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Battery mAh Runtime Calculator
Estimate how long a battery lasts from its capacity, the device’s current draw and efficiency.
Battery capacity
Device current draw
Efficiency
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How it works
Runtime (h) = capacity (mAh) ÷ draw (mA) × efficiency
Runtime is capacity divided by current draw, trimmed by an efficiency factor for conversion losses and the fact that batteries don’t deliver 100% of rated capacity under load.
Worked example
A 5,000 mAh battery at a 500 mA draw and 85% efficiency lasts about 8.5 hours.
Frequently asked questions
Why apply an efficiency factor?
Voltage conversion and heat waste some energy, and real capacity under load is below the rating, so 80–90% is realistic.
Does a higher draw shorten it proportionally?
Roughly, yes — double the current draw roughly halves the runtime.
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This calculator is for educational purposes. Double-check important results before acting on them.