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Introduction #
Lumber sizes, grades, and milling marks are three separate systems, and mixing them up is how the wrong board ends up on the truck. The Western Wood Products Association publishes the softwood lumber sizes and grade rules this follows; the hardwood system beside it works differently.
Order a 2x4 off the rack and you get a piece of wood that is not 2 inches by 4 inches. Order a 1x4 and it is not 1 inch thick. This is not a trick and it is not a mistake at the yard. It is how lumber has been named for more than a century, and if nobody explains the system to you, every piece of dimension lumber you pick up looks like it is lying to you.
A 2x12 stair tread does not give you a 12 inch step. On our garage steps it gives you a 10 inch tread with a 1-1/4 inch lip hanging over the riser below it. A board of "select" pine at the box store is not a species, it is a grade, a sorting bin the mill dropped the board into based on how clean the face looks. None of this is guesswork once you know the rules behind it, and the rules are written down: one book for softwood, another for hardwood.
This guide covers how lumber gets sized, graded, and finished at the mill, before species or profile even enter the picture. A separate guide in this series covers what species you are actually holding, and another covers how a piece of casing differs from a flat board or a baseboard. Here the question is simpler: what does the name on the tag actually promise, and how do you read what the mill already told you on the stamp.
Before You Begin #
Prerequisites #
Grab the same tape measure that already lives in your bag. Then find a cutoff of a 1x4 and a cutoff of a 2x4, real scraps off the truck, not a picture in this guide. Measure both while you read. The numbers below stop being abstract the moment your own tape confirms them.
What You Need To Know #
Every board you pick up carries three separate facts, and they answer three different questions. Size tells you how big it is. Species tells you what tree it came from. Grade tells you how clean and how strong the mill decided it is. A board can be big, cheap, and low grade, or small, expensive, and high grade, in almost any combination, because these three things stack on top of each other instead of being tied together. The one place size and grade tie together is hardwood grading, where the top grade needs a minimum board size, covered below.
Most of this gets printed or stamped somewhere, on the wood itself or on the ticket that came with it: a grade stamp on framing lumber, a pattern number on a bundle tag for trim, a species name on the yard's invoice. Finished trim off the shelf usually skips the grade stamp, because that call got made back at the mill. So look for the stamp or the tag before you guess, and treat a store shelf label like "select" as a clue, not a settled fact. This guide teaches you where to look.
Nominal Size vs Actual Size: The Rule, Not Just the Numbers #
The nominal size is the name on the board. A 2x4 is a 2x4 because that is what the mill calls it, not because it measures 2 inches by 4 inches. The actual size is what your tape shows after the mill dries the wood and planes it smooth. On dressed lumber, actual is always smaller than nominal, and the rule that governs exactly how much smaller is the American Softwood Lumber Standard, PS 20, the rulebook the graded softwood mills build to.
Two things shrink the board on its way from the log to the shelf. Drying pulls water out of the wood, and wood shrinks as it dries. Planing then shaves both faces smooth and square, cutting real material off on top of the drying loss. A rough green 2x4 leaves the saw bigger than the finished board, though a modern mill already saws it a little under a true 2 by 4. By the time it is dry and planed, it measures 1-1/2 inches by 3-1/2 inches, and that gap is not a defect. It is the standard working the way it was built to work. Pressure-treated lumber is the common exception: it usually ships wet, so it can measure a hair fuller until it dries in place.
End views drawn to scale, 1 inch equals 24 pixels. The name on the tag is not the number on the tape.
You will run into this constantly on real jobs. Order a 1x12 for a stair skirt board and you get a board 11-1/4 inches wide, not 12, and in most cases that 11-1/4 is what a drop-in skirt board needs. See How to Install Stair Skirt Boards for the full layout.
Garage steps do the same thing sideways. Stringers and treads there run 2x12, and the actual 11-1/4 inch width gives you a 10 inch tread once you account for a 1-1/4 inch nosing overhang out front. See How to Build Garage Steps.
Closet shelving hides the same trap in its depth. The three standard depths are called 12, 16, and 24 inches, but the actual measured depth runs under that nominal number and it shifts by supplier: a 12 typically comes in at 11-1/4 or 11-1/2, a 16 at 15-1/4 or 15-1/2, a 24 at 23-1/4 or 23-1/2. That actual number is what your cleat cuts have to match, not the label. See How to Build Basic Closet Shelves, Part 1.
