Power Bank mAh to Wh: How to Check It Before You Fly
Divide mAh by 1,000 and multiply by the cell voltage: 10,000mAh at 3.7V is 37Wh. Why 3.7V and not 5V, how to read the label, and the 100Wh airline limit.
To find a powerbank's watt-hour (Wh) rating, divide its mAh by 1,000 and multiply by the cell voltage. A 10,000mAh powerbank with 3.7V cells is 10 x 3.7 = 37Wh. Airlines work in Wh, not mAh, and the line that matters is 100Wh. For a typical 3.6V or 3.7V pack that works out to roughly 27,000mAh. Below that you are fine. Above it, IATA's 2026 passenger guidance says the powerbank does not fly.
The formula is the easy part. People get it wrong on the voltage, and on labels that print two different mAh figures for the same battery.
The mAh to Wh formula, straight from the FAA and IATA
The FAA's guidance for airline passengers gives the method in two lines: "multiply the volts (V) by the amp hours (Ah)", and "for milliamp hours (mAh), divide mAh by 1000 (to get to Ah) and then multiply by the V." IATA's 2026 guidance document for passengers writes the same thing as Wh = Ah x V.
So the whole calculation is:
Wh = (mAh / 1,000) x volts
A 20,000mAh pack at 3.6V: 20 x 3.6 = 72Wh. A 5,000mAh pack at 3.7V: 5 x 3.7 = 18.5Wh. That second one is the powerbank inside every Brick station, and the same arithmetic tells you how many phone charges it holds.
Which voltage to use, and the 5V trap
Use the nominal voltage of the cells inside the pack. Don't use the 5V, 9V or 20V printed next to the USB ports.
On both of the labels below, the headline mAh is measured at the cells' own voltage. Anker's explanation of its powerbank labels says its rated capacity is given "at the rated voltage of 3.6V". Ugreen lists its PB311 as "10,000mAh at 3.7V". Brick's own powerbank cell is 3.7V.
If you multiply that front-of-box mAh by 5V because 5V is the number on the USB port, you get the wrong answer and it is always too high. A 20,000mAh pack comes out at 100Wh instead of its real 72Wh. That can make a perfectly legal powerbank look like it is sitting on the limit.
If the label gives no cell voltage at all, 3.7V is the safer assumption, because it gives a slightly higher Wh than 3.6V and so never flatters the pack.
How to read a real power bank label
The printing on the back of a powerbank is often called the silk screen. Look for three lines.
- A Wh figure. If it is printed, you are done. IATA's guidance says "all lithium-ion batteries are required to have the Watt-hour rating marked on the outside of the battery case."
- Cell or battery capacity, in mAh with a voltage beside it. Anker's example reads "Cell Capacity: 5,000mAh, 14.4VDC, 72Wh" for a pack with four cells in series. That is 5Ah x 14.4V = 72Wh, the same energy as 20,000mAh at 3.6V, written a different way.
- Rated capacity, sometimes a second, smaller mAh figure. Ugreen prints "6,250mAh typical at 5V/3A" on a 10,000mAh pack. That is what reaches the USB port after the voltage is stepped up to 5V and some energy is lost on the way.
That second figure confuses people. It is not a different battery. 6,250mAh at 5V is about 31Wh, which is what comes out of a 37Wh pack once conversion losses are taken. For airline purposes, use the battery figure (37Wh), since that is the energy stored in the cells. The smaller output number is more useful for working out how many charges you will get.
mAh to Wh table for common power bank sizes
| Capacity (mAh) | Wh at 3.6V | Wh at 3.7V | Under 100Wh? |
|---|---|---|---|
| 5,000 | 18.0 | 18.5 | Yes |
| 10,000 | 36.0 | 37.0 | Yes |
| 20,000 | 72.0 | 74.0 | Yes |
| 25,000 | 90.0 | 92.5 | Yes |
| 26,800 | 96.5 | 99.2 | Yes, just |
| 27,000 | 97.2 | 99.9 | Yes, just |
| 30,000 | 108.0 | 111.0 | No |
| 40,000 | 144.0 | 148.0 | No |
Anything from 20,000mAh down clears the limit comfortably. Between about 26,000 and 27,000mAh, the printed Wh figure decides it, so check the label rather than the box.
Why airlines care about Wh, not mAh
mAh measures charge. Wh measures energy, and energy is what matters if a battery overheats. Two packs with the same mAh but different cell voltages hold different amounts of energy, so regulators set every limit in Wh.
For powerbanks the limits have tightened. IATA's 2026 passenger table allows powerbanks up to 100Wh in carry-on only, at most two per person. Powerbanks above 100Wh and up to 160Wh are listed as "Forbidden". The US pages have not caught up in the same words: the TSA's page on larger batteries still describes spares of 101 to 160Wh as allowed "with airline approval". Your airline applies whichever rule is stricter. In 2026 that means 100Wh is the working ceiling for a powerbank, and the approval route is for spare batteries built for a specific device, such as a camera or a drone.
Our guide to the 2026 airline rules for powerbanks covers the rest: the two-per-person cap, no recharging on board, and the airlines that have banned in-flight use entirely.
What if the Wh rating is worn off or missing?
This is the one problem the maths cannot fix. IATA's guidance is blunt: "If passenger handling staff are unable to verify the Watt-hour rating by checking either the battery, or the user documentation, the operator may not permit the carriage of the lithium battery."
Doing the maths yourself does not help at the gate if the staff cannot see the number. Three things do:
- Photograph the label while it is still readable, and keep the photo on your phone.
- Keep the product page or manual that states the Wh figure, since IATA accepts "user documentation".
- If the label is scuffed and you have no records, leave the pack at home. An unmarked powerbank is a coin toss at security.
The other option is not to fly with one. A power bank sharing network lets you rent a charged powerbank where you land and drop it at any station when you are done. Brick's are 18.5Wh, far below any limit, and you never carry one through security. Our renting vs buying comparison covers when owning still makes more sense, and brick.tech shows how renting works.
Power bank Wh: quick answers
How many Wh is a 10,000mAh power bank?
About 36 to 37Wh, depending on whether the cells are rated at 3.6V or 3.7V. That is well under the 100Wh carry-on limit.
How many Wh is a 20,000mAh power bank?
72Wh at 3.6V, 74Wh at 3.7V. That sits comfortably inside the 100Wh carry-on limit.
What is the biggest power bank I can take on a plane?
100Wh, which is roughly 27,000mAh at 3.7V. IATA's 2026 guidance lists powerbanks above 100Wh as forbidden on passenger aircraft, and you can carry at most two.
Should I use 3.7V or 5V to calculate Wh?
Use the cell voltage, usually 3.6V or 3.7V, printed beside the capacity. Multiplying the front-of-box mAh by 5V overstates the energy. A 20,000mAh pack would come out at 100Wh instead of 72Wh.
Why does my power bank show two different mAh numbers?
The larger one is the capacity of the cells. The smaller "rated" figure is what reaches the USB port at 5V after conversion losses. Use the cell figure for airline limits and the rated figure to estimate phone charges.
Can I fly with a power bank that has no Wh label?
You might not be allowed to. IATA says the airline may refuse carriage if staff cannot verify the Wh rating from the battery or its documentation. Carry the manual or a photo of the label.
