Formula
Wh = mAh × V ÷ 1000
Most lithium-ion cells are rated at a nominal 3.7 V; power banks usually advertise mAh at this cell voltage.
Worked example
A 20,000 mAh power bank with 3.7 V cells: Wh = 20000 × 3.7 ÷ 1000 = 74 Wh — under the common 100 Wh airline carry-on limit.
Reference table
Quick reference
| mAh | Wh @ 3.7 V | Wh @ 5 V |
|---|---|---|
| 1,000 | 3.7 | 5 |
| 2,000 | 7.4 | 10 |
| 3,000 | 11.1 | 15 |
| 5,000 | 18.5 | 25 |
| 10,000 | 37 | 50 |
| 20,000 | 74 | 100 |
Where this shows up in the real world
Airlines enforce battery limits in watt-hours — typically 100 Wh in carry-on without approval — while power banks advertise mAh, so every traveler eventually does this conversion at a checkout or a check-in desk. It's also the honest way to compare batteries across devices with different voltages, from earbuds to e-bikes.
Common mistakes to avoid
The marketing trap: mAh figures use the 3.7 V cell voltage, not the 5 V USB output, and DC conversion losses then take 20–35%. So a '20,000 mAh' bank is 74 Wh of storage delivering maybe 50–60 Wh to your phone. Check airline rules in Wh, compare products in Wh, and treat giant mAh claims at miracle prices as a red flag.
Frequently asked questions
Why do airlines use Wh instead of mAh?
mAh alone doesn't define energy — voltage matters too. Most airlines allow batteries up to 100 Wh in carry-on without approval; 100–160 Wh often needs airline approval. Check your airline's current policy before flying.
Which voltage should I use?
Use the battery's nominal cell voltage — 3.7 V for typical lithium-ion power banks — not the 5 V USB output.
Why does my power bank charge my phone fewer times than the math suggests?
Voltage conversion and heat lose 20–35% of the stored energy, so a 74 Wh bank delivers roughly 50–60 Wh to your device.