This is a question we get asked on a daily basis, so we thought we would try to give some advice as to just what size foam should be cut at to fit your covers.
There is not a set formula for every case, it depends on the type of foam you are using, whether the foam has a polyester wrap, how much 'give' there is in the cover and whether the foam is going on a drop in seat.
Let's deal first of all with a drop in seat. These are found primarily on dining chairs. The finished seat pad has to drop in to a wooden frame which will be a very close fit on the seat pad. In this case your foam should be the size of the board or the frame that it is going on. Any extra would mean that the seat pad would not drop in to the space available for it. With drop in seats the fabric cover is fixed with staples/tacks to the board or frame.
If you have any other type of seat pad where the fabric is fixed to the board or frame, as opposed to the foam going in to separate cover (i.e. one with a zip in it) then we would suggest having the foam cut slightly larger than the board or frame just so you don't feel the edge of the frame/board through the fabric.
In all other cases the finished cushion should be larger than the cover it is going in to. If the foam is having a polyfibre wrap on it then you can cut the foam the exact size of the cover and the polyfibre wrap will give you the extra size required to make the cushion look nicely filled.
If the foam is not having a polyfibre wrap (the foam can have just a stockinette liner but that is a different matter entirely and is not taken in to consideration when measuring) then the foam should be cut slightly larger than the cover size. The softer the foam the more can be added to the measurements of the cover size. For instance when using our medium/soft or RX Flex foams we could easily add 0.5"/1cm to the length, width and depth measurements of a seat pad sized cushion (dining chair size) and for larger cushions over 1m long we could add 2cm to the length. If however you are using our medium firm or high firm foams, then these do not compress as much as the softer foams but even so adding 1cm to the length, width and depth measurements of the cover would produce a nice full looking cover. With the firmer foams we would only add 2cm to the length when the cover measurement was around 2m.
Summing up them; if you have a cover that measures 18" x 18" x 2" (46cm x 46cm x 5cm) then you should either order your foam those sizes and order a 4oz polyfibre wrap on the foam or order the foam 18.5" x 18.5" x 2.5" (47cm x 47cm x 6cm) when having no polyfibre wrap. Cushions that have a polyfibre wrap on them have a more rounded appearance on the corners rather than just plain foam which has more of a squared/block look. Either is fine, it is just a case of which one suits your style of furniture.
You need to be aware that foam cannot make your cover any larger than it is when stretched but If you just order your foam just the size of the covers it can look very baggy.
You can of course send us your covers for foam to be cut the correct size but the hints given above should help you make the correct decision regarding the sizes to order.