mAh to Wh: your power bank's watt-hours
The one number airlines care about, and the label trap that hides it.
Airline limits are in watt-hours (Wh): 100 Wh without approval, 160 Wh with it. Power banks are sold by milliamp-hours (mAh). To convert, you need the battery's voltage:
Wh = mAh × V ÷ 1000Lithium-ion and lithium-polymer cells are 3.6, 3.7 or 3.85 V. So a 10,000 mAh bank at 3.7 V is 37 Wh, and a 20,000 mAh bank is 74 Wh.
Common sizes
| Battery capacity | at 3.6 V | at 3.7 V | at 3.85 V | Flies without approval? |
|---|---|---|---|---|
| 5,000 mAh | 18.0 Wh | 18.5 Wh | 19.2 Wh | Yes |
| 10,000 mAh | 36.0 Wh | 37.0 Wh | 38.5 Wh | Yes |
| 20,000 mAh | 72.0 Wh | 74.0 Wh | 77.0 Wh | Yes |
| 25,600 mAh | 92.2 Wh | 94.7 Wh | 98.6 Wh | Yes |
| 26,800 mAh | 96.5 Wh | 99.2 Wh | 103.2 Wh | Depends on the voltage |
| 27,000 mAh | 97.2 Wh | 99.9 Wh | 104.0 Wh | Depends on the voltage |
| 30,000 mAh | 108.0 Wh | 111.0 Wh | 115.5 Wh | No |
| 40,000 mAh | 144.0 Wh | 148.0 Wh | 154.0 Wh | No |
Banks sold as "26,800 mAh" are designed to land just under 100 Wh (26,800 × 3.6 = 96.5 Wh), but at 3.85 V the same mAh would be 103 Wh. That's why the voltage on the label matters.
The "rated capacity" trap
Many labels print two capacities:
Battery: 20000mAh 3.7V 74WhRated capacity: 12000mAh (5V⎓2.4A)
The second is what the bank delivers through USB at 5 V, after conversion losses. Multiplying it by 5 V (60 Wh) or by 3.7 V (44 Wh) under-states the battery. Airlines use the battery's own capacity, the first line. If a label shows only the rated capacity, the battery inside holds more than that: on labels that print both, the battery's Wh is about 20–30% higher.
Where to find it on the label
- Look on the back or bottom for "Wh". Air-transport rules have required lithium-ion batteries to be marked in Wh since 2012, so most are.
- If there's no Wh, look for the mAh next to a 3.6–3.85 V figure ("Li-ion 3.7V 10000mAh", "Cell capacity").
- Moulded or engraved text is hard to read: shine a light across it at an angle.
Power Bank Check reads the label with your iPhone's camera, picks the right numbers, and tells you what your airline allows. See the 2026 rules by airline.