The Softwood 1-By and 2-By Actual-Size Tables #
Here are the actual, dry, dressed sizes for the boards and dimension lumber you will measure most, straight out of PS 20.
| Board (nominal) | Actual size |
|---|---|
| 1x2 | 3/4 x 1-1/2 |
| 1x3 | 3/4 x 2-1/2 |
| 1x4 | 3/4 x 3-1/2 |
| 1x6 | 3/4 x 5-1/2 |
| 1x8 | 3/4 x 7-1/4 |
| 1x10 | 3/4 x 9-1/4 |
| 1x12 | 3/4 x 11-1/4 |
| Dimension lumber (nominal) | Actual size |
|---|---|
| 2x2 | 1-1/2 x 1-1/2 |
| 2x3 | 1-1/2 x 2-1/2 |
| 2x4 | 1-1/2 x 3-1/2 |
| 2x6 | 1-1/2 x 5-1/2 |
| 2x8 | 1-1/2 x 7-1/4 |
| 2x10 | 1-1/2 x 9-1/4 |
| 2x12 | 1-1/2 x 11-1/4 |
The width loss follows one simple split. Nominal widths under 8 inches lose 1/2 inch. Nominal widths of 8 inches and wider lose 3/4 inch. That is why a 1x6 loses only 1/2 inch (5-1/2 actual) while a 1x8 loses 3/4 inch (7-1/4 actual), even though both are called "1-by" boards.
A 4x4 follows the same width rule as a 2x4, so it dresses down to 3-1/2 inches square, not 4. Timbers, stock 5 inches and thicker in its smallest dimension, follow their own line in PS 20: nominal minus 1/2 inch in both directions, so a 6x6 is 5-1/2 inches square and an 8x8 is 7-1/2. This guide does not chase framing timbers any further. A separate guide in this series covers framing lumber in depth.
The 5/4 and 6/4 Answer: How to Actually Get 1-Inch-Thick Material #
Say it the way the yard says it: five-quarter, not "five fourths." Softwood board stock thicker than a standard 1x is named in quarters of an inch, measured on the rough board before it is dried and planed, and that name rides all the way to the invoice even after the actual thickness changes.
A 1x board dresses to 3/4 inch, so ordering "1x" off the rack will not get you a true 1 inch of thickness. You have to order 5/4 (five-quarter). A 5/4 board dresses to a true 1 inch. Step up to 6/4 and you get 1-1/4 inch. Step up to 8/4 and you get 1-1/2 inch. These come from the same PS 20 standard as the tables above, just applied to board stock instead of dimension lumber.
The Hardwood Quarter System (4/4 Through 16/4), and Why It Is Not the Softwood System #
Hardwood borrows the same quarter naming softwood board stock uses, then the two systems part ways. Softwood 5/4 and 6/4 are named off the standard board tables above. Hardwood is named off its own rough-sawn thickness, starting at 4/4 (1 inch rough) and climbing in quarter-inch steps through 16/4 (4 inches rough).
The finished, S2S (surfaced two sides) thickness runs thinner than the rough call, the same shrink-and-plane loss you already saw on softwood, just with its own numbers. A 4/4 hardwood board nets 13/16 inch. A 5/4 board nets 1-1/16 inch. An 8/4 board nets 1-3/4 inch. Hardwood widths and lengths are usually random too, sold board by board off the actual piece rather than picked from a fixed catalog of stock widths the way a 2x4 or a 1x6 is. Poplar is the exception you will meet most: it comes milled S4S in fixed widths, 1x2, 1x4, 1x6, the same way pine does, because it is the paint-grade hardwood.
| System | 6/4 nets to |
|---|---|
| Softwood board stock (PS 20) | 1-1/4 inch |
| Hardwood, S2S (NHLA) | 1-5/16 inch |
Same name, two different numbers, because the two numbers come from two different rulebooks. If somebody says "6/4" without saying which system, ask which one they mean.
