Estimate how long a battery will last based on its capacity and your device's current or power draw, including a real-world efficiency factor.
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This tool estimates how long a battery will last before it discharges, by dividing its capacity (in milliamp-hours, mAh) by the device's current consumption rate (in milliamps). If the device's consumption is known in watts instead of milliamps, the tool first converts it to an equivalent current using the battery voltage (I = P ÷ V). Because real batteries don't discharge their rated capacity at 100% efficiency — due to the gradual voltage drop during discharge, internal resistance losses, and performance changes with temperature and load — an "actual efficiency percentage" (85% by default, adjustable) is applied to reduce the capacity used in the calculation, reflecting real-world performance instead of just the theoretical number printed on the battery. This distinction between rated and usable capacity matters most for anyone designing a battery-powered device or estimating realistic runtime, since relying on the printed rating alone consistently overestimates actual field performance.
A battery rated at 2000 mAh sounds like it should deliver exactly 2000 milliamp-hours of usable current before dying, but real-world runtime almost always falls short of that theoretical figure — a gap that surprises many people until they understand what the rated capacity actually represents versus how batteries behave in practice.
The mAh rating printed on a battery is measured under controlled laboratory conditions: typically a slow, steady discharge rate at room temperature, conditions that rarely match how a device actually draws power in real use. Real devices often draw current in variable bursts, operate across a range of temperatures, and continue drawing power even as the battery's voltage sags toward the end of its discharge cycle — all factors that reduce the effectively usable capacity below the rated number.
Internal resistance is a major contributor to this gap. Every battery has some internal resistance that converts a portion of its stored energy into heat rather than delivering it to the device, and this effect becomes more pronounced at higher discharge currents — a battery discharged quickly loses proportionally more capacity to internal heating than the same battery discharged slowly, which is part of why high-drain devices see a larger gap between rated and actual capacity.
Temperature also plays a significant role: battery chemistry, particularly in common lithium-ion and alkaline cells, performs measurably worse in cold conditions, sometimes delivering 20-30% less usable capacity at low temperatures compared to room-temperature performance, which is why devices used outdoors in winter often show noticeably shorter battery life than the same device used indoors.
The efficiency factor used in this calculator — commonly around 85% as a reasonable planning estimate — accounts for this combined real-world gap between theoretical and actual usable capacity. Engineers designing battery-powered products routinely apply a similar derating factor when estimating runtime for a new design, since promising a runtime based on the raw rated capacity alone would set unrealistic expectations that the finished product would then fail to meet in typical use.
Real batteries lose some effective capacity to internal resistance, voltage sag under load, temperature effects, and self-discharge, so applying a realistic efficiency factor (commonly 80-90%) gives a more accurate real-world estimate than the theoretical maximum.
Check the device's datasheet or specifications for its rated current/power consumption, or measure it directly with a multimeter or USB power meter if available.
Voltage itself doesn't appear in the basic capacity-over-current formula, but it's needed to convert a power (Watts) specification into an equivalent current draw, since Power = Voltage × Current.