Mega ADK box says 7-18V, is it really 18V (the site here says 12V)

Right on the box it says Input Voltage 7-18v

The R3 boards for the Mega (non-ADK) and the UNO say 7-12v. Which is it?

The regulator can probably handle 18 but the higher the voltage the more power gets wasted as heat in the regulator. The actual maximum will depend on hoe much current your circuit draws because that will determine how much power the regulator will have to dissipate into the air and thus how hot the regulator gets. At some point the regulator will overheat and fail.

The hardware pages explain it clearly. The on board regulator is rated for 20V. http://arduino.cc/en/Main/ArduinoBoardUno

The general reocmmendation is 12V max, but as John says, there is a lot of "it depends" built into the answer.

I know enough electrical engineering to know what regulators do and how to read a datasheet. Some of it depends on the actual circuit and thermal management.

My point was THE BOX SAID IT COULD GO UP TO 18 VOLTS.

If this is a misprint on the box, someone should say so and fix the box printing.

What is there to fix? The box is correct.

The web page says 7-12v The box says 7-18v Both are under "specifications" Unless mathematics has devolved 12 != 18. Is the official specification 12v or 18v?

tz: Is the official specification 12v or 18v?

The explanation was already given. You said you understand. Both are correct.

Where did you buy it from?

I do not understand how something can be simultaneously 12 and 18. If there were two speed limit signs, one at 50, and a second at 75 how would you reconcile it?

I understand that one or the other is in error and that both cannot be correct in context. The part has extra margin for higher voltages, but 150% is a bit much, and the application is automotive but low power so 13.5v or even up to 15+ is possible.

I bought it at Radio Shack. It is the standard packaging and has a sticker which I assume is the serial number. Here is a blogpost with a picture - the main difference is that on the back, it says "INPUT VOLTAGE: 7-18V". I checked multiple packages. The UNOs and Megas said 7-12,. The two ADKs both said 7-18v.

Their product summary page says 6-18V