Will I need external power? How to calculate it?

Using Arduino Project Guidance
7

Rookie2:

rweber95:
I think a USB port device is limited to 0.5 A at 5 VDC.

0.5 for USB 2.0 and 0.9 for 3.0

PeterH:
You need to know in advance what the voltage and current requirements are for anything you connect to the Arduino

Actually the .5amp 5vdc limit is enforced with a on-board 500ma thermofuse on the arduino board, so the limit applies to both USB 1,2, or 3. Note that if you are powering the board via the arduino external power connector then you can draw a little more current then the 500ma USB limit, depending on the actual voltage applied to the external power connector or Vin pin.

Lefty

I guess so, I could also use an Amperemeter connected in series with the +5v line of the USB cable, right?