Battery Life Calculator
Enter a battery’s rated capacity and your circuit’s current draw to estimate how many hours (and days) it will run.
Estimated runtime
Reading a battery’s mAh rating
A milliamp-hour (mAh) rating tells you how much charge a battery can deliver over time — a 2000mAh battery can, in theory, supply 2000mA for one hour, 1000mA for two hours, or 100mA for twenty hours. Dividing the mAh rating by your circuit’s current draw in mA gives the ideal runtime in hours.
Real devices rarely reach that ideal figure, which is why this calculator applies a default 0.85 efficiency factor to model the usable capacity before a device’s low-battery cutoff kicks in. Set it to 1 to see the theoretical maximum instead.
Frequently Asked Questions
How is battery life calculated?
The basic formula divides the battery's rated capacity (in milliamp-hours, mAh) by the current your circuit draws (in milliamps, mA): hours = capacity ÷ current. A 2000mAh battery powering a 100mA load lasts about 20 hours by this ideal figure.
Why does the calculator apply a derating factor?
A battery's printed mAh rating is measured under specific lab conditions and assumes you use it down to a low cutoff voltage. In practice, most devices stop working before the battery is fully drained, and capacity drops with age, temperature, and higher current draws. A common rule of thumb is to plan for about 85% of the rated capacity, which this calculator uses as its default.
Does a higher current draw shorten battery life more than expected?
Often yes — this simple mAh ÷ mA formula assumes capacity stays constant regardless of load, but real batteries deliver somewhat less total capacity at high discharge rates (a behavior described by Peukert's law for some chemistries). For a rough estimate at typical hobbyist currents, the linear formula is a reasonable starting point.
Where do I find a battery's mAh rating?
It's usually printed directly on the battery or its packaging. Alkaline AA batteries are commonly rated around 1800–2700mAh, while a typical single-cell lithium-ion battery ranges from a few hundred to a few thousand mAh depending on its size.
Educational estimate only. Actual runtime varies with battery chemistry, age, temperature, and how steady the current draw is — always follow the battery manufacturer’s handling and disposal guidance.