Battery watt-hour calculator
Free tool
Airlines write battery limits in watt-hours; manufacturers print milliamp-hours. This is the bridge. Check the numbers, then read the full battery rules guide or the power bank rules.
🔋 Battery watt-hour calculator
Power banks show mAh, airlines limit Wh. Under 100 Wh flies free in carry-on; 100–160 Wh needs airline approval.
The formula, once, so you own it
Watt-hours = (mAh ÷ 1000) × voltage. The voltage lives on the same label as the capacity — for lithium power banks it is almost always 3.7 V (the cell’s nominal voltage), not the 5 V USB output. Using 5 V overstates your Wh and can talk you out of a bank that is actually legal. When a label already prints Wh, trust that number: it is the legally relevant one.
Common capacities, pre-computed
| Label (3.7 V) | Watt-hours | Verdict |
|---|---|---|
| 10,000 mAh | 37 Wh | ✓ Carry-on OK |
| 20,000 mAh | 74 Wh | ✓ Carry-on OK |
| 27,000 mAh | 99.9 Wh | ✓ Carry-on OK |
| 30,000 mAh | 111 Wh | ⚠ Airline approval (max 2) |
| 40,000 mAh | 148 Wh | ⚠ Airline approval (max 2) |
| 50,000 mAh | 185 Wh | ✕ Banned on passenger planes |
How do I calculate watt-hours from mAh?
Wh = (mAh ÷ 1000) × volts. A 20,000 mAh power bank at 3.7 V is 74 Wh. The voltage is printed on the battery label, usually 3.7 V.
Is 27,000 mAh allowed on a plane?
Yes — 27,000 mAh at 3.7 V is 99.9 Wh, just under the 100 Wh free limit, which is exactly why so many power banks top out at that capacity.
What is the power bank limit for flights?
Up to 100 Wh: free in carry-on. 100–160 Wh: airline approval required, maximum two. Over 160 Wh: banned from passenger aircraft. All power banks are carry-on only, never checked.