S4S, S2S, and S1S2E: What "Surfaced" Means and How Many Faces #
S4S means surfaced four sides: every face and edge has run through the planer, smooth and flat. A standard 1x board off the shelf is S4S. Most S4S boards have square edges, but an eased edge is a separate spec (marked EE or E4E) that some stock is sold with, so S4S by itself does not promise a sharp corner. S2S means surfaced two sides only, the two wide faces, with the edges left rough from the saw. S1S2E means surfaced on one face and two edges, smooth on one side and along both long edges, rough on the back.
We run S4S constantly. A 1x8 S4S board laid flat on a ledge is what this crew calls the nosing, the base the handrail sits on before the newel posts go in. See How to Install Wood Nosing. We also run 1x4 S4S as the framing behind a granite support, and closet cleats are 1x4 MDF or pine.
Casing, baseboard, and a plain S4S board can look almost the same from across a room, but they are three separate manufactured products, and a tape on the thickness usually settles it. A 1x4 S4S board is 3/4 inch thick. Flat casing usually runs 11/16 inch. Flat MDF baseboard usually runs 9/16 inch, and real flat base spans about 1/2 to 5/8 depending on the line. For the common flat paint-grade products, those three numbers do not overlap, so thickness tells you what you are holding. Two exceptions to know: some yards sell 3/4 inch "flat stock" as casing or base, a square-edged board sold as trim, and some solid-wood or tall MDF base profiles are milled a full 3/4. When the thickness matches, go to the edge and the back.
Width can fool you more often than thickness. A 1x4 and WM 412 flat casing are both exactly 3-1/2 inches wide, and some base profiles are stocked at 3-1/2 as well, so width alone will not separate them. Base stock more often runs 3-1/4 inches, close enough to confuse at a glance.
The second tell is the edge. A plain 1x board off the shelf usually has a hard, square corner, the arris, on every edge. Casing and baseboard usually carry a deliberate eased edge instead, a small radius. It is a helpful tell, not a rule: flat stock casing and base are sold square-edged on purpose, and some S4S boards come with eased edges.
The back of the piece tells you a third thing, but be careful what it actually tells you. Casing and baseboard both get a shallow groove, or hollow, milled into the back, called back relief, so the flat face sits tight against a wall that is not perfectly flat. Both pieces get it. Back relief separates molded trim from a plain S4S board, and it does not separate casing from baseboard, they both carry it the same way. Two caveats worth keeping in mind: relief on MDF trim is not consistently documented and can run shallower than on solid stock, so a flat-feeling back does not by itself prove you are holding a plain 1x, and a tape on the thickness is still the best first test.
Rough Sawn and Resawn #
Rough sawn lumber comes straight off the saw with little or no planing. It runs close to its full nominal size, feels fuzzy to the hand, and usually shows saw marks on every face; a skip-planed piece may have a face or two trued up. It shows up on rustic beam work and specialty stain-grade runs where the rough texture is the point, not a flaw to plane away. Our own ceiling beams are smooth boxes of clear pine or white oak, not rough timbers.
Resawn is different. It means a board, rough or surfaced, that has been split through its thickness on a bandsaw into two or more thinner pieces, often to get a fresh rough face on the new pieces. The cut does not have to land in the middle. Rough sawn describes how the piece left the mill. Resawn describes cutting an existing board thinner.
Board Foot vs Lineal Foot: Why Hardwood and Softwood Are Priced Differently #
A board foot is a volume measurement, not a length. One board foot equals 144 cubic inches of wood, the same volume as a piece 1 inch thick, 12 inches wide, and 12 inches long. The formula: nominal thickness in inches, times width in inches, times length in feet, divided by 12.
Try it on a real board you could check on your phone. A 4/4 hardwood board, 6 inches wide, 8 feet long: 1 (thickness) times 6 (width) times 8 (length) equals 48, divided by 12 equals 4 board feet. Hardwood gets sold this way because it comes in random widths and lengths, so volume is the fair way to price it board to board.
Softwood dimension lumber, S4S boards, and stock molding do not sell that way at the yard counter. They sell by the piece or by the lineal foot, which our guides also write as linear foot, because the width and thickness are already fixed by the catalog. Once those two are fixed, length is what you are paying for; species, grade, and treatment set the price per foot. (Mills and wholesalers still trade softwood by the thousand board feet, MBF, but by the time it reaches you it is per piece or per foot.)
