Milwaukee M12 Vs. M18 Batteries: What's The Difference?

Milwaukee M12 and M18 batteries work with a collection of power tools worthy of adding to your collection in 2025. The first difference to notice between the two battery systems is that they operate at different voltages. As their names imply, the M12 battery system operates at a nominal 12 volts, while the M18 system is rated at 18 volts. This means that as a general rule, there is no crossover between Milwaukee's M12 and M18 battery systems. While there are adapters available that allow you to use an M18 battery to power M12 Milwaukee power tools, some of the reviews for them indicate that the adapters may be less than ideal.

In addition to the difference in voltage between them, their entirely different shape and size of the battery packs makes interchanging the M12 and M18 Milwaukee batteries unlikely. Battery capacity, measured in amp-hours (Ah), is another difference between the M12 and M18 battery systems. The M12 system holds the lowest capacity battery between the two systems with its M12 Redlithium CP1.5 battery pack rated at 1.5 Ah. Mid-range capacity ratings for both battery systems are similar with a variety of offerings from 2.0 Ah through 6.0 Ah. The M18 battery system pulls ahead at the higher end with 8.0 and 12.0 Ah battery packs available.

The Milwaukee M18 battery has higher voltage and available battery packs with extra amp-hour capacities, offering longer run times than the M12 system. In an effort to put other advantages of the M18 system into perspective, we'll use ⅜-inch Milwaukee Impact Wrenches from both systems as our benchmark.

Starting with Milwaukee's specification for the M12 Fuel Stubby ⅜-inch Impact Wrench. It stands 7.2 inches tall, spans 2.4 inches at its widest part, and is 4.8 inches in length from the square drive to the back of the case. Overall, without a battery attached, the tool weighs 2.2 pounds, and it's capable of spinning up to 3,000 rpm.

Representing the Milwaukee M18 Tools system is the M18 Fuel ⅜-inch Mid-Torque Impact Wrench. Dimensional specs provided by Milwaukee are not as complete for the M18 model as they were for the M12 Impact Wrench, but it's generally larger, and we do know that it measures six inches in length versus the 4.8 inches of the M12, and it weighs 3.4 pounds without a battery, 1.2 pounds heavier. It also has a lower maximum speed, spinning at up to 2,575 rpm.

While the M12 Impact Wrench is smaller, lighter, and faster, the M18 version is ultimately more powerful. However, the difference may be less than you'd think. Both models feature 550 lf-ft of maximum "Nut-Busting" torque as Milwaukee calls it. However, the M18 has a significant advantage in "Fastening" (running) torque with its 600 lb-ft compared to the M12's 400 lb-ft of torque.