Battery Runtime (mAh) Calculator
Estimate how long a battery lasts under a given load, from its mAh capacity, voltage and the current your device draws — with real-world derating.
How it works
Runtime (h) = capacity(mAh) × usable% ÷ load(mA) · Energy = mAh × volts ÷ 1000 (Wh) · Power = mA × volts ÷ 1000 (W)
A battery’s capacity in milliamp-hours divided by the current a device pulls gives an ideal runtime in hours — 5,000 mAh at 500 mA is nominally ten hours. Real cells never deliver their full rating, though: internal resistance, temperature, age and the cutoff voltage cost 10–20%, so a usable-capacity factor makes the estimate honest. Multiplying capacity by voltage converts mAh into watt-hours, the fairer way to compare packs of different voltages.
Worked example
A 5,000 mAh cell driving a 500 mA load runs 5,000 ÷ 500 = 10 hours ideally. Applying an 85% usable factor gives 8.5 hours. At 3.7 V the pack stores 18.5 Wh and the load draws about 1.85 W.
Frequently asked questions
Why is real runtime shorter than mAh ÷ load?
The simple division assumes a perfect battery delivering every rated milliamp-hour down to zero. In practice voltage sag, heat, cell age and the device’s low-voltage cutoff leave some capacity unused, which the usable-percentage factor accounts for.
How do I compare batteries of different voltages?
Convert to watt-hours: mAh × volts ÷ 1000. A 5,000 mAh pack at 3.7 V (18.5 Wh) holds less energy than a 5,000 mAh pack at 7.4 V (37 Wh), even though the mAh number is identical.
Does a higher load reduce total capacity?
Often, yes — the Peukert effect means very high discharge currents extract fewer usable amp-hours than gentle ones. For moderate loads this calculator’s derating covers it; for heavy loads relative to capacity, lower the usable percentage.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.