How long will your battery last?

What the Battery Life Calculator Does

This Battery Life Calculator estimates how long a battery can power a device before it runs flat. You enter the battery capacity in milliamp-hours (mAh) and the device's current draw, or load, in milliamps (mA), and it returns the expected runtime in hours.

It is built for hobbyists, electronics students, and makers working on projects like Arduino boards, LED strips, drones, flashlights, RC cars, and portable IoT sensors. If you are choosing a battery pack or trying to predict how often you'll need to recharge, this tool gives you a fast first answer.

How It Works: The mAh Formula

Battery life is calculated by dividing capacity by load. Both values use the same unit (milliamps), so the units cancel and you are left with hours:

Battery life (hours) = Capacity (mAh) / Load (mA)

Capacity in mAh tells you how much charge a battery holds: a 2000 mAh cell can theoretically supply 2000 mA for one hour, 1000 mA for two hours, or 100 mA for twenty hours. The load is how fast your device pulls that charge. The result is an ideal estimate that assumes the battery delivers its full rated capacity at a steady draw.

Worked Example

Suppose you have a 3000 mAh lithium battery powering a small device that draws a steady 250 mA. Plug the numbers in:

3000 mAh / 250 mA = 12 hours.

Now imagine you add a brighter display that pushes the draw up to 600 mA. The runtime drops sharply: 3000 / 600 = 5 hours. Doubling and tripling the load cuts your runtime by the same proportion, which is why reducing current draw is often the easiest way to extend battery life.

Factors That Affect Real Runtime

The formula gives an ideal, best-case number. In practice you will usually get less, because several real-world factors are at play:

• Discharge efficiency: most batteries cannot deliver 100% of their rated capacity, so a common rule of thumb is to multiply the result by about 0.7 to 0.85 for a realistic figure.
• Temperature: cold weather reduces usable capacity, while heat can shorten battery lifespan.
• Age and cycles: an older battery holds less charge than its label suggests.
• Variable load: devices that switch between sleep and active modes draw an average current, not a constant one.
• Voltage and converters: step-up or step-down regulators waste some energy, increasing the effective draw on the battery.

Tips and Common Mistakes

The most frequent error is mixing units. Capacity is in mAh and load must be in mA; if your datasheet lists current in amps, multiply by 1000 first (0.4 A = 400 mA). Similarly, a battery rated in amp-hours (Ah) converts the same way: 2.5 Ah = 2500 mAh.

Another mistake is confusing mAh with watt-hours (Wh). mAh alone ignores voltage, so it only compares batteries of the same voltage. To compare across voltages, use Wh = (mAh / 1000) x volts. Finally, treat the calculated hours as a planning estimate, not a guarantee, and always size your battery with some headroom.