What Size Needle Do I Use For This Yarn?

If you are trying to figure out what gauge you would get by combining two or three yarns, here are a couple of tricks to help.
Math version: Add the stitches per inch together of the proposed yarns, and divide by 3.
Example 1: Combine 2 strands of fingering (sock). If each strand of fingering knits up at 7 st/in, the math is 7+7=14, 14/3=4.7, so the combined yarns are about worsted weight. If each strand of fingering knits up at 8 st/in, the math is 8 + 8 = 16, 16/3=5.3 so the yarns combine to a DK weight.
Example 2: Combine 1 strand of worsted and 1 strand of fingering. The worsted knits at 5 st/in and the fingering at 8 st/in. The math: 5 + 8 = 13; 13/3 = 4.3 st/in or a nice Aran equivalent.
Hands-on version: Take a knit gauge and thread the selected yarns through the hole which just fits the yarn. This gives you the needle size to use.