Battery Life Calculator
Estimate how long a battery lasts from its capacity and the current your device draws
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Estimates how long a battery runs a device, including devices that sleep most of the time, and converts mAh to Wh, sizes a battery and counts power bank charges.
How to use
- On "Runtime", enter the "Battery capacity" and the "Device consumption"; the runtime appears at once.
- If the units differ (mAh and W), enter the "Battery voltage".
- Change the "Real-use factor" if you know how much of the capacity is usable.
- For a device that wakes and sleeps, use "Intermittent use" and press "Add phase" for each phase.
- "Convert and size" covers mAh and Wh, capacity needed, power bank charges and series or parallel packs.
How it works
- Runtime (h) = capacity (mAh) × factor ÷ current (mA). Energy (Wh) = mAh × V ÷ 1000.
- The factor (0.8 by default) and the power bank efficiency (0.65) are rough assumptions you can change.
- Average current = sum of current × time per phase ÷ total time. Phases are saved in this browser.
- Results are estimates. Charging and protection circuits are not covered.
Questions and answers
- How long will a 2000 mAh battery last at 100 mA?
- 20 hours in theory; with the default factor of 0.8 the page estimates 16 hours.
- How do I convert mAh to Wh?
- Multiply by the battery voltage and divide by 1000. A 10000 mAh power bank rated at 3.7 V stores 37 Wh.
- How many times can a 10000 mAh power bank charge my phone?
- With a 4000 mAh phone battery at 3.85 V and efficiency 0.65, about 1.6 times; the page compares in Wh.
- What happens to voltage and capacity in series and parallel?
- In series voltages add and capacity stays the same; in parallel capacity adds and voltage stays the same. Energy adds in both.