Battery Runtime Calculator

Estimate how long a battery lasts for a given current draw, with an allowance for the capacity you can never really use.

Battery capacity (mAh)

Average current draw (mA)

Usable capacity (%)

Estimated runtime --

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The formula

hours = (capacity_mAh x usable%) / current_mA

Example:
2000 mAh at 80% usable, drawing 80 mA
= 1600 / 80 = 20 hours

How it works

Printed capacity is measured under laboratory conditions at a gentle discharge rate. Real circuits do not get all of it, which is what the usable percentage is for — 80% is a reasonable default, and lower if the device must keep working as the voltage sags.

Treat the answer as an upper bound. Current is rarely constant: a board that sleeps between radio transmissions has an average draw far below its peak, and averaging correctly matters more than this calculator does. Capacity also falls in the cold and declines as the cell ages.

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