Battery Life Calculator
Estimate battery runtime from capacity and load.
Accounts for losses; default 85%
Results update as you type.
How it works
This calculator estimates how long a battery will power a device from the battery's capacity and the device's current draw. You enter the capacity in milliamp-hours, the load in milliamps, and an efficiency factor, and it returns the estimated runtime in hours. The basic idea is simple: capacity in milliamp-hours divided by current in milliamps gives hours of runtime, since a milliamp-hour is exactly one milliamp flowing for one hour. So a 3000 milliamp-hour battery driving a 200 milliamp load lasts about 15 hours in the ideal case. Real batteries fall short of this, though, so the efficiency factor, typically around 80 to 90 percent, trims the estimate to account for losses in voltage regulators, heat, self-discharge, and the way batteries deliver less than their rated capacity under real loads and at temperature extremes. This gives a more realistic runtime than the raw division. The estimate is useful for sizing batteries in electronics projects, gauging how long a phone, drone, or IoT device will run, and comparing battery options. For accuracy, use the actual average current your device draws, since it often varies with activity. Enter the capacity, load, and efficiency to estimate battery life.
Runtime = (capacity / load) × efficiency
Battery runtime is the capacity in milliamp-hours divided by the current draw in milliamps, which gives the ideal hours, multiplied by an efficiency factor for real-world losses. Efficiency accounts for voltage conversion, heat, and the fact that batteries do not deliver their full rated capacity under load, so it trims the ideal figure to a realistic estimate.
Source:Electronics
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter battery capacity and device load.
- 2
Adjust the options
Adjust efficiency if they apply — sensible defaults are already set.
- 3
Read your result
Estimated runtime updates instantly as you type — no button to press.
Frequently asked questions
How do I calculate battery life?
Divide the battery capacity in milliamp-hours by the device's current draw in milliamps to get the ideal runtime in hours, then multiply by an efficiency factor for real-world losses. A 3000 milliamp-hour battery at 200 milliamps gives 15 ideal hours, about 12.75 at 85 percent efficiency. The calculator does this for you.
What is a milliamp-hour?
A milliamp-hour is a unit of charge equal to one milliamp of current flowing for one hour. Battery capacity in milliamp-hours tells you how much charge the battery holds, so dividing capacity by the current draw gives how many hours that current can flow, which is the basis of the runtime estimate.
Why apply an efficiency factor?
Batteries never deliver their full rated capacity in practice. Voltage regulators lose energy as heat, self-discharge drains charge, and capacity drops under heavy loads and at hot or cold temperatures. An efficiency factor of around 80 to 90 percent trims the ideal figure to reflect these losses, giving a more realistic runtime estimate.
What current draw should I use?
Use the average current your device actually draws during use, not its peak. Many devices vary their consumption with activity, so an average over a typical usage cycle gives the best estimate. If you only know the peak, the runtime will be underestimated; if you use idle current, it will be overestimated.
Does higher capacity always mean longer life?
For the same load, yes: a higher-capacity battery lasts proportionally longer. But runtime also depends on the current draw, so a power-hungry device can drain a large battery quickly. To extend life, you can either increase capacity or reduce the device's current consumption, and both together give the best results.