How do I convert mAh to watt-hours (Wh)?
Multiply the milliamp-hours by the battery voltage, then divide by 1000. For a power bank, use 3.7 volts (the lithium-ion cell nominal), not the 5 volts printed on the USB output. So a 20,000 mAh bank is 20,000 times 3.7 divided by 1000, which is 74 watt-hours. Using 5 volts would wrongly give 100 watt-hours, overstating it by about 35 percent. If your battery already shows a watt-hour rating on its label, use that number directly, since manufacturers print the true figure.
Why do I use 3.7 volts and not the 5 volts on the USB output?
Because 3.7 volts is where the energy is actually stored. A power bank's cells sit at about 3.7 volts (the lithium-ion nominal), and the printed mAh is rated at that cell voltage. The 5 volts you see on the USB port is produced by a boost converter that steps 3.7 up to 5 and loses some energy in the process, so it is neither the storage voltage nor a lossless number. Calculating watt-hours at 5 volts inflates the result by about 35 percent, which matters because it can push a flight-legal bank over the 100 watt-hour line on paper. Airlines and the FAA use the 3.7 volt basis.
How many watt-hours is a 20,000 mAh power bank? And a 26,800 mAh?
A 20,000 mAh power bank is about 74 watt-hours (20,000 times 3.7 divided by 1000), comfortably under the 100 watt-hour airline limit. A 26,800 mAh bank is about 99 watt-hours, which is why 26,800 mAh is the common flight-friendly maximum: it sits just under 100. Anything much above about 27,000 mAh at 3.7 volts crosses the 100 watt-hour line and then needs airline approval to fly.
Is my power bank allowed on a plane? Can it go in checked luggage?
A power bank under 100 watt-hours flies in your carry-on with no approval needed. Between 100 and 160 watt-hours it is allowed only with your airline's approval and limited to two spare batteries. Over 160 watt-hours it is banned from passenger aircraft. In every case a power bank must be in your carry-on, never in checked luggage, because it is treated as a spare lithium battery and those are prohibited in the hold. Some airlines also cap how many power banks you can carry regardless of size, so confirm with yours.
What is the biggest power bank I can fly with?
Without any airline approval, the practical maximum is a bank at or just under 100 watt-hours, which is about 27,000 mAh at 3.7 volts (this is why 26,800 mAh models are so common). If you are willing to get your airline's approval before the flight, you can carry one up to 160 watt-hours, but only two spare batteries and still carry-on only. Above 160 watt-hours there is no flying with it on a passenger plane. Always confirm the exact rule with your specific airline, since some are stricter.
How many times will a power bank charge my laptop?
Take the bank's watt-hours, keep about 80 to 90 percent of it for real-world conversion and cable losses, and divide by your laptop battery's watt-hours. A 74 watt-hour bank (20,000 mAh) gives roughly 60 to 67 usable watt-hours, so it charges a 52 watt-hour MacBook Air about 1.1 to 1.3 times, or a 100 watt-hour 16-inch MacBook Pro only about 0.6 to 0.7 of a full charge. The calculator above does this for your exact bank and laptop. Treat it as a planning estimate, since cable quality, temperature, and what you are doing while charging all move the number.
How many power banks can I bring on a plane?
For banks at or under 100 watt-hours, the FAA sets no specific number for personal use, and IATA allows up to about 20 spare batteries of all types per person. For the larger 100 to 160 watt-hour banks, the limit is two. The important caveat is that individual airlines often set their own stricter cap on how many power banks you may carry regardless of watt-hours, and some have tightened these rules recently, so always check your airline's current policy before you pack. Since 2026 many airlines also restrict using or charging a power bank during the flight, and where you stow it, separately from how many you may carry.