3138: Dimensional Lumber Tape Measure

Dimensional Lumber Tape Measure

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Title text: A person with two watches is never sure what time it is, especially if I got them one of the watches.

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Explanation

This is one of 52 incomplete explanations: This page was created by AN ARMY OF TWO INCH BY FOUR INCH LUMBERS THAT SAW FIVE CM BY TEN CM LUMBERS. Don't remove this notice too soon. If you can fix this issue, edit the page!

A 2×4 is a type of dimensional lumber, meaning it is cut to a specified cross-section. In the case of a 2×4, despite implicitly specifying dimensions of 2 inches by 4 inches, its actual dimensions are 1.5 inches by 3.5 inches. The Dimensional Lumber Tape Measure “fixes” this inaccuracy by changing the length of some of its indicated inches so that a 2×4 is measured as 2 inches by 4 inches. On the dimensional tape the 1st, 2nd, and 8th division are made shorter than on the standard tape. Note that the comic states 7.125 inches as the width of a 1×8, when in reality the width is 7.25 inches.

As explained on Wikipedia, the nominal dimensions of a piece of dimensional lumber (US)/timber (UK) are those to which, in history, the wood was cut from green logs. Over time, the wood would shrink from loss of water. Consequently, a board cut to 2×4 inches would shrink to some fraction of those dimensions. The nominal dimensions also refer to the rough cut lumber—the final product is typically planed smooth, which further reduces its dimensions. The actual final dimensions would vary based on the type of wood, the amount of water lost and other such factors, with a greater or lesser amount of predictability. Over time, the actual dimensions of the wood became standardized at some regularly-achievable value less than the nominal dimensions. Different types of construction material sometimes use different measures, for instance, “1 inch” plywood is typically not 1 inch thick, but it is also not 3/4 inches thick (the thickness of a 1-inch board).

A person not familiar with this history may be puzzled at the disconnect between the nominal and actual dimensions of lumber/timber, perhaps to the point of thinking that some underhanded short-measure had gone on. To such persons, the comic’s Dimensional Lumber Tape Measure makes sense, or at least addresses the disconnect. It would not, however, have any practical use, and attempts to employ it would likely lead to constructions going dangerously awry. Necessary lengths of timber, as well as other cuts that fine-tuned a supplied timber to fill a space, would be intrinsically inconsistent with the gaps they were intended to tightly fit within. At best, every single component of a construction would be measured and cut according to this particular measure and the resulting structure would be self-consistent but subtly undersized compared to the original plan.

Additionally, leaving it in someone else’s toolbox without informing them would likely lead to them incorrectly measuring things, as the Dimensional Lumber Tape Measure appears visually similar to a standard tape measure and has similar enough units that it is plausible someone could use the Dimensional Lumber Tape Measure and assume it indicated full inches.

The title text is a play on the adage “A man with two watches is never sure what time it is”. That adage is a rephrasing of Segal’s law.

Transcript

This is one of 27 incomplete transcripts: So far, only the raw text was added. Needs description of the pictures and layout. If you can fix this issue, edit the page!

Dimensional Lumber Tape Measure

Dimensional lumber sizes are tricky. A “2×4” is actually 1½" by 3½", and a “1×8” is ¾" by 7⅛".

(A drawing of a rectangular wooden block labelled “2×4”, with notes indicating the length of the sides being 1½" and 3½".)

If you know someone into carpentry or woodworking, get them our dimensional lumber tape measure.

(A drawing of two tape measures seen from the side. One is labeled 12', and the other is labeled 12'*.)

(Two drawings of the tape measures’ tapes is cutoff in the right panel:)

Normal tape measure: (The measure is divided into inches evenly.)

Dimensional lumber tape measure: (The measure is divided unevenly, such that 1st and 2nd marked ‘inches’ are each equal to a ¾-inch but the 3rd to 7th marks each equate to a single 1 inch. The 8th mark is once again ¾-inch after the 7th.)

(Neither measure indicates the unit being used.)

Don’t tell them you got it—just leave it in their toolbox. They’ll appreciate the surprise when all their measurements work out!

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Discussion

I was under the impression this was actually a thing that exists, somewhere. (Separate and apart from so-called "shrink rules" used by patternmakers who create patterns for metal castings). No? JohnHawkinson (talk) 00:36, 6 September 2025 (UTC)

https://www.inchcalculator.com/actual-size-of-dimensional-lumber/ for reference --- MEL

Wikipedia has a similar table. Interestingly, if the values on this table are correct, the xkcd measure fails for the 8 x 8 board. 2605:59C8:160:DB08:988B:772A:4E5:B209 02:18, 6 September 2025 (UTC)

>the xkcd measure fails for the 8 x 8 board Studs and joists are routine repetitive structure and should be the same as their neighbors. 8x8 are non-routine; even in say a heavy mill building 8x8s are costly enough for the carpenter to measure or trim every column. --PRR (talk) 17:10, 8 September 2025 (UTC)

My house has near full-size 2x4s and 2x6. Very-dry trees were sawn on site, the saw set for 2.0" centers. Band saw has very narrow kerf. About 1.9". A profitable saw-mill would use a coarser blade and push the size down as much as customers would accept (and even a junior carpenter can tell an undersize stud by feel). But here they were clearing land as much as saving money. --PRR (talk) 17:10, 8 September 2025 (UTC)

"I was under the impression this was actually a thing that exists, somewhere" - if doesn't exist now, it soon will. 70.115.234.146 03:59, 6 September 2025 (UTC)

Thinking that there might be a typo in the comic - It says : A "1x8" IS "3/4 BY 7 1/8", yet it should be "3/4 BY 7 1/4".

