physically fit each other's tools, and Milwaukee doesn't make an adapter between them.
What does work
Any M18 battery fits every M18 tool, and any M12 battery fits every M12 tool, old or new. A small CP battery and a big HD battery both work; the bigger one just runs longer and weighs more.
Some chargers charge both M12 and M18 batteries, so you only need one charger if you own both systems.
Should you own both?
Lots of pros do: M12 for small tools that ride on your belt all day, M18 for saws, big drills and impact wrenches. If you're just starting, pick one system first. Our battery quiz can help, and the battery shop compares every size.
What about MX FUEL?
MX FUEL is a third, separate system for heavy jobsite equipment like cut-off saws, core drills and concrete vibrators. Its batteries don't fit M18 or M12 tools either.
Specs quoted are the manufacturer’s figures as listed by tool dealers. Team Red Tools is an independent fan site, not affiliated with Milwaukee Tool.