What Does 4/4 Lumber Mean? Quarter Thickness, Rough and Surfaced

Lumber math · Quarter sizes · Updated 2026-09-13

4/4 lumber is hardwood that measured one full inch thick when it came off the mill saw. The fraction is not a ratio, not a grade and not a product code. It is a count of quarter inches of thickness, said out loud as four quarter, and it always describes the board in the rough, before a planer has touched it. Eight quarter, written 8/4, is two inches rough. Twelve quarter is three inches rough.

That one word, rough, is where the money goes. Hardwood is tallied and priced on the rough thickness, so a 4/4 board that reaches you at 13/16 in thick is still bought, sold and invoiced as 4/4. Woodworkers who have not been told this spend years quietly assuming every yard is shorting them. The more expensive version of the same mistake is ordering 4/4 for a part that needed a full inch, and finding out at the planer with the wood already paid for.

This guide gives you the full quarter table in decimal and fraction, the surfacing allowance that produces those numbers, what rough sawn, skip-planed, S2S and S4S each mean for the thickness in your hands, and how to work backwards from a finished thickness to the quarter you should order. The arithmetic that turns those thicknesses into volume lives in how to calculate board feet.

Quarters count inches of rough thickness

Read the fraction as a count, not as division. The bottom number is always 4 because the unit being counted is a quarter of an inch, and the top number is how many of those quarters the sawyer cut. So 4/4 is four quarter inches, which is one inch. 5/4 is five quarter inches, or 1-1/4 in. 6/4 is 1-1/2 in, 8/4 is 2 in, 10/4 is 2-1/2 in, 12/4 is 3 in and 16/4 is 4 in. Nobody reduces the fraction, and that is deliberate: writing 4/4 rather than 1 tells you at a glance that you are looking at a rough hardwood thickness and not a finished dimension. Quarter notation covers thickness only. Hardwood is normally sold random width and random length, so there is no matching notation for the other two dimensions and you measure them off the board itself.

The S2S column is not one yard's private decision. It comes from the surfacing allowance published by the National Hardwood Lumber Association, and it is a flat subtraction from the rough thickness rather than a percentage. Stock 1-1/2 in and under loses 3/16 in. Stock 1-3/4 in and over loses 1/4 in. Run the whole quarter ladder through those two rules and you get the table above, cell for cell, which is why 13/16 in is the S2S thickness you usually meet on 4/4 from one yard to the next.

• Stock 1-1/2 in and under loses 3/16 in, which is 0.1875 in. That covers 4/4, 5/4 and 6/4.
• Stock 1-3/4 in and over loses 1/4 in, which is 0.2500 in. That covers 8/4 and everything above it.
• 4/4: 1.0000 minus 0.1875 gives 0.8125 in, which is 13/16 in.
• 6/4: 1.5000 minus 0.1875 gives 1.3125 in, which is 1-5/16 in.
• 8/4: 2.0000 minus 0.2500 gives 1.7500 in, which is 1-3/4 in.
• The allowance is per board, not per face. It is the total taken off both faces together.
• Because it is a flat subtraction, the bite is proportionally worst at the thin end: 4/4 gives up 18.75 percent of its thickness to surfacing, while 12/4 gives up 8.33 percent.

Rough sawn, skip-planed, S2S and S4S

Skip-planing is the useful middle ground and worth asking for by name. It cleans enough of the face that you can read grain, color and defects before you buy, without spending the thickness that a full S2S pass costs. If you own a jointer and a planer, rough or skip-planed stock is usually the better buy, because you decide how much comes off each face and you can keep a cupped board as thick as possible. If you do not, S2S saves you the milling but hands you a board that has already spent most of its margin, which matters enormously when the part you need is close to the surfaced thickness.

You are billed on the rough thickness

Here is the part that starts arguments at the counter. A hardwood tally is taken on the rough board. Thickness is the rough quarter size rather than anything you can measure with calipers after surfacing, and on random-width stock the width is normally read to the nearest inch. Your invoice therefore charges a 4/4 board at 1.0000 in even though 13/16 in of wood is what you carry to the truck when that board is surfaced. Every yard prices this way, the rate per board foot is set with the convention already baked in, and the alternative would simply be a higher rate applied to a smaller number. What it does mean is that two quotes only compare if both are stated the same way, so ask whether a price is per board foot rough before you put them side by side. Lumber price per board foot walks through lining two quotes up properly.

Quarters without the mental arithmetic. Board Foot Calculator₊ puts quarter notation on a picker with a rough and surfaced toggle, so you can price a board the way the yard bills it and still see the volume of wood you are actually taking home. Every total expands to show its working line by line, with your own inputs echoed back and a unit label on each number. Learn more about Board Foot Calculator₊.

Buying backwards from a finished thickness

1. Write down the finished thickness: The dimension on your cut list after all milling and sanding: a door rail at 3/4 in, a table top at 7/8 in, a leg at 1-3/4 in. Start here, not at the quarter.
2. Add what milling removes: Flattening and thicknessing a flat S2S board usually costs 1/16 to 1/8 in. A cupped, bowed or twisted board costs more, and a long board costs more than a short one because the twist has further to travel.
3. Pick the quarter whose surfaced thickness clears that total: Use the table below. If the margin left over is under 1/16 in, either move up a quarter or buy the board rough instead.
4. Decide rough or surfaced: Rough hands you the full quarter thickness and the freedom to take wood evenly off both faces. S2S saves a milling step but has already used up the surfacing allowance on your behalf.
5. Keep thickness margin separate from material waste: Margin for milling is a thickness question. How many extra board feet to buy for defects, snipe and short offcuts is a quantity question, covered in lumber waste factor.

