Battery life calculator
Enter the battery capacity in milliamp-hours, the average current the device draws in milliamps, and the share of the rated capacity you expect to use. The ideal runtime is capacity divided by current; the realistic runtime applies your usable-capacity factor.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the battery capacity from its label, in mAh. For ampere-hours, multiply by 1000.
- Enter the average current the device draws, in mA.
- Leave the usable capacity at 80 % or set your own assumption.
- Read the estimated runtime in hours and days.
Runtime (h) = capacity (mAh) ÷ current (mA) × usable capacity (%) ÷ 100
Worked example
A 2000 mAh battery powers a sensor that draws 100 mA. Ideal runtime is 2000 ÷ 100 = 20 h. At 80 % usable capacity it is 20 × 0.8 = 16 h, which is about 0.67 days.
Tips
- Use the average current over a full cycle, including sleep periods, not the peak.
- A low-power device that sleeps most of the time can have an average far below its active current.
Frequently asked questions
How do you calculate battery life?
Divide the capacity in mAh by the average current in mA to get hours. A 2000 mAh battery powering a 100 mA load lasts 2000 ÷ 100 = 20 hours in theory. This tool then multiplies by the usable-capacity factor you set.
What usable-capacity value should I choose?
The default of 80 % is a rule of thumb, not a measured figure. Real batteries rarely deliver the full rated capacity because of age, cold, high discharge current, the cut-off voltage of the device and converter losses. Values between about 70 and 85 % are common assumptions; use 100 % for the ideal figure.
Why is my real battery life shorter than the result?
The calculator assumes a constant average current. Devices that spike when a screen, radio or motor switches on use more than the average you enter, and cold weather and old cells lower the usable capacity. Measure the current if you can, and lower the percentage for a safer estimate.
Does the battery voltage matter?
For a device that draws a known current directly from the battery, no. If you know the power in watts instead, convert to current as watts ÷ volts first, or work in watt-hours with the mAh to Wh converter.
Can I use it for several cells?
Cells in series add voltage but keep the same mAh. Cells in parallel add their mAh. Enter the resulting pack capacity and the current drawn from the pack.