# Actual vs Nominal Plywood



## Jsunnb (Apr 4, 2019)

Hello all,

First time poster here. I'm ramping up to make a bathroom vanity, simple shaker style. But I'm struggling with how I handle the difference between actual wood thickness and nominal thickness.
My background is mechanical engineering and I've fully designed the cabinet in Solidworks (3D CAD) and plan to make some flat drawings and start making my first cabinet. But when I lookup 3/4" plywood at a home improvement store like Home Depot, I find that 3/4" plywood is actually 23/32" nominal (seems being under by 1/32 is normal because it's sanded flat) but from there, the actual thickness is even less still, Home Depot says 3/4" birch is actually 0.703", so much closer to being 3/64" undersized.

It seems to me while 1/32" isn't impossible, working in 1/16 as your smallest increment is probably most realistic since a pencil line can be nearly 1/32 thick. I'm wondering how this is handled? My dad who does a lot of woodworking, but mainly does things like jewelry boxes, says that you'll eat up that gap easily with glue and imperfections and just plan as if it was 3/4". But when I look at how thick 3/64" is on my calipers, it seems like an awful lot to eat up.

How is this normally handled in cabinet making, or is there plywood I can buy that is actually 3/4" that I should be using instead of what I find at Home Depot?

Thank you in advance for any help you can give
Jason


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## jmartel (Jul 6, 2012)

Buy some real plywood from a lumber dealer if you want true 3/4". Plus it's not crappy plywood as well.

Also, that plywood at Home depot is actually 18mm, and marketed as 3/4" since that's the closest equivalent in US units.


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## Jsunnb (Apr 4, 2019)

Awesome! That explains a lot then. Thank you! I'll see what I can find.


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## bondogaposis (Dec 18, 2011)

This is pet peeve of mine, there is no reason to have actual and nominal sizes in plywood. If they can make particle board and MDF in 3/4" they can make plywood as well. The US is a big enough market, plywood manufacturers could make it as they used to. It makes no sense in cabinet grade especially.


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## Jared_S (Jul 6, 2018)

Buy plywood, measure actual thickness and adjust your drawing/cuts to suit.


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## Jsunnb (Apr 4, 2019)

> Buy plywood, measure actual thickness and adjust your drawing/cuts to suit.
> 
> - Jared_S


The main reason I don't want to go this route if its not necessary is that I'd be trying to adjust measurements by 0.047" which is less than a 1/16", I just feel like the tools I have would never support that kind of accuracy. But if no local lumbar yards have true 3/4" plywood, I'll have to give it my best shot with the 18mm (0.706") plywood.


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## Jared_S (Jul 6, 2018)

> Buy plywood, measure actual thickness and adjust your drawing/cuts to suit.
> 
> - Jared_S
> 
> ...


Good luck, i haven't found it in American made ply from reputable suppliers.

Mdf all day long at at .750", cabinet ply .710 to 725 and anything in between. Half inch will be .480" give or take.

Cutting to 1/64 is about as good as you need on a plywood box. Faceframe, door and drawer components are more critical.


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## maverik (Dec 30, 2015)

+1 about the only time you need to be that critical is with dado's and grooves. just adjust the dado stack to the plywood you have, make some test cuts on scrap wood for a good fit.


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## YesHaveSome (Sep 10, 2017)

Just draw it full size and then use relative dimensioning once you start making cuts and putting it together. Even if it was exactly 3/4", if you just start cutting out all of the pieces based on your plans alone and try to put it together you'll probably run into issues.


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## Jsunnb (Apr 4, 2019)

> Just draw it full size and then use relative dimensioning once you start making cuts and putting it together. Even if it was exactly 3/4", if you just start cutting out all of the pieces based on your plans alone and try to put it together you ll probably run into issues.
> 
> - YesHaveSome


Hmm, that might be the best way to go. Thank you for the suggestion!


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## MrRon (Jul 9, 2009)

I make all my drawings using Autocad. I work to a thousands tolerance, even in wood. I will be off, but it is only by a few thousands. I've worked to thousands all my life, so it is not a problem for me. When I draw a plan, I take thicknesses of all the wood I use into account as if they were metal. I know others will not do as I do, but I am very comfortable with my way of doing. I build large scale trains and the details involved require precision. For furniture, it isn't necessary.


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## TungOil (Jan 16, 2017)

As mentioned above the ply is 18mm….so invest in a metric tape measure and build to mm dimensions instead. Many of us have switched especially for cabinetry. All the Blum hinges and slides a metric as well.

Also, do yourself a huge favor and find an industrial plywood supplier and purchase pre-finished birch ply for your cabinet project. You will thank me later. Cover the exposed ends with a raised panel.


