ConverterHub

Power Bank Charge Calculator

Enter the power bank and device battery capacities to see how many full charges you get.

How it is calculated

Both capacities are in mAh, and efficiency is a percentage. The usable capacity is usable = bank × η / 100, and the number of full charges is charges = usable / device. Here bank is the printed power bank capacity, device is the battery capacity of the phone or gadget, and η is the share of the power bank's energy that actually reaches the device. The tool does a straight proportion in mAh and does not convert between voltages itself, so the efficiency you enter has to cover all the losses. The result is shown to two decimals and the usable mAh to whole numbers. Efficiency must be above 0 and at most 100, and both capacities must be above zero.

Worked example

A 20,000 mAh power bank, a phone with a 4,500 mAh battery, and an assumed efficiency of 80 %. Usable capacity = 20,000 × 80 / 100 = 16,000 mAh. Charges = 16,000 / 4,500 = 3.56. Without the loss the plain division 20,000 / 4,500 would show 4.44 charges, so the loss costs close to one full charge.

Try this example →

Common mistakes

  • Dividing the two mAh figures on the boxes and counting that as the result. The efficiency step is exactly what separates the real number from the optimistic one.
  • Entering the efficiency as a fraction. The field takes a percentage, so 80 means 80 %, and 0.8 would be read as 0.8 %.
  • Entering the phone's charger or cable rating in place of its battery capacity. The device field is the battery size in mAh.
  • Expecting the result to be a whole number of top-ups. The 3.56 means three full charges and more than half of a fourth, not four charges.
  • Comparing a battery rated in watt-hours with one rated in mAh. Both fields must be in mAh, so convert one of them first.

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.

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