Battery Life Calculator
Estimate runtime from battery capacity and device draw — mAh or Wh.
The Battery Life Calculator estimates how long a battery powers a device: enter capacity and draw in the same unit pair — mAh and mA for small electronics, Wh and W for laptops, power banks and UPS units — and read the runtime in hours and days.
Runtime math is a simple division: a 3,000 mAh phone battery feeding a 300 mA average draw lasts 10 hours; a 50 Wh laptop battery under a 10 W load lasts 5. The tool handles both unit systems because real-world questions come in both — phones and earbuds in mAh, power tools and batteries in Wh.
Real devices draw intermittently (radios burst, processors idle), so practical life often exceeds a constant-load estimate. Treat the figure as the planning baseline. Runs locally in your browser.
Estimated runtime
10 h 0 m
3000 mAh @ 300 mA
In days
0.4
continuous operation
Runtime = capacity ÷ draw (mAh/mA or Wh/W). Real devices draw intermittently (sleep, radio bursts), so practical battery life often exceeds a constant-load estimate — and never exactly matches marketing numbers. Voltage matters too: Wh = Ah × V.
How to use the Battery Life Calculator
- Choose the unit system: mAh + mA, or Wh + W.
- Enter the battery capacity.
- Enter the average device draw.
- Read the runtime in hours and minutes.
- Check the days view for always-on devices.
Frequently asked questions
What is the formula for battery life?
Runtime = capacity ÷ draw, in matching units: 3,000 mAh ÷ 300 mA = 10 hours, or 50 Wh ÷ 10 W = 5 hours. Convert mA to A and mAh to Ah for consistent math in other formulas.
What is the difference between mAh and Wh?
mAh measures charge; Wh measures energy. Wh = (mAh ÷ 1000) × voltage. Wh is the honest unit for comparing batteries at different voltages — two 5,000 mAh packs at 3.7 V and 12 V store very different energy.
Why does real battery life differ from the estimate?
Devices rarely draw a constant current — radios, screens and processors pulse — and batteries lose capacity as they age and in the cold. The estimate is the ideal constant-load baseline.
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