Decking question - how much space to allow for carpet?

I am replacing the floor and decking of my boat. The wood that was there was rotted so I can't make a good template from the old wood. I need to make a new one. How much space should I allow on either side for carpet?

For example, if the width of the floor is 48", how much narrower should I really cut the wood to allow for carpet? 1/4"?

I plan on using the carpet from Lowe's.

I guess I would have the same question when cutting the hatches. Leave 1/4" gap on all sides?

buy your carpet - fold it double - measure the thickness - there is your gap

Thank you.

Hatches you are double thick on all sides. Cut 1/4 off of each side of your hatches. Pretty much 1/8 = 1 layer of carpeting.