Power Bank Charge Calculator

About this calculator

A power bank's printed capacity (say, 20,000mAh) is measured at the internal battery's own voltage, usually around 3.7V, but it has to boost that up to 5V (or higher, for fast charging) to push power out through a USB port. That voltage conversion is not free: some energy is lost as heat in the process, and typical real-world conversion efficiency runs somewhere around 70–90% depending on the power bank's quality and the charging speed used.

This calculator takes the power bank's rated capacity, your device's battery capacity and a conversion efficiency, and divides the usable output energy by what your device actually needs to fill up, so you can see how many real full charges to expect — not the inflated number you would get from naively dividing the two mAh ratings on the box.

It is useful before a trip or a camping outing when you are deciding how large a power bank to buy, or when you already own one and want to know whether it will realistically get your phone through a long day away from an outlet.