peaceandjoy

First - read ecorug's directions carefully. I used the eco fiber, it is 100% wool and is available in the 1/2" thickness (Nature's Cushion only goes up to 3/8", if that 1/8" matters to you).

From the website: "How much shorter should a felt rug pad be than your actual rug?
Approximately 1-1.5 inches on all sides. Rug pads will be cut 1-1.5 inches smaller on all sides."

You are giving the measurements for your "carpet". This product is sold as a carpet pad. So when you give the measurements for a carpet, the pad for said carpet will be 1" to 1-1/2" smaller on each side (so 2'-3" less total in each direction. The pad would then not show under the edge of your carpet.

Since we aren't using it for this purpose, you need to Add to the measurement to get the size you really want. I was able to easily cut the one I ordered. I ordered 2' wide to fit a Better Board, but 2' longer than the Better Board so I would also have a piece to travel with.
When you put the measurements in, the first column is for feet. So if you want an 18" piece, you put in 1' x 8" for the rug size. Put that measurement in for both length and width. Remembering that the cut will be smaller (see above) than your rug. You can enter this for eco fiber - it's $25.95 before the 15% discount (which has been on there every time I've ever looked). For the Nature's Cushion, the $44.95 price is for any size up to 3' 11". So you either get a larger one and cut it down (I cut mine with my paper scissors, went through the whole thing easily), or put directions in the special notes section. Maybe call or email to get pricing on one that small. They might have a scrap they could send?