The capacity specifies how much electricity a rechargeable battery stores, and is usually printed on the outside of the battery. For small rechargeable batteries, this is usually given in milli-ampere-hours (mAh), and in ampere-hours (Ah) for large batteries.

A wide variety of factors determines how long a battery lasts. In a camera, using the zoom, flash and LCD display will use extra power and shorten the run time of the battery. By using the run time you're getting from your current battery, the calculator considers your personal usage pattern when calculating.

This estimate assumes that your old battery is still in good working order, and that your usage pattern is the same when using the old or the new battery.