Power bank flight checker: mAh to Wh and airline rules
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Type the capacity printed on your power bank and pick your airline. You get a plain verdict (fine, ask first, or not allowed), the arithmetic, and what that airline says about use in flight. Nothing you type leaves your browser.
Your power bank
How the answer is worked out
Airlines limit the energy in a lithium battery, not its mAh. Energy in watt-hours is mAh × volts ÷ 1000. Most power banks have 3.6 to 3.85 V cells, so 20,000 mAh is about 72 to 77 Wh. If the Wh figure is printed on the bank, use that and ignore the estimate. The common limits are 100 Wh (carry without approval) and 160 Wh (never above, and 100 to 160 Wh needs your airline's approval). Where you give only mAh the tool assumes 3.7 V and warns you when the real voltage could move the answer across a line: 27,000 mAh is 99.9 Wh at 3.7 V but 103.95 Wh at 3.85 V.
What it checks, and what it does not
It checks capacity, number of banks, and use in flight against five rule sets taken from the airlines' own notices or the best primary report available, each with a date it took effect and the date I last checked it. The full table with sources is on the airline power bank rules page. It cannot know about your route's other airline, a codeshare, or a rule changed this week, so confirm with the airline before you fly. A bank with no readable capacity label may be refused at security whatever the maths says.
Not sure what to buy? Read how to choose a power bank that is fine in the cabin.
Developers: the same checker is a free, zero-dependency JavaScript library and command-line tool, power-bank-flight-check (MIT licence; the public repository is coming soon).