tnleafer

I own a custom frame shop.
32 x 40 and 40 x 60 are factory size board. They generally run up to 1/8 over.
While this is not a problem for a framer, it is an issue for DIY.
If you need exact sizes you will need to be prepared to trim yourself,, or order from a frame shop, it will cost you more.
Your frame needs to be 1/8 larger in both directions than the mat board size. This is only 1/16 extra on each side and allows for expansion. If you ^jam^ the mat into a tight frame, chances are it will buckle and look really bad over time.
There are some online services that will size and cut mats for you, but it would be better to go to a local independent frame shop and develop a relationship. They can provide mats, glass, frames, and all the advice you need to learn to do it properly. Hacked together framing will not enhance your art, your image,, or make a nice presentation.
A lot of photographers tend to be very detailed and picky about their work, yet drop the ball when doing their own framing. I've owned frame shops since the 80's, we do onsite fine art printing, archival framing, etc., and have framed thousand of photo's for myself and others over the years.
You can enhance even a mediocre photo with proper matting and framing. You can also wreck a great photograph and your reputation with a sloppy presentation.
In most cases a simple mat and and minimalist frame will make your work pop and not distract the viewer.
A few tips, use archival mat board, never ever use cardboard as a backer, no packing tape ever, learn to do it correctly, remember if you are proud of your work, proudly present it as the whole package speaks volumes about you.

clint f. Loc: Priest Lake Idaho, Spokane Wa

The lions share of the size is based on planing. All studs are S4S meaning each side is sanded (planed). This and the drying make 2x4. Hardwood is sold by the 1/4. Rough sawn 4 quarter is actually 1”. 2” is 8 quarter etc. S3S is sanded 3 sides with the 4th side is field ripped to the final dimension.

bleirer

Interesting link here https://www.uglyhedgehog.com/tpr?p=11914869&t=681934

frankraney Loc: Clovis, Ca.

Just backing you up and agreeing.

chasgroh Loc: Buena Park, CA

You sound like a knowledgable woodworker (I'm also a cabinetmaker...but still primarily a rough carpenter) and mentioning surfacing is entirely correct...but, still, *drying* doesn't have anything to do with the assignment of size to framing lumber. I buy S2S for my art framing material...lots cheaper but another operation jointing (which I do on my trusty Powermatic 66!).

TriX Loc: Raleigh, NC

A classic and excellent cabinet saw. I bought a Delta Unisaw because I liked the fence, but I do envy you the finish on the table of the Powermatic

clint f. Loc: Priest Lake Idaho, Spokane Wa

I’m using 4quarter S3S to build a queen sized bed headboard. We have 2 Windsor Plywood stores in town. They sell “Shorts” that are S3S, 6-7’ long at a very low price. They let you pick through the pallet to get the widths. It’s all plain sawn. I have to rip it anyway so I cut off the rough surfaced edge to the pre-jointing dimension. For my quarter sawn I go to interstate and get 4 quarter and plain and joint. Windsor has other species in the shorts area, walnut, maple, alder etc.

WDCash Loc: Milford, Delaware, USA

It likely to allow for, a frame that is not exactly square or an exact measurement, possible damage to an edge of the matt material that can be trimmed off, an allowance in production that insures they are not Undersized. We can always trim down but its hard to make it grow.

Makers of many types of sheet goods, intended to be fabricated into a custom size or shape, are often sold slightly oversize. Sunbrella marine canvas for example. Sold as 45 or 60" width will arrive from the mill 46 and 61" wide.

TomHackett Loc: Kingston, New York

Thanks for the good information, especially the tips. I've gotten pretty good cutting smaller mattes and foam boards in the past, but this time I guess I bit off more than I could chew (and got a bad taste in my mouth).

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

All you have to do is spend a few minutes trimming. To avoid ragged edges, use a new mat knife, a cork-backed straightedge, and take multiple light scoring cuts. I've been matting and mounting images for 55 yrs - that's how it's done. Tattered edges come from trying to take too deep a cut and/or the straightedge moving. A dozen pictures should take you maybe 5-6 minutes total to trim. I would let your supplier know about the error for future orders, or find a different supplier.

TomHackett Loc: Kingston, New York

Thanks!

dat2ra Loc: Sacramento

Not meaning to be a pedant, but actually, unmilled (aka "rough") lumber such as is commonly used outdoors, is 2" x 4". This is then milled (planed) smooth on all sides to 1 1/2 x 3 1/2 for interior use.

Mr. Evans Loc: Ohio

I can think of two reasons: first, the trimmer of the mats might figure the size is "close enough." And second, if they give you a slightly larger size, it allows you to trim the edge if the corners have been dinged or bent. I prefer it that way. Trimming a mat board is not too hard with the right tools and very hard to to do with the wrong tools. Use a good straight edge. Fletcher makes a good one that will keep your fingers safe. Use a utility knife with a NEW blade. A blade is only good for about 20' of cutting and then it's shot. An X-Acto knife is not a good tool for this kind of cutting. The blade is too long and can bend. The knife is held in a pencil-thin handle because it should be used with the leverage of a pencil. And most importantly, the surface you are cutting on is important. Self-healing cutting surfaces are great, but expensive. The blade has to go into the cutting surface slightly to make a clean cut. And last, don't try to cut through 4-ply board with one cut; it takes too much force and your hands can slip. Two or three cuts makes it easier.