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Battery Life Calculator

Enter the battery capacity, the device’s current draw and the usable share.

mAh
mA

From the device’s datasheet. For a power rating in watts, divide by the voltage to get amps.

%

Real batteries rarely give their full rated capacity. About 80% to 90% is typical.

This result is an estimate only. Double-check important figures before relying on them.

How it is calculated

  • Run time (hours) = capacity (mAh) × usable % ÷ current (mA)
  • Days = hours ÷ 24

For electrical power, use the watts, amps and volts calculator.

Worked example

A 5,000 mAh battery, 85% usable, powering a 250 mA load:

  • Run time = 5,000 × 0.85 ÷ 250 = 17 hours

Why your number may differ

Cold temperatures, battery age, and a draw that changes with use all reduce real run time. Treat the answer as a rough guide.

Frequently asked questions

How do I calculate battery life?

Divide the battery capacity in mAh by the current the device draws in mA. A 5,000 mAh battery powering a 250 mA load lasts about 20 hours before allowing for losses.

Why use a usable percentage?

Batteries lose capacity to heat, conversion and low-voltage cut-off, and the draw can change. Using 80% to 90% gives a more realistic estimate.

How do I convert watts to mA?

Divide watts by the battery voltage to get amps, then multiply by 1,000. 3.7 W from a 3.7 V cell is 1 A, or 1,000 mA.

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