Does my data get uploaded anywhere?
No. Everything runs locally in your browser and nothing is sent to a server.
Is this free to use?
Yes. It is completely free, with no account, no signup, and no usage limits.
Does it work on a phone?
Yes. The page runs in any modern mobile browser, on iPhone and Android alike, with no app to install.
How many coulombs are in one mAh?
Exactly 3.6 C. A milliamp is 0.001 A and an hour is 3,600 seconds, and charge is current times time, so 0.001 x 3,600 gives 3.6 coulombs. An amp hour is 1,000 times that, or 3,600 C.
Why can I not convert mAh to watt hours here?
Because it needs a voltage, which this page has no field for. The formula is Wh equals mAh times volts divided by 1,000. A 5,000 mAh cell at 3.7 V is 18.5 Wh, and the same 5,000 mAh at 3.85 V is 19.25 Wh.
Can I take a 10,000 mAh power bank on a plane?
Almost certainly. Airlines limit spare lithium batteries by energy, not charge, usually to 100 Wh. A 10,000 mAh bank at a 3.7 V cell voltage is 37 Wh, well under the limit. Anything above 100 Wh needs airline approval, and above 160 Wh is usually banned.
How do I convert Ah to mAh?
Multiply by 1,000. A 2.5 Ah cell is 2,500 mAh, which is 9,000 coulombs. Enter 2.5 with Amp hour selected and the table gives you all three at once.
What is a coulomb, in practical terms?
It is the charge moved by one ampere in one second. A phone charging at 2 A moves 2 C every second, so a full 18,000 C charge (5,000 mAh) takes at least 9,000 seconds, or 2.5 hours, at that rate before any charging losses.