How is battery life calculated?

Capacity (mAh) divided by draw (mA) gives hours, times efficiency.

Why does it last less than calculated?

Efficiency isn't 100%, and cold and aging reduce capacity.

Battery Life Calculator

Estimate a device's battery life from battery capacity (mAh) and current draw.

Battery capacity: 4,000 mAh

Current draw: 500 mA

Efficiency: 85 %

6.8 h

Battery life

Battery life	6.8 h
Life in minutes	408 min

ℹ️ Life = capacity / draw × efficiency. In reality temperature and battery age reduce it.

The battery life calculator estimates how long a device will run from its battery capacity and current draw. It suits phones, power banks, drones and electronics projects where you need a quick runtime figure.

How the calculator works and what it’s for▾

The formula in brief

Runtime is the battery capacity divided by the current draw, multiplied by the efficiency. With capacity in milliampere-hours and draw in milliamperes, the result comes out in hours.

For example, a 4000 mAh battery drawing 500 mA would ideally last 8 hours. At 85 percent efficiency the realistic estimate drops to about 6.8 hours.

What you enter and what you get

You enter the battery capacity in milliampere-hours, the device's current draw in milliamperes and an efficiency in percent. The result shows the estimated runtime in both hours and minutes.

Efficiency accounts for losses in voltage conversion and heat. A realistic value is often around 80 to 90 percent, which trims the theoretical runtime a little.

When it helps

The calculator is useful for comparing batteries, sizing a power bank or estimating how long field sensors will keep running. It is also handy for hobbyists and DIY electronics builds.

Good to know

The figure is indicative only. In reality temperature, battery ageing and fluctuating load shorten runtime noticeably. Treat the estimate as a starting point rather than a guarantee.

🔄 Reviewed June 2026