The stock tires on my '06 QC are 275/60/20. That equates to 10.83" wide and 32.99" tall. A 315 is 12.4" wide. The formula is simple:
275/25.4 (mm/inch) = width
width * aspect ratio (60% in my case) = sidewall height
sidewall height *2 + wheel size = diameter
You can't tell the height of the tires without knowing all three measurements (section width, aspect ratio and wheel size).
Of course, there will be some variation, but that is the way to determine tire size.