I no longer want to be a lumberjack! 2A02:2455:1960:4000:748F:2291:F005:1989 06:57, 6 September 2025 (UTC)

I'm a lumberjack and I'm OK. I sleep all night and I work all day! ;-) --Kynde (talk) 05:31, 8 September 2025 (UTC)

This reminds me of when I changed my friend’s text replacements to be slightly misspelled whenever she tried to type a common word in college. She was getting a degree in linguistics and it was SO FUNNY 《プロキシ》(XKCD中毒者) 13:29, 6 September 2025 (UTC)

She should have claimed that she was undertaking "applied linguistics" and investigating how to create a deliberate language change! 92.17.62.87 20:23, 6 September 2025 (UTC)

I once set a stoned friend's keyboard to French. 90% of the letters and 20% of the special characters are the same, so he spent multiple minutes getting frustrated why he kept "missing" the correct keys. :D Fabian42 (talk) 07:26, 9 September 2025 (UTC)

I was told (and maybe this is wrong), that the dimensions are intended to represent the final thickness of a wall when drywall (usually 0.5" thick) is attached to the studs. Shamino (talk) 20:58, 6 September 2025 (UTC)

I think that's just a convenient side effect. Of course, if the drywall is 1/2 thick, a wall with 2x4 studs will be 4.5 inch thick. RegularSizedGuy (talk) 22:22, 6 September 2025 (UTC)

That would be pretty ahistorical, so I think it is indeed wrong. Drywall is a relatively modern invention, and I think the standardization of 2x4s as 1.5″ thick predates it (need to check that…but even if it didn't, then it would be worse). Wood lath and plaster walls are more like 5/8″ from the stud face, if not more. And, of course, in modern American multifamily residential construction 5/8″ walls are more common, or even double-5/8″ walls (making 1 1/4″) in fire-rated assemblies. So it does not even end up being "convenient," not that a 4" wall assembly is particulary more "convenient" than a 4.5″ or a 4.125″ or a 4.75″ wall assembly…very little turns on the thickness of the stud plus wallboard, but a lot turns on the thickness of the stud cavity (insulation, space for utilities, &c.) or the thickness of the drywall (spacing of electrical outlets, mudrings, etc.). JohnHawkinson (talk) 04:20, 7 September 2025 (UTC)

Once again, the UK wins on ridiculousness. I bought some fencing materials yesterday. The panels were 1.83m x 1.22m, so they could be metric but nevertheless 6' x 4'. I got some presawn posts that were 2400mm long (so kind of 8', or close enough), and they were sold as 75mm x 75mm, so they were 3x3, but they fit perfectly into the 70mm x 70mm post supports I got to go with them. Using metric to sidestep the need for traditional-measurement nonsense...but just keeping the nonsense and throwing new numbers at it. Actually, that should rendered into Latin and put on a scroll as part of a national coat of arms. Yorkshire Pudding (talk) 09:49, 7 September 2025 (UTC)

And in the meanwhile, when you buy wood here in Germany, it's the size it's specified. Buy a 40mm×60mm×4m beam from Obi, and that's what you'll get. --Slashme (talk) 14:46, 27 November 2025 (UTC)

Since the tape measure uses variable length inches, the watch (provided by the cartoonist) might be similar to Vetinari's clock. (Where individual ticks are of random duration.) I looked to see if xkcd had covered such a clock before (for possible link), but didn't find one. 2600:387:4:803:0:0:0:90 19:16, 7 September 2025 (UTC)

As an aside, I am always bemused about Americans being so stuck on imperial measurements when metric is so much easier. Oh except money...Americans are happy with metric money :o). 59.101.181.77 20:42, 7 September 2025 (UTC)

Oh, undoubtedly. It's literally just counting. The system we all use for enumerating everything (including feet and inches, or furlongs and chains, or drachms and scruples, or whatever else) is base 10. So just use base 10 and give names to 1,10,100,1000, etc. of length/capacity/mass/etc. units, and nobody needs to know anything beyond counting to deal with absolutely everything. Anybody who says pounds and ounces, or yards and miles (or whatever) is superior is objectively wrong.

But.

People get used to things.

So you get the mess I mentioned above.

