Battery Life Calculator

Divide the capacity by the current draw, both in the same milli unit, then multiply by an efficiency factor. In full: hours = (mAh / mA) times (efficiency / 100). At 100% efficiency a 2000 mAh pack driving 200 mA lasts 2000 divided by 200, which is 10 hours.

Rated capacity is measured under an ideal, slow discharge at a friendly temperature. In real use, voltage regulators waste some energy, cells warm up, and a faster discharge yields less than the label promises. The default 85% derating turns the optimistic textbook figure into a more honest estimate.

For everyday electronics 80 to 90 percent is a reasonable range, so 85% is a sensible default. Set it to 100% only when you want the theoretical ceiling for comparison. Very high-drain or cold-weather use can justify a lower figure, while a well-matched, gently loaded circuit can sit near the top of the range.

Use the average current across the whole cycle you care about, not the peak. A device that idles most of the time and only bursts occasionally will outlast what its peak draw suggests, so measure or estimate an average over a realistic period of use.

Not for a runtime in hours. Because capacity in mAh and draw in mA already assume the same nominal voltage, the volts cancel out of the division. Voltage only matters when you convert to energy in watt-hours or compare packs of different voltages.

The decimal hours are converted to total minutes and rounded to the nearest minute before splitting into hours and minutes. So 8.5 hours reads as 8h 30m, and a value like 9.4444 hours rounds to 9h 27m. The decimal figure beside it keeps more precision.

Capacity and current draw must be greater than zero, and efficiency must be between 0 and 100 percent. A zero current draw would imply infinite runtime, which is not a meaningful answer, so the tool asks for a positive value instead.

Treat it as a planning estimate, not a guarantee. Real runtime shifts with temperature, battery age, discharge rate and how bursty the load is, so measured life can land above or below the number. It is most useful for comparing options and sizing a battery, not for predicting the exact minute a device dies.

No. The math runs entirely in your browser, so the capacity, current and efficiency you enter never leave your device and nothing is stored.