Softwood Grades: Appearance Grades and Structural Grades Are Two Different Systems #
Softwood carries two grading systems you will meet, and they answer two different questions. Appearance grades ask how clean the face looks. Structural grades ask how much load the piece can carry. A board can rank high on one system and never enter the other at all, because the two systems are not measuring the same thing. (There is a third, factory and shop grades, for lumber headed to a door or window plant; it does not show up on a trim ticket.)
Appearance, or board, grades cover material picked for how it looks: trim, paneling, shelving. At the top sit the Selects: B&BTR (B and Better, the two cleanest grades combined), C Select, and D Select, each carrying a few more small knots and blemishes than the last. Below the Selects come the Common board grades, No.1 Common down through No.5 Common, with more knots and more waste at every step down. "#2 pine" in our own material lists is this same system: it is a No.2 board grade (No.2 Common under the western rules, plain No.2 under the southern pine rules), not a species.
Structural, or dimension, grades cover framing lumber, sorted by strength instead of looks: Select Structural, No.1, No.2, No.3, Stud, and the Construction, Standard, and Utility group used in light framing. A framing 2x4 stamped No.2 has nothing to do with a trim board stamped No.2 Common. Same number, two different systems, two different questions.
Retail labels like "select," "clear," "premium," and "knotty" at the box store are shorthand the store puts on the rack. Some line up with a real agency grade (Select and Premium are both real grade names in the rulebooks) and some are just descriptions. Either way, clear and knotty describe a grade, how many knots and how big, not a species. A board can be clear pine or knotty pine and still be the exact same tree underneath.
Hardwood NHLA Grades: FAS, FAS 1-Face, Selects, No.1 Common, No.2A Common #
Hardwood grading runs on a different idea entirely: how much clear, usable face you can cut out of the board. The National Hardwood Lumber Association (NHLA) sets the rules most North American hardwood lumber follows. Hardwood flooring and tropical species like mahogany are graded under their own separate rules.
FAS, the top grade, requires boards 6 inches and wider, 8 feet and longer, and yields 83-1/3 percent clear face cuttings (a few species, walnut among them, get smaller minimums). FAS 1-Face asks for that same FAS face on one side and allows the No.1 Common standard on the other. Selects match FAS on clear yield but allow a smaller board, 4 inches and wider, 6 feet and longer. No.1 Common drops to boards 3 inches and wider, 4 feet and longer, yielding 66-2/3 percent clear. No.2A Common drops further still, the same 3 inch by 4 foot minimum board, yielding 50 percent clear.
Here is what actually matters day to day: most of what this crew touches is hardwood molding and S4S board stock, already milled to a finished thickness long before it reaches us, so the grade call was made at the mill. The grade name matters most when somebody is ordering a custom run of raw stock, and it still matters when you are hand-picking S4S boards for a stain-grade run. It stops mattering once you are looking at a bundle of finished, profiled trim.
Reading a Grade Stamp: Species, Grade, Mill Number, Moisture Content, Grading Agency #
A softwood grade stamp packs five facts into one small mark, and once you know where to look you can read most new framing lumber cold. An offcut or a weathered piece may have lost its stamp.
- Species. The tree or species group, like DF for Douglas Fir or SPF for spruce-pine-fir.
- Grade. The grade name or abbreviation, like SEL STR, NO.2, or STUD.
- Mill number. Traces the piece back to the specific mill that ran it.
- Moisture content. How dry the piece was when it got stamped.
- Grading agency. Who supervised the grading. WWPA, SPIB, NLGA, and WCLIB are the names you will see most, all of them accredited through the American Lumber Standard Committee (ALSC).
The moisture mark is worth memorizing on its own. S-GRN means the piece was over 19 percent moisture content when stamped, unseasoned. S-DRY or KD means 19 percent or under. MC15 or KD15 means 15 percent or under, a tighter standard. KD-HT means kiln-dried and heat-treated, run hot enough to meet pest-control rules, more relevant to shipped or imported material than to local framing stock.
