Originally Posted by timmhaan
OP, if you believe the formula on that page, you will need to add one chain link:
SIMPLE EQUATION: L = 2 (C) + F/4 + R/4 + 1
L = Chain length in inches. Round the final result to closest whole inch figure.
C = Chain stay length in inches, measure to closest 1/8”. Use chart below to find decimal measurement.
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.
You're increasing "R" by 4, so the formula result will be exactly 4/4=1 greater with your new cassette.
Edit: FYI, when I went from a big cog of 24 to 27, I added a link.