Short answer: multiply the power bank's mAh by its cell voltage (usually 3.6 V or 3.7 V, printed on the label) and divide by 1,000 to get watt-hours (Wh). Plan on roughly two-thirds of those watt-hours reaching your phone's battery. Divide that by your phone's battery in Wh and you have a realistic number of charges.
Our power bank guide gives the rule of thumb (60 to 70 percent of the label) and the sizes to pick. This guide shows the maths behind it, so you can check any label yourself.
Step 1: turn mAh into Wh
IATA, the airline industry body, gives the standard formula: watt-hours equal the rated capacity in amp-hours multiplied by the nominal voltage. Divide mAh by 1,000 to get amp-hours first.
| Printed capacity | Cell voltage | Energy |
|---|---|---|
| 10,000 mAh | 3.7 V | 37 Wh |
| 10,000 mAh | 3.6 V | 36 Wh |
| 20,000 mAh | 3.6 V | 72 Wh |
| 20,000 mAh | 3.7 V | 74 Wh |
| 25,000 mAh | 3.6 V | 90 Wh |
| 27,000 mAh | 3.7 V | about 100 Wh |