Battery Life Calculator
Estimate how long a battery will run a device from its capacity and current draw.
About the Battery Life Calculator
Enter the battery's capacity in mAh, the device's average current in mA and the share of capacity you can actually use. The calculator gives the run time in hours and days.
Run time is capacity divided by current, reduced for usable capacity. High currents and cold temperatures reduce usable capacity, especially for alkaline cells (the Peukert effect). For devices that sleep, use the average current over a full cycle.
The formula
Hours = capacity × usable % ÷ current
Worked example
A 3,000 mAh battery powering a 150 mA device, 85% usable
- Usable: 3,000 × 0.85 = 2,550 mAh
- 2,550 ÷ 150 = 17 hours
Questions people ask
What about watt-hours?
Wh = mAh × voltage ÷ 1000; use Wh to compare batteries of different voltages.
Why doesn't my phone match?
Its current varies widely with screen and radio use.
How do I find a sleeping device's average current?
Weight each current by its share of time: 10 mA for 1% of the time and 0.05 mA otherwise averages about 0.15 mA.