When is 3/4 not 3/4?

[BobA]

When you are talking about plywood of course.  I was just measuring my plywood to center my slot cutting bit and everything was a bit off.   So after googling a bit I came up with this.

The most common sizes of plywood sold in the United States are 3/4", 1/2" and 1/4". However, that's not exactly correct. A 3/4" sheet of plywood is really 23/32" (and is now typically labeled as such). Similarly, 1/2" should be correctly labeled as 15/32" and 1/4" labeled as 7/32". However, everyone still refers to them as 3/4", 1/2" and 1/4".

Most of the time it doesn't matter as much but when getting the tmolding centered it does.

[Chris G]

Yes, very frustrating fact.   

[Jouster]

Personally I think its a conspiracy to get use cab builders to purchase laminate to even it out.

This is outrageous!!

Jouster

[fixedpigs]

when...?

usually always...

measure a 2x4 sometime...

that common size refers to the nominal size...not the finished size...

in the case of plywood...the faces have to be sanded down and such...this removes material and results in a slightly smaller overall size...

[SavannahLion]

Yes, it's nominal size and where are they marking boards as their "true" size?

I saw something on the history channel that explains how we came about our nominal sizes. For instances, lumber isn't their stated size for a lot of lame reasons (the post by balesd). In fact, some places actually sell special rulers that have "adjusted" measurements to accommodate the tweaked measurements of lumber.

[ScottS]

I find that it's best not to assume anything about dimensional lumber and plywood. Don't, for example, assume that every piece of 3/4" plywood is actually 23/32" or that all 2x4s are 1.5x3.5"! I've seen pieces of plywoow that were actually 3/4" thick and others that were 22/32 or even 21/32 or something in between. These days, I'll measure the thickness with a fractional dial caliper at a couple of places along the edge and do my cutting and layout based on the actual size of the piece I'm working with...

[ChadTower]

I've seen ply sheets at Lowe's marked with actual size like that, which made it odd when I asked the guy specifically for 5/8" MDF and he had no idea where to find it.

[BobA]

You will notice that almost all your posts deal with nominal vrs dimensional sizes of lumber.  This difference and the various reasons for it have been the subject of thousands of posts that can be googled.  While this is well known it is much harder to find such a size chart for plywood.  After looking for such info I tend to agree with ScottS and measure each sheet that I am working with. 

[Dervacumen]

In fact, some places actually sell special rulers that have "adjusted" measurements to accommodate the tweaked measurements of lumber.

Can you give me an idea where I can find one of these?  I've not been able to find these anywhere and I've been looking around for a couple years.  I'd love to get my hands on one.

[SavannahLion]

In fact, some places actually sell special rulers that have "adjusted" measurements to accommodate the tweaked measurements of lumber.

Can you give me an idea where I can find one of these?  I've not been able to find these anywhere and I've been looking around for a couple years.  I'd love to get my hands on one.

When I worked at Home Depot, they came in once in a while, usually around Christmas or Father's Day. Some of them are in really funky shapes made to work with a specific "size" of wood such as a 2x4. There's an accompanying how-to video just to teach people how to use those particular ones. I've also seen the occasional T-square looking thing and a sliding ruler looking thing.

I've seen the sliding ruler thing at a nearby tool shop (can't recall the name), so I don't think it's a THD exclusive.

I never really saw myself justifying purchasing something like that when, building something, I would need exact measurements forcing me to obtain oversized wood and cutting/planing it down or the project is loose enough that exact measurements wouldn't mean squat.