Estimate how long a battery will run a device: capacity in mAh divided by draw in mA, with a real-world efficiency factor built in.
How runtime is estimated
On paper, runtime is simple: battery capacity divided by the device’s current draw. In practice, conversion losses, voltage sag and capacity fade mean real devices never hit the paper number — so this calculator applies an efficiency factor (0.85 by default, a sensible engineering rule of thumb) and shows both the ideal and the realistic figure side by side.
How to use it
- Enter the battery capacity in mAh (printed on the battery or in the spec sheet).
- Enter the device’s average draw in mA.
- Adjust the efficiency factor if you know your setup’s losses.
The formula
A worked example
A 3,000 mAh battery feeding a device that draws 250 mA: the ideal figure is 12 hours, and at 85% efficiency the realistic estimate is 10.2 hours — 10 h 12 min. If the device’s draw varies (screens, radios and motors all spike), use its average draw; a plug-in USB meter or the spec sheet’s typical-use figure is the honest source.