Shaped molding almost never shows a grade stamp; the stamp is a framing-lumber thing, and where a finish board is stamped at all it is on the back or an end where it will not show. A flat 1x board sold as trim can carry a stamp, so keep the stamped face to the wall. The trim equivalent is the pattern number printed on the bundle tag or the ticket, covered later in this guide.
Moisture Content and Acclimation: Why Trim Sits Inside Before It Goes Up #
Wood keeps trading moisture with the air around it for its whole life. Interior trim wants to reach roughly the same moisture level as the room it will live in before you nail it down, not the level it happened to be at when the truck delivered it. Kiln-dried trim typically arrives in the 6 to 9 percent moisture content range, close to what a heated, occupied house runs, but not always an exact match.
Wood moves almost entirely across the grain, not along its length, which is why a 12 foot piece of casing does not noticeably grow or shrink in length season to season, while a wide flat board can visibly cup or bow. Wood shrinks about twice as much in one direction across the grain as it does in the other, and that mismatch is why a wide board tends to cup toward the bark side as it dries. A board that was already dried flat and then picks up moisture tends to cup back the other way.
This crew's own practice matches the science: let material acclimate inside the heated house for 24 to 48 hours before it goes up. Acclimate means the material sits and adjusts to the temperature and humidity of the room it will live in before you install it. See How to Install Baseboard for where that step falls in the sequence.
Pattern Numbers (WM Numbers): How a Profile Gets Ordered by Number, Not by Sample #
The Wood Moulding and Millwork Producers Association (WMMPA) publishes the WM pattern book, a numbered catalog of profile cross sections. A WM number is not a style name, it is a specific cross section from the pattern book: set dimensions and a set profile shape, meant to be the same piece whichever mill cuts it, though differences of a fraction of an inch can show up between mills or runs.
Builder selection sheets already order this way. One in the company files calls out WM-418, WM #25 for window sills, a WM 6 base, and the words "FLAT MDF". The WM 412 mentioned earlier in this guide is a flat casing profile, 11/16 by 3-1/2, the one that matches a 1x4 in width. A profile, meaning a pattern number, is how a mill knows exactly what to cut without you shipping them a physical sample.
Stock vs Custom Run-to-Match Molding, and What a Knife Charge Is #
Stock molding is whatever is already in the catalog, cut, bundled, and sitting on the rack, sold by the piece. You order a WM number and it ships.
Custom, or run-to-match, molding works differently. A mill grinds a new steel knife to match a profile you give them, either a physical sample piece or a careful tracing, then runs your order on that knife. That setup is a real cost the mill has to recover, so a custom run usually carries a one-time knife or setup charge on top of the material, plus a minimum footage to make the setup worth the mill's time, plus a real wait while the knife is ground and the order runs. Stock molding skips the knife charge and most of the wait.
This is standard practice across the millwork trade, not a markup specific to one mill. Even a shop willing to work from a sample or a scale drawing still needs the profile grinding and setup time before the first stick runs.
Lumber Sizes Quick Reference #
The ten sizes a trim carpenter measures most, and six lines worth memorizing.
| Nominal | Actual |
|---|---|
| 1x2 | 3/4 x 1-1/2 |
| 1x3 | 3/4 x 2-1/2 |
| 1x4 | 3/4 x 3-1/2 |
| 1x6 | 3/4 x 5-1/2 |
| 1x8 | 3/4 x 7-1/4 |
| 1x12 | 3/4 x 11-1/4 |
| 2x4 | 1-1/2 x 3-1/2 |
| 2x6 | 1-1/2 x 5-1/2 |
| 2x12 | 1-1/2 x 11-1/4 |
| 5/4x6 | 1 x 5-1/2 |
- 5/4 softwood dresses to a true 1 inch thick (hardwood 5/4 nets about 1-1/16).
- Board foot equals nominal thickness in inches, times width in inches, times length in feet, divided by 12.
- S4S means surfaced four sides, smooth and flat, no profile. Square edges are usual, not guaranteed.
- The thickness tell: 1x4 is 3/4 inch, flat casing is usually 11/16, flat MDF base is usually 9/16. If the thickness matches, check the edge and the back.
- S-DRY means 19 percent moisture content or under.
- Clear and knotty are grades, not species.
