Lumber Sizes: Nominal vs Actual, Straight From the Standard

The chart everyone publishes, and the three columns they leave out

Nominal versus actual is probably the most reprinted table in American construction. A 2x4 is 1-1/2 by 3-1/2, a 2x8 is 1-1/2 by 7-1/4, and there are several hundred pages saying so.

They are right, as far as they go. What they mostly skip is that all of these numbers come from one document — the American Softwood Lumber Standard, PS 20, published by NIST — and that document contains a good deal the reprints drop:

• a second set of sizes for green lumber, which is bigger
• a different rule for timbers, 5 inches nominal and up
• and small-board thicknesses that break the simple subtraction rule people quote

The tables above are Table 3 of that standard, read from the standard.

Green lumber is dressed larger, and it is not a rounding error

Here is the column that goes missing. A dry 2x4 is 1-1/2 by 3-1/2. A green 2x4 is 1-9/16 by 3-9/16 — a sixteenth of an inch bigger on both dimensions. A green 2x12 is 11-1/2 where the dry one is 11-1/4, a full quarter inch wider.

The mill is not being generous. It is being predictive. The standard draws the line at 19 percent moisture content: at or below that, lumber is dry; above it, green. Wood shrinks across the grain as it gives up moisture, so a board dressed green is cut oversize on the understanding that it will come down to roughly the dry dimension once it has sat in a heated building for a season.

That is worth knowing for two practical reasons. The first is that a stack of green 2x10s framed tight today can leave gaps later. The second is the opposite complaint, heard more often: someone measures a board, finds it a sixteenth over the chart, and concludes the chart is wrong. The chart is fine. The board is wet.

The reduction is not one rule, it is three

The shorthand in circulation — and in Google's own summary of this question — is: subtract a quarter under 2 inches, a half from 2 to 6, three quarters at 8 and up.

That is right in the middle and wrong at the bottom. From the standard:

• Under 1 inch nominal: only 1/16 to 1/8 comes off. A 3/8 board dresses to 5/16.
• 1 inch to 1-1/2: 1/4 comes off. A 1x is 3/4. Five-quarter stock finishes at exactly 1 inch.
• 2 through 7 inches: 1/2 comes off. This is the familiar band.
• 8 inches and over: 3/4 comes off.

That last step is the one that costs money. Carry the half-inch rule past 7 and you predict a 2x8 at 7-1/2. It is 7-1/4. Over a stair stringer, a rim joist or a run of shelving, a quarter inch per board adds up into something you have to explain.

Timbers play by different rules again

Anything 5 inches or more in nominal thickness is a timber in the standard's language, and it gets its own line in Table 3 plus a note that timbers are often manufactured and sold without drying.

For 5x and 6x material, 1/2 inch comes off whether it is dry or green — so a 6x6 is 5-1/2 either way, which is why that one feels reliable. From 7 to 15 inches the two diverge: 3/4 off dry, but only 1/2 off green. A green 8x8 measures 7-1/2. A dry one measures 7-1/4.

So if you have a post that seems a quarter inch fatter than the internet promised, you are probably holding green timber, and it is exactly the size the standard says it should be.

What this actually changes on site

Sizes are trivia until they are added together. Then they decide things:

A 2x4 wall is 4-1/2 inches finished. Three and a half of stud, half an inch of drywall on each face. With 5/8 board it is 4-3/4. That is the number a door jamb has to match, the depth of every window return, and the amount of floor a partition costs you — and it is not on any lumber label.

A built-up header is two 2x members and a piece of 1/2 inch plywood for a reason that is pure arithmetic. Two 2x pieces are 1-1/2 each, three inches together, half an inch shy of the 3-1/2 inch stud depth. The plywood makes up exactly that half inch, so the header finishes flush and the drywall lies flat across it. It stiffens the header too, but the thickness was chosen by the sum.

Four 2x4s face to face are 6 inches, not 8. Obvious once said, and still the source of built-up posts that do not fill the pocket someone framed for them.

A 2x10 floor joist is 9-1/4 deep. Every floor built from nominal sizes finishes lower than its name suggests, and the gap compounds when you stack a system: joist, subfloor, underlayment, finish floor.

Where the standard stops

Two limits worth stating plainly.

The dressed sizes in PS 20 are minimums. A mill may run over them, and a board that has spent a month in a wet yard measures what it measures. Where a fraction decides whether something fits, put a tape on the actual piece rather than trusting any chart, this one included.

And this standard covers sawn softwood lumber only. Plywood and OSB are sold at or near their true thickness, and usually a shade under — a sheet called 1/2 inch is very often 15/32. LVL, PSL and I-joists are manufactured to their maker's own dimensions and are generally exactly what the label says. Nominal-versus-actual is a solid-sawn problem, and one of the few places where engineered lumber is simply easier.