Notice how thin the margin is at the top of that table. Buying 4/4 S2S for a 3/4 in finished part leaves 1/16 in in total, which is 1/32 in per face. That is plenty for a genuinely flat board and nowhere near enough for a cupped one, which is exactly why so many 3/4 in parts end up at 5/8 in. Buy 4/4 rough for the same part and you have a full 1/4 in to spend. The same logic explains why 8/4 is the standard answer for a 1-1/2 in leg: 6/4 rough measures exactly 1-1/2 in, so it leaves you nothing at all, and 6/4 S2S at 1-5/16 in cannot get there by any route.

Softwood does not use quarters

Construction softwood is sold in nominal sizes, a different convention with the same underlying habit of naming a board after something bigger than it is. A dry, surfaced 2x4 measures 1-1/2 in by 3-1/2 in, a 2x10 measures 1-1/2 in by 9-1/4 in, and a 1x6 measures 3/4 in by 5-1/2 in. Nominal 1 in thickness dresses to 3/4 in and nominal 2 in dresses to 1-1/2 in, nominal widths through 6 in lose 1/2 in, and nominal widths of 8 in and up lose 3/4 in. When board feet are quoted for softwood the math is conventionally done on the nominal size, so a 2x4 counts as 2 in by 4 in. That is not sloppiness, it is the same idea as hardwood rough thickness: the trade prices the board it cut, not the board it dressed. The one rule is never to mix the two systems inside a single estimate.

What to ask for at the counter

• Is the price per board foot on the rough thickness? It almost always is, and it is worth hearing out loud before you compare two suppliers.
• What surfacing is on this lift: rough, skip-planed, S2S, S3S or S4S?
• What thickness do you surface 4/4 to? Most yards work to 13/16 in, some run thinner, and the answer decides whether the board can still make your part.
• Is width measured on the rough board, and rounded to what?
• Can I take this rough if I am milling it myself, and is it priced the same?
• What grade is this lift? Grade governs how much clear cutting comes out of a board and is a separate question from thickness, covered in hardwood lumber grades explained.
• Is there a minimum quantity for this species at this thickness?

Once the thickness convention is settled in your head, everything after it is arithmetic. Keep the quarter chart above within reach, pull the wider reference tables from the board foot chart when you are estimating a whole job, and run the numbers at the rack with Board Foot Calculator.

Frequently asked questions

What does 4/4 lumber mean?

4/4 means the board was one inch thick in the rough, before any planing. The fraction counts quarter inches of thickness, so 4/4 is four quarters or 1 in, 5/4 is 1-1/4 in and 8/4 is 2 in. It describes thickness only, which is why hardwood is still sold random width and random length alongside it.

Is 4/4 lumber actually 1 inch thick?

Rough 4/4 is a full 1 in thick, but surfaced two sides it typically arrives at 13/16 in. The difference is the standard surfacing allowance of 3/16 in on stock 1-1/2 in and under. If you need a full inch of thickness, buy 5/4 or buy 4/4 in the rough and mill it yourself.

How thick is 8/4 lumber?

8/4 is 2 in thick rough and 1-3/4 in surfaced two sides. Stock 1-3/4 in and over uses the larger surfacing allowance of 1/4 in rather than the 3/16 in applied to thinner boards. That makes 8/4 the usual choice for a 1-1/2 in finished leg or apron.

Why am I charged for 1 inch when the board is only 13/16 in thick?

Because hardwood is tallied and billed on the rough thickness, not the surfaced thickness. The rate per board foot is set with that convention already built in, so the yard is not overcharging you any more than a lumberyard is when it sells a 2x4 that measures 1-1/2 by 3-1/2 in. Just make sure any two quotes you compare are both stated per board foot rough.

What is the difference between S2S and S4S lumber?

S2S has both faces planed flat and parallel while the edges stay rough and the width still varies. S4S has both faces planed and both edges jointed and ripped, so it comes out at a uniform width. Both land at the same thickness, so S4S costs you width and usually price, not thickness.

Is a 2x4 the same as 8/4 lumber?

No. A 2x4 is a softwood nominal size that actually measures 1-1/2 in by 3-1/2 in, while 8/4 is a hardwood rough thickness of a genuine 2 in. Softwood board-foot math is conventionally done on the nominal size and hardwood on the rough quarter size, and mixing the two conventions inside one estimate is a reliable way to buy the wrong pile.

What thickness should I buy for a 3/4 in finished top?

Buy 4/4. Rough 4/4 gives you 1 in and a full 1/4 in of milling margin, while 4/4 S2S at 13/16 in leaves only 1/16 in in total, or 1/32 in per face. That surfaced margin is fine on a flat board and not enough on a cupped or twisted one.

Keep reading

• How to Calculate Board Feet: The Formula, Worked Examples and the Traps
• Board Foot Chart: Common Hardwood and Softwood Sizes
• Lumber Price Per Board Foot: How Hardwood Is Quoted and How to Compare Two Quotes