Battery Life Calculator
Enter a battery capacity and the current your device draws to get the ideal run time in hours and minutes — and learn why real-world battery life is shorter.
Hours and minutes at once
Enter the capacity in mAh and the load current in mA and the calculator returns the run time in hours, plus the same figure in minutes.
Match your units
Use milliamp-hours (mAh) for capacity and milliamps (mA) for the draw — if your capacity is in Ah, multiply by 1000 first.
What is a battery life calculator?
Capacity divided by current draw
A battery life calculator turns two numbers — the battery capacity in milliamp-hours (mAh) and the current the device draws in milliamps (mA) — into how long the device will run. It is the figure behind every "lasts up to N hours" claim: divide the charge stored in the cell by the rate at which the device uses it and you get the run time. Capacity tells you how much energy is in the tank; the load current tells you how fast it empties. The calculator reports the result in hours and, for short-lived loads, the same time in minutes.
Enter a capacity in mAh and a load current in mA to get the ideal battery life in hours and minutes instantly.
Battery life is the capacity divided by the load current, with the minutes figure being the hours multiplied by 60.
Battery life (h) = capacity (mAh) ÷ load current (mA)Suppose a 3000 mAh battery powers a device that draws 200 mA. Dividing 3000 by 200 gives 15 hours of ideal run time, and multiplying by 60 gives 900 minutes. Halve the current draw to 100 mA and the battery lasts twice as long; double it to 400 mA and it lasts half as long. Run time is simply capacity shared out over the rate of use, so a bigger battery or a lower draw both stretch it.
The formula is exact, but it describes an ideal that real hardware never quite reaches.
Real-world runtime is shorter than the ideal
This calculator assumes a constant current draw and a battery that delivers its full rated capacity. In practice, expect roughly 70–85% of the figure: voltage regulators and conversion lose energy, cold or hot temperatures cut usable capacity, and the discharge curve means the cell cannot give up every last mAh. Treat the result as an upper bound — useful for comparing devices, not a guaranteed run time. Keep your units consistent (mAh and mA), or convert amp-hours to mAh by multiplying by 1000 first.