About this tool
Battery Life Calculator is a free, in-browser tool that estimates how long a battery will run a given load. Enter the capacity and the current or power the device draws, and it returns the run time in hours plus a days / hours / minutes breakdown. Nothing is uploaded — every figure is computed locally as you type.
In mAh mode the run time is (capacity in mAh ÷ load in mA) × derating; in Wh mode it is (capacity in Wh ÷ power in W) × derating. The derating factor (default 0.7) accounts for real-world losses — voltage sag, temperature, converter inefficiency and unusable reserve capacity — because a battery rarely delivers its full rated capacity.
Use it to size a battery for a project or to sanity-check a manufacturer's runtime claim. Lower the derating factor toward 0.5 for demanding or cold conditions, or raise it toward 0.9 for a fresh cell driving a light, steady load.
Frequently asked questions
What is the derating factor and why default to 0.7?
It is the fraction of rated capacity you actually get in use. Real batteries lose capacity to voltage drop, heat, converter inefficiency and reserve you cannot safely use, so 0.7 (70%) is a common, conservative planning figure. Set it to 1 to see the ideal, loss-free run time.
When should I use Wh/W instead of mAh/mA?
Use Wh and W when the device voltage differs from the battery voltage, such as anything behind a DC-DC converter or inverter. Energy in watt-hours cancels the voltage, so Wh ÷ W gives hours directly without needing to match voltages.
Why does mAh ÷ mA give hours?
Milliamp-hours divided by milliamps leaves hours: 3000 mAh ÷ 250 mA = 12 hours at full capacity. Multiplying by the derating factor gives the realistic figure, here 12 × 0.7 = 8.4 hours.
Is my data sent anywhere?
No. The calculation runs entirely in your browser; capacities and loads never leave your device, and the tool works offline.
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