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01Converting mAh to Wh, and Wh back to mAh
To turn mAh into Wh, multiply by the battery's voltage and divide by 1,000. To go the other way, multiply Wh by 1,000 and divide by the voltage. For a phone power bank, use 3.7 V unless the label says otherwise.
Wh = mAh × V ÷ 1,000 · mAh = Wh × 1,000 ÷ VExample: 10,000 mAh × 3.7 V ÷ 1,000 = 37 Wh.
The reason you need the voltage: mAh counts charge, while Wh counts energy. Energy is charge times voltage. Two batteries with the same mAh hold different amounts of energy if one runs at a higher voltage.
- Power bank to Wh: 20,000 mAh × 3.7 ÷ 1,000 = 74 Wh.
- 100 Wh to mAh: 100 × 1,000 ÷ 3.7 = about 27,000 mAh. That's why banks marketed at 26,800 mAh are so common: they sit just under the airline line.
- Rated output to Wh: a label showing 6,000 mAh at 5 V works out to 30 Wh. Same method, different voltage.
02Check your power bank's Wh
The voltage setting matters most near the 100 Wh line. Here's how common sizes shift between 3.6 V and 3.7 V cells:
| Advertised capacity | At 3.6 V | At 3.7 V |
|---|---|---|
| 5,000 mAh | 18 Wh | 18.5 Wh |
| 10,000 mAh | 36 Wh | 37 Wh |
| 20,000 mAh | 72 Wh | 74 Wh |
| 26,800 mAh | 96.5 Wh | 99.2 Wh |
| 30,000 mAh | 108 Wh | 111 Wh |
03The two units side by side
| mAh (milliamp-hours) | Wh (watt-hours) | |
|---|---|---|
| Measures | Electric charge | Energy |
| Includes voltage? | No | Yes |
| Fair across devices? | Only at the same voltage | Yes |
| Where you see it | Phones, power banks, earbuds | Laptops, camping batteries, airline rules |
| Best for | Comparing two phones or two power banks | Comparing a bank with a laptop, or checking flight limits |
A laptop battery makes the difference obvious. Laptop packs wire several cells in series, so they run at roughly 11 to 15 V instead of 3.7 V. A laptop battery with a similar mAh figure to a big power bank can hold three or four times the energy. Compare them in mAh and you'd get it badly wrong; in Wh the gap is plain.
For phones, mAh is still handy. Phone batteries all use similar single-cell lithium chemistry, so comparing a 4,300 mAh Galaxy with a 5,000 mAh one is fair. If you're new to the unit itself, start with what mAh means.
04Four mix-ups that give the wrong answer
- Using 5 V for the headline mAh. The big number on the box is measured at the cells' 3.6 to 3.7 V. Run it through the formula at 5 V and a 20,000 mAh bank looks like 100 Wh when it's really about 74 Wh.
- Comparing mAh across device types. A tablet, a laptop and a camping battery may all run at different voltages. Only Wh lines them up fairly.
- Dividing bank mAh by phone mAh. That ignores conversion losses. Expect about two-thirds of the bank's capacity to reach the phone over a cable, less wirelessly.
- Treating Wh as charging speed. Wh is how much energy is stored. Watts (W, no "h") is how fast it moves. A 74 Wh bank can still charge slowly if its output is only 10W.
If you only remember one thing: mAh needs a voltage before it means anything outside its own category, and Wh already has one baked in.
05Why airlines count watt-hours
Fire risk scales with how much energy a battery can release, not how much charge it stores at some voltage. So aviation rules are written in Wh: up to 100 Wh in your carry-on, 101 to 160 Wh only with airline approval, and nothing larger as passenger baggage. Spare batteries never go in checked bags.
Security staff look for the Wh figure on the label. If your bank only shows mAh, they may work it out at 3.7 V, or they may simply not let it through. Rules tightened in 2026, including a two-bank limit; the full picture is in power bank rules for flying.
Magnetic battery packs and battery cases are spare batteries as far as airlines are concerned. Same Wh limits, same carry-on rule.
06What to check on the label before you buy
Then match the size to your day. Two-thirds of the rated capacity reaching your phone is a realistic expectation; the charge calculator turns that into full charges for your model.
Everyday banks from 10,000 to 20,000 mAh, all comfortably under 100 Wh.
Questions people ask
How do I convert mAh to Wh?
Multiply mAh by the battery voltage, then divide by 1,000. For a power bank with 3.7 V cells, 10,000 mAh is 37 Wh and 20,000 mAh is 74 Wh.
How many mAh is 100 Wh?
About 27,000 mAh at 3.7 V, or roughly 27,800 mAh at 3.6 V. That's why 26,800 mAh banks are popular with travelers: they stay just under the 100 Wh carry-on limit.
Is Wh or mAh more accurate?
Both are accurate; they measure different things. Wh is more useful for comparing batteries that run at different voltages, like a power bank and a laptop. mAh is fine for comparing two phones or two power banks.
What voltage should I use to convert power bank mAh?
Use the cell voltage printed on the label, usually 3.6 V or 3.7 V. Don't use the 5 V USB output voltage for the headline mAh figure, or you'll overstate the Wh.
Is a 20,000 mAh power bank under 100 Wh?
Yes. At 3.7 V it's about 74 Wh, well under the limit, so you can pack it in your carry-on.