Device Battery Life Calculator
Estimate battery runtime from capacity and current draw.
Average current the device draws while running.
Results update as you type.
How it works
A device battery life calculator estimates how long a battery will power a device from two figures: the battery's capacity in milliamp-hours (mAh) and the device's average current draw in milliamps (mA). This simple ratio underlies runtime estimates for phones, tablets, wearables, drones, flashlights, and countless electronics projects, giving a quick sense of how long a charge should last under a given load. The estimate is useful for consumers comparing devices and for makers designing their own. Someone choosing a power bank wants to know how many phone charges it delivers; a hobbyist building an Arduino or Raspberry Pi project needs to size a battery for a target runtime; a drone pilot estimates flight time before the pack runs down. Because runtime scales directly with capacity and inversely with draw, the calculator makes the trade-off between a bigger battery and a hungrier device immediately clear. The calculation works because milliamp-hours measure stored charge and milliamps measure the rate charge is consumed, so dividing capacity by draw leaves a result in hours. Real batteries fall short of this ideal: efficiency losses in voltage conversion, capacity that fades with age and cold, and draw that varies with activity all reduce actual runtime, typically to 70 to 90 percent of the theoretical figure. Treat the result as an optimistic upper bound. Consumers, hobbyists, and engineers use a battery life calculator to estimate and compare device runtimes.
runtime (hours) = battery capacity (mAh) ÷ current draw (mA)
Battery capacity in milliamp-hours states how much charge the cell holds: a milliamp-hour is the charge delivered by one milliamp flowing for one hour. Dividing the capacity by the current the device draws in milliamps gives the number of hours that current can flow before the charge is exhausted. The units cancel cleanly because milliamp-hours divided by milliamps leaves hours, making this a direct estimate of runtime at a steady draw.
Source:Battery capacity and current relationship (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 current draw.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Estimated runtime updates instantly as you type — no button to press.
Frequently asked questions
What does mAh mean on a battery?
mAh stands for milliamp-hours, a measure of how much electric charge a battery stores. One milliamp-hour is the charge delivered by a current of one milliamp flowing for one hour. A 4,000 mAh battery can, in theory, supply 4,000 milliamps for one hour, or 500 milliamps for eight hours. A higher mAh rating means more stored energy at a given voltage, and therefore longer runtime for the same device.
Why is real battery life shorter than calculated?
Several losses reduce actual runtime below the theoretical figure. Voltage regulators and converters waste some energy as heat. A device's current draw varies with activity, often exceeding the average you enter during bursts. Batteries lose capacity as they age and in cold temperatures, and manufacturers rate capacity optimistically. Together these mean real runtime is usually 70 to 90 percent of the calculated value, so treat the result as an optimistic ceiling.
How do I find my device's current draw?
Check the device's specifications or datasheet, which often list typical and peak current consumption. For electronics projects, a multimeter or USB power meter placed in the circuit measures actual draw directly, which is the most accurate method. Draw varies with what the device is doing, so use an average that reflects your real usage. If only power in watts is given, divide it by the battery voltage to estimate current.
Does voltage matter for this calculation?
This simple formula assumes the battery and the device's rated draw share the same voltage, which holds when comparing a device to its own battery. If voltages differ, or you are converting between them, you should work in energy terms using watt-hours instead, found by multiplying capacity in amp-hours by voltage. For matched-voltage estimates, dividing mAh by mA gives hours directly, which covers most everyday comparisons.
How can I extend my device's battery life?
Reduce the current draw, since runtime is inversely proportional to it. Lower screen brightness, disable unused radios like Bluetooth and GPS, close background apps, and enable power-saving modes. For projects, put microcontrollers into sleep between tasks, which slashes average draw dramatically. Keeping the battery at moderate temperatures preserves capacity. A larger-capacity battery also helps, but cutting the load is often the most effective and immediate improvement.