Quick answer: multiply mAh by 3.7 and divide by 1,000 to get watt-hours (Wh). Expect about 65% of that to reach your device, so a 10,000mAh bank (37Wh) charges a typical phone about 1.4 times. Banks up to 100Wh (about 27,000mAh) can fly in carry-on with no approval. Enter your numbers below.
Power bank calculator
How the math works
Power banks are rated in milliamp-hours at the 3.7 volts of their lithium cells, but USB delivers power at 5 volts or more. Converting to watt-hours puts every battery on the same scale:
Wh = mAh × 3.7 ÷ 1,000, then charges = (bank Wh × efficiency) ÷ device Wh.
The efficiency figure covers the voltage conversion inside the bank, the second conversion inside your phone, and heat. 65% is a realistic default; high-end banks charging a laptop over USB-C PD can reach 80%. Our full explainer, Power Bank mAh vs Wh, walks through why the box number overpromises.
| Advertised | Wh | Phone charges (4,500mAh) | Airline carry-on |
|---|---|---|---|
| 5,000mAh | 18.5Wh | 0.7 | Allowed |
| 10,000mAh | 37Wh | 1.4 | Allowed |
| 20,000mAh | 74Wh | 2.9 | Allowed |
| 26,800mAh | 99Wh | 3.9 | Allowed (under 100Wh) |
| 30,000mAh | 111Wh | 4.3 | Airline approval |
| 50,000mAh | 185Wh | 7.2 | Not allowed |
Frequently Asked Questions
How do I convert mAh to Wh?
Multiply the mAh rating by the battery voltage (3.7V for power banks) and divide by 1,000. A 20,000mAh bank is 20,000 × 3.7 ÷ 1,000 = 74Wh.
How many times will a 20,000mAh power bank charge my phone?
About 2.5 to 3 times for a typical 4,500mAh phone. The bank holds 74Wh, roughly 65% of that reaches the phone, and a 4,500mAh phone battery is about 17Wh.
What size power bank can I take on a plane?
Up to 100Wh (about 27,000mAh) in carry-on with no approval. 100 to 160Wh needs airline approval and is usually limited to two per passenger. Over 160Wh is not allowed, and no power bank may go in checked luggage.
Why does my power bank deliver less than its rating?
The rating describes the cells at 3.7V. Stepping up to USB voltage, stepping back down inside the phone, and heat losses consume 30 to 40% of the energy, so you get roughly 60 to 70% of the advertised capacity.
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