RO,
I suggest that you add more detail to your question.
Year of boat would help.
Also, you can probably find a manual on this site. Download & save it. It will answer your question & it contains important info for use. Click on the, "Boat Info" tab up top.
It also helps if you create your public profile. This is useful in helping you with answers to future questions like, your boat info, area of sail, fresh or salt, experience etc.
My 1980 measurement is, 11.50 Ft. also known as 100% for sizing your head sail. Any larger, the common sail sizes will read, 115%, 130/135%, 150/155%.
CR