mAh to Ah: divide by a thousand
Divide milliamp-hours by a thousand and you have amp-hours. A table for common power banks, plus why 20000 mAh does not charge a phone six times.
Milli just means a thousandth. 20000 mAh is 20 Ah, and that is the whole calculation — no voltage needed, nothing else to look up.
The trouble starts one step later. A power bank labelled 20000 mAh does not hold 20 Ah at the voltage you are picturing. Lithium cells sit at 3.7 V, so what you actually have is 74 Wh. The USB socket puts out 5 V and the converter takes its cut on the way, which leaves roughly 14000 mAh measured at the output — that is the figure worth comparing against your phone.
One row in the table looks arbitrary until you know why it exists. Power banks are often sold at 26800 mAh because that works out to 99.16 Wh, just under the 100 Wh mark above which airlines want the carrier's approval before a battery goes into the cabin.
Common power banks
| Capacity | In amp-hours | Energy at 3.7 V |
|---|---|---|
| 5000 mAh | 5 Ah | 18.5 Wh |
| 10000 mAh | 10 Ah | 37 Wh |
| 20000 mAh | 20 Ah | 74 Wh |
| 26800 mAh | 26.8 Ah | 99.16 Wh |
| 30000 mAh | 30 Ah | 111 Wh |
Do not compare mAh across different voltages
A 20000 mAh power bank and a 12 Ah motorcycle battery are not comparable figures. The first runs at 3.7 V and holds 74 Wh; the second at 12 V and holds 144 Wh. Only watt-hours tell you which one stores more energy — and it turns out to be the one that looked smaller.