A 20000mAh power bank holds about 74 watt-hours at the usual 3.7 V cell rating, which is under the 100 Wh limit for carry-on bags on US flights. On paper it charges a 4000mAh phone five times. In practice you get fewer, and the airline rule is the part you can rely on.
Below you get the math, a table of real capacities against the FAA limits, and what to check before you buy.
How many charges does a 20000mAh power bank give you
Divide the bank’s capacity by your phone’s battery. The Galaxy S25 lists a 4000mAh battery on Samsung’s US page, so 20000 divided by 4000 gives five full charges as a ceiling.
Real numbers land lower. The bank boosts its cell voltage to 5 V or higher to charge your phone, and that conversion wastes some energy as heat. Makers rarely publish the efficiency, so treat five as the best case and plan for fewer.
If you are comparing phones, the Pixel fast charging story shows why charging speed and capacity are two separate specs.
How to convert mAh to watt-hours
Airlines count watt-hours, not mAh. The formula is capacity in mAh times voltage, divided by 1000. Lithium-ion cells are rated at roughly 3.7 V, so 20000 mAh works out to 74 Wh.
Use the voltage printed on the bank’s label. If it prints a Wh figure directly, that number wins over your own math.
Video: Anker Innovate & Educate
Power bank sizes against the carry-on limit
The Wh column below is my arithmetic at 3.7 V. The limits come from the FAA PackSafe page, and the phone battery comes from Samsung.
| Capacity | Watt-hours at 3.7 V | FAA carry-on status | Charges of a 4000mAh phone, ceiling |
|---|---|---|---|
| 10000 mAh | 37 Wh | Allowed | 2.5 |
| 20000 mAh | 74 Wh | Allowed | 5 |
| 26800 mAh | 99.2 Wh | Allowed, just under 100 Wh | 6.7 |
| 30000 mAh | 111 Wh | Airline approval needed, two spares max | 7.5 |
Can you take a 20000mAh power bank on a plane
Yes. The FAA allows lithium-ion batteries up to 100 Wh, and spare batteries including power banks must travel in carry-on baggage only, never checked.
Batteries from 101 to 160 Wh need airline approval, with a limit of two spares per person. Anything above 160 Wh is forbidden. The FAA also notes that many airlines set stricter quantity limits, so check yours before you fly.
What to check before you buy
Pick a bank that prints its Wh rating on the case. An unmarked bank leaves you to prove its capacity at the gate.
Look at the ports next. USB-C power delivery output matters more than raw capacity for a laptop or tablet, and a bank that cannot output enough watts will charge slowly. For a MagSafe-style phone, the MagSafe battery pack is a smaller, lower-capacity example of the tradeoff.
Finally, ask whether you need 20000mAh at all. A single phone for a weekend trip is covered by half that, and our iPhone 13 battery charging coverage shows how much of a phone’s charging story is about speed, not capacity.
