Most standard futon covers that you'll find on the shelf in a store, or online will come with a standard depth of 6″, and will fit futon mattresses that are 6″ to 8″ thick. A 6″ deep futon cover will actually fit an 8″ futon best, and it will have a very nice snug fit. If your mattress is thicker or thinner, you may need to custom order your cover. The following information will help you determine the best size futon cover to order, to get the best fit.
Futon covers are normally ordered with a depth of 2 inches less than the thickness of the mattress. This is because when the futon cushion is flat, a cover is at it's tightest. Unless the futon pad is only used for a bed, and it doesn't fold like it would in a futon frame. If your cover is for bed use only, you should order a cover that is the same thickness as the mattress. If your cover is for a folding futon sofa, that makes a bed, when it's folded into a sofa position, the bend creates slack in the cover. So, by ordering covers a little on the tight side, they will fit best, and look perfect, when your futon is in the sofa position.
There are some exceptions to this rule and those are some innerspring futons, or some super soft, or super firm futons. Find a futon expert somewhere, and ask for their advice. Most covers can be customized to fit any futon mattress, no matter how thick, or what the exact size is.
This is the best way to measure a futon mattress. If your futon mattress is squared on the edges, like a conventional mattress, just measure the length, width, and depth. If your futon mattress is more rounded on the edges, it's a little harder to measure because you have to use an imaginary line where the thickest part of the futon mattress is. So, measure the length, width, and thickness, and included the part of the futon mattress that is bulging, or rounded on the edges. Don't just measure the top seams. Remember though, a futon mattress is not a solid block of wood. If you are off by an inch or so, the cover will still fit, and will look fine. If you are extremely particular about the fit of your cover, then by all means, seek out the advice of your local futon store, or find a futon specialty shop and discuss it with them.
These are the standard sizes of futons. The thickness varies by the quality, but these sizes are standard.
Twin Size 39″ X 75″ – Full Size 54″ X 75″ – Queen Size 60″ X 80″
Standard Chair 28″ X 54″ – Standard Ottoman 28″ X 21″
Twin Chair 39″ X 54″ – Twin Chair Ottoman 39″ X 21″
Full Loveseat 54″ X 54″ – Full Loveseat Ottoman 54″ X 21″
Queen Loveseat 60″ X 60″ – Queen Loveseat Ottoman 60″ X 20″
California King 72″ X 84″ – Eastern or Standard King 78″ X 80″
If you are extremely particular about the fit of your cover, or you have any other questions, by all means, seek out the advice of a futon specialty shop.