Battery Life Calculator
Estimate how long a battery will run a device from its capacity (mAh or Wh), the device's current or power draw, and usable capacity.
How to use
- 1Enter the battery capacity and voltage.
- 2Enter the device's average current (mA) or power (W).
How it's calculated
Runtime (h) = capacity (mAh) × usable fraction ÷ load (mA). With watts: runtime = Wh × usable fraction ÷ W, where Wh = mAh × V ÷ 1000.
Frequently asked questions
Why doesn't my battery last as long as calculated?
Real capacity drops with high current, cold temperatures and age, and converters waste some energy. The 'usable' setting accounts for this.
How do I convert mAh to Wh?
Wh = mAh × voltage ÷ 1,000. A 10,000 mAh power bank at 3.7 V holds 37 Wh.
How do I find a device's current draw?
Check its specs, or measure it with a USB power meter or multimeter; use the average for devices that sleep and wake.