header sizing for 8' shed wall opening ?

[cbc58]

I am planning to build a shed with an 8' opening and am trying to figure out the required size of the support header.  The shed itself will be 12' wide and 16' long.  There are sizing charts but they only appear to show buildings a minimum 20' wide - not 12.  I am using 2x4's for framing.  I will probably have an 8' opening at both ends of the shed.  Build would be in NC with low snow loads.

What would I need to use for a header?  Would 3 2x8's  suffice?  Or 2 2x10's ?  Any info appreciated. 

[Don_P]

Making some assumptions... the door is on a 16' wall with ~14' rafters bearing half their weight on the header, half on the opposite 16' wall.

If so the area tributary to the header is ~8'x~7'=56 square feet supported by the beam. I'm in the mountains near Boone, call it 30 psf snow+10psf dead load= 56sf x40psf=2240lbs

Design for Bending (timbertoolbox.com)

I'm getting a pass with a double 2x8 in #2 SPF or better

[cbc58]

Thank you Don P.  That's a great website...