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## Kelster58 (Dec 2, 2016)

I'm with the draw it 3/4" and when cutting grooves, dados, and rabbits adjust your dado set to fit the plywood you are using. If quality matters I would not consider using plywood from a box store. Find cabinet grade plywood from a specialty lumber store. Home Depot and Lowes plywood is not in any shape, way, or form worth your time and effort. I drive 75 miles to a specialty lumber store for cabinet grade plywood. Cabinet grade plywood has 7 layers, typically the inner layers are poplar, very few if any voids, and has finished layers on both sides that are thick enough to take sanding and finishing. The plywood I purchase is USA made, 3/4" x 96-1/2" x 48-1/2", and is right around $100.00 per sheet for red Oak. Well worth the money…..........


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## Jsunnb (Apr 4, 2019)

I think my biggest take away from this thread is to go to a lumber yard. That is my Saturday plan. It's only about 20 miles away, so not too far. And second is to measure as I go.

I've been truly impressed with the responses I've been getting from everyone, excited to be a member of this forum, tons of knowledge to learn from!


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## jonah (May 15, 2009)

You really don't have to adjust much of anything to account for the difference in thickness. Sure, you cut your dados and rabbets differently, but you'd do those to the size of the actual material anyway, so there's no point stressing over it. The missing thickness just means your final piece is an eighth inch smaller than you wanted it to be in your drawing.

As much work as possible in projects should be done with relative dimensioning - with the actual materials to be used - rather than going by a plan.


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## therealSteveN (Oct 29, 2016)

Can you state clearly where working with pieces of 2 different sizes is causing you issues? What is it you are trying to do, that this discrepancy you see not allowing you to? I understand you have 3/4" solid stock, and undersized plywood is causing an issue for you, where is it the issue? IOW what assembly, or part of an assembly are you stuck on.

I know where the ply goes, as opposed to the solid stock on cabinets I make, and I am not seeing where these 2 meet that is causing an issue? Perhaps a glimpse of your plan, or a sketch of what you see the joint where you are seeing a problem would help resolve this.

I am probably wrong, but I am wondering if you are thinking of making a panel, either a door, or a side where you abut solid stock on the edge of plywood? Is that even close to what you are thinking? The only other place where they come together on my cabinets would be as an edge banding, on the edges of plywood, and for those either a tape, or solid wood you want them to be oversize compared to the ply as you can then trim them to be exact. Actually there is another place I would see them, on an inset door/panel, but if you have 3/4" solid stock, you would want 1/4", 3/8" or at the biggest 1/2" thick plywood for them.


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## wormil (Nov 19, 2011)

Build to fit. Guy on youtube did an excellent video on this a year or so back, he built a vanity upside down. First time I had ever seen that. He made the top, flipped it over, and built the cabinet on top of it. Went together really fast, almost no measuring. It worked because the top was the only piece that had to fit the space, the base only had to fit the top, it's actual measurements were irrelevant. That's what I love about woodworking. I went looking for that video awhile back and sadly couldn't find it, but maybe it plants an idea.


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## oldnovice (Mar 7, 2009)

My son, who has a considerable more experience with Fusion 360 than I do, showed me a very simple example how to use parametrics in Fusion 360.

I designed a small circular container with design parameters for diameter, height, and wall thickness. 
By going to the parameter table I can change any or all of the parameters (height, diameter, wall thickness), and these changes propagate through the entire design.

*Fusion 360 is free* to hobbyist but the learning curve is relatively steep!

I don't know id Autocad is parametric maybe *MrRon* can lend some insight.


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## SMP (Aug 29, 2018)

That used to drive me crazy when I would dado for shelving with a router bit and some plywood would fit and some would be loose! No more cheap plywood for me. I only buy at the lumber yard now.


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## Kirk650 (May 8, 2016)

I buy the skinny plywood and I bought router bits specifically for the skinny stuff. Works great. I do have to keep those specially router bits separate from the standard ones. Get the Whiteside bits.


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## CWWoodworking (Nov 28, 2017)

Menards and HD both carry American made ply in oak and maple that is pretty darn close to 3/4. Same as a real lumber yard.

The import stuff is thinner.


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## BlueRidgeDog (Jan 2, 2019)

I find…others may be different, that relative dimensioning is the rule for most furniture as I don't mill hardwood to a certain size…I generally mill it till it is flat and true and all the parts that need to be the same thickness are. Everything is then built based on the resulting size that you could get from the stock (within reason).

Sadly the same is now true for sheet goods as you can't assume a given size anymore, so a drawer that needs to be a certain width is made by accounting for the width of the sides, then making the back accordingly. No different than using a story board.


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## ocean (Feb 13, 2016)

#1 buy cabinet grade ply. Leave HD and Lowes in you rear view mirror. #2 don't stress out on ply thickness, as others have said it really only applies to the dado cuts. Having worked in a cabinet shop I was really use to working with tape measurements and pencil and never had a CAD drawing. A lot of good advise in the posts - take it to heart .


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## tomsteve (Jan 23, 2015)

your solidworks CAD drawing is no different that the thousands of plans on the market- every dimension is a suggestion- not the law.


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## oldnovice (Mar 7, 2009)

> your solidworks CAD drawing is no different that the thousands of plans on the market- every dimension is a suggestion- not the law.
> 
> - tomsteve


Please explain!
In my opinion parametric modeling is very effective solution handling variable and fixed characteristics!


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