Pack math
Pack energy and a runtime ceiling
Two lines of arithmetic, done for you. Energy is the pack's nominal voltage times its amp-hours. Divide by the load and you get the longest the pack could possibly run it. Real runtime is shorter, and this page will not pretend to know by how much.
Why the volts look low
Makers sell packs by their maximum voltage: 20V MAX, 40V, M12. Energy math uses the nominal voltage, which is lower: 18 V for DeWalt 20V MAX, 36 V for Ryobi 40V and 10.8 V for Milwaukee M12. A FLEXVOLT pack runs at 60V MAX in a 60V tool and at 20V MAX in a 20V tool, and its energy is the same either way, but the label lists a different amp-hour figure for each: type the 60V-mode figure when you pick FLEXVOLT here. If your pack's label lists watt-hours, trust the label; you can type its numbers straight in.
Where the load comes from
Cordless makers seldom publish running watts, and the draw swings with the cut, the bit and the feed. Treat the load as your own estimate, and use the ceiling to compare packs against each other, not to plan a day.
DeWalt shelf · M12 shelf · Ryobi 40V shelf · Packs side by side