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More Boat Door Size Advice
Okay, you've decided want a new boat door. Now it's time to figure your door size, and here are the things you need to consider:
How big is your cut-out? If you're replacing an old boat door, just take it off and measure the hole that's left. 
If you're installing a door where no door existed before, you need to figure out how big a cut-out you need (or have room for). Once you know that cut-out dimension (24" x 16" in this example), it's easy to figure out your total door size, including the frame.
Most of the doors we sell have frames that are 2" wide -- but if you're pinched for space, we also make doors with frames 1-1/2" wide. For now, let's say you want your door to have a 2" frame.
Your cut-out size plus the frame width equals your door size. In this case, add 2" 
to your cut-out dimensions -- so your 24" x 16" cut-out requires a 26" x 18" door. That door will overlap your 24" x 16" cut-out by 1" on all sides.
When you center your door over the cut-out, about half of the width of the frame should cover the surrounding area -- and the other half should extend inside the cut-out opening.
You have a little wiggle-room: Your cut-out can come almost to the inside edge of the frame and still look and function just fine. But remember, if the cut-out is too wide or too tall, you won't have enough room to put fasteners into your frame.