Pack Runtime Calculator

Estimate how long a cordless tool pack will run your tool, from its volts and amp-hours.

Amp-hours alone don't tell you how much energy a pack holds. Multiply by the voltage to get watt-hours: Ah × V = Wh. This gives an idealised runtime estimate from that. Everything is worked out in your browser. Nothing is saved or sent anywhere.

1. Your battery pack

Use the voltage and amp-hour rating printed on the pack. "Max" voltages (such as 20V Max) are the fully charged figure; the nominal voltage is lower and is what's used here.

100% for a new pack. An older pack might hold 70–80%, or use a capacity test result.

2. Your tool and how you use it

Tools rarely draw their full rated power all the time. Enter the average watts while the trigger is pulled, and how much of your working time the tool is actually running.

A guess is fine. Light drilling is far less than cutting or grinding under load.
Of your working time. 100% means running non-stop, like a fan or light.

3. Results

How it was worked out

    What this does and doesn't tell you

    • This is an idealised estimate. Real runtime is usually shorter, and it varies with cold weather, heavy loads, pack age and the pack's low-voltage cut-off.
    • Heavy loads drain a pack faster than the watt-hours suggest, because the cells heat up and their voltage sags. The tool or pack may also cut out early to protect itself.
    • Comparing packs across brands? Compare watt-hours, not amp-hours. A 4Ah 36V pack holds twice the energy of a 4Ah 18V pack.
    • Lithium-ion tool packs can short, overheat and catch fire if they're damaged, wired wrongly or charged on the wrong charger. Only use the maker's charger for the pack, and recycle any pack that is swollen, burnt, leaking, crushed or has been in water.

    Next steps

    Pack running out much sooner than this? Read Why Won't My Pack Charge? and Battery Testers for the Bench, then work out a recharge with the Pack Charge Time Calculator.

    ↑