Adopt the metric system, but then sell milk in 568ml bottles, because that's a pint, and milk inherently belongs in pints, so people have to have that much milk as a unit. Sell syrup and treacle in 454g and 907g cans, so 1lb and 2lb cans of sugary stuff can still exist. Nobody would be able to cope with 400g, 450g, 500g, 900g or 1kg! Keep selling beer and cider in pints, but change spirits to 25ml or 35ml (which you choose is up to you as a licensed bar) from 1/6 of a gill (or 1/5 of a gill in Scotland). Sell fuel in litres, but advertise vehicle fuel consumption in miles per gallon. Yorkshire Pudding (talk) 22:59, 7 September 2025 (UTC)

I know, obviously not the full shilling! 92.17.62.87 23:16, 7 September 2025 (UTC)

I was amused to see, watching the women's rugby world cup at the weekend that distances were given in metres.--86.163.160.215 08:56, 9 September 2025 (UTC)

I will venture the hypothesis that folk in this conversation are not cooks. It's in the kitchen that the difficulties with the metric system are most frequently encountered. Many Imperial measures are factors of two; it's easy and intuitive to double something or halve something and have the result make sense, as several kitchen veterans have told me over the years, with varying degrees of irritation. A pound (16 ounces), half a pound (8), a quarter pound (4), yada. Too many halvings in the metric system, and you're into fussy decimals. Moreover, if the recipe calls for a pound of butter, and you feed it half a kilogram, thinking that's the metric equivalent and close enough, the biscuits/cookies ain't gonna come out the same, and folk are gonna come after you. I learned years ago to check the cup measure carefully to see if it was graded in ounces or milliliters, and whether the recipe it was supposed to be serving came from Yankeeland or Godzone. Or else. The metric system may be logically superior, but may not be practically superior in all contexts. It might be well to seek reasons, other than the usual dismissive ones, for why, for example, Canada took 15 years to fail to fully convert to the metric system.2605:59C8:160:DB08:F102:9332:DCBD:89C6 03:10, 8 September 2025 (UTC)

I very much am a cook, and that is, I'm afraid, nonsense. Cooking is the perfect example of why metric is better! A recipe written in imperial, or the almost-identical US Customary Units, doesn't work if you substitute a round number of grams in for it, no. Obviously. But that presupposes that recipes naturally occur in imperial, with conversion being necessary if grams are used. Plenty of recipes exist natively in metric though, and are a clumsy mess if performed in avoirdupois.

And "fussy" decimals aren't a problem. If you need to halve 325ml (for example), no recipe will be affected by your using 163ml instead of 162.5 – much as fluid ounce measurements aren't accurate to the half millilitre, millilitres don't need to be either. Being a Brit, I learned to bake bread in imperial because we're across two systems here, and were even more so when I was young, but I forced myself to change, because metric is, inarguably, vastly superior. Working with percentages of hydration when you're in fluid ounces of water and pounds of flour (or the entirely nonsensical volumetric cup system) is utterly ridiculous when you could just use numbers that are exactly equivalent to each other. 1kg of flour, at a 66% hydration ratio? Why, that'll be 660ml of water, which can simply be weighed into the bowl at 660g. Can pounds, ounces, pints, fluid ounces and cups do that? Very much no.

And your halveable measures are all well and good...if you're halving those particular numbers. Anyone can think of numbers that are easily halved though. But what if it's a 2 egg recipe with 3oz of flour, and you want to make 3 eggs' worth? Well, then you need 4.5oz of flour. A bit...fussy, no? Imperial and US-measure recipes feel like examples of pounds working neatly, because they've been constructed around easy-to-use quantities in that system. But metric recipes behave just as neatly, and are far more readily scalable, because the numbers are all just base 10, which everybody uses for everything all the time.

I know both. I can use both. I started out with imperial. But I choose to use metric, because metric is so very obviously superior. Yorkshire Pudding (talk) 15:17, 8 September 2025 (UTC)

I am also a cook. I do all or part of the cooking on a regular basis for multiple families. The system I am accustomed to is the "obviously superior" one. While I can use both, multiplication by two (or three or five or 19) is just as easy as by ten. More importantly, grabbing the "nonsensical volumetric cups", while more than a little inaccurate, is very easy compared to grabbing the scale and making sure it's tare'd correctly for each ingredient. You also ignore the intrinsic issues with the metric system as well, given your example: two eggs is very easy to work with, while 114gm of eggs just begs for the same half-again issues. While the metric system has many advantages, I'm responding to your very clear tone that you feel superior since you "upgraded" which system you use. Perhaps most importantly, as a cook AND a baker, I'm also well aware that no measurements really need that much precision in cooking. Source: I do almost all my "measuring" by "feel" which is so very obviously superior to both. AlexaDTink 172.223.58.201 16:19, 5 October 2025 (UTC)

Of course, American recipes get round this problem by measuring things by volume, using a standard cup size.--86.163.160.215 11:15, 9 September 2025 (UTC)

See Technology Connection's latest video where he goes through a ridiculous series of imperial conversions to get from 192g of water to "about 0.2L". 80.189.2.17 (talk) 23:49, 8 September 2025 (please sign your comments with ~~~~)

This is, of course, already covered by 2585: Rounding and 3065: Square Units, in which a value of [exactly?] X <unitOne>s is described as [roughly?] Y <unitTwo>s then the premise becomes that it's [exactly!] Y <unitTwo>s or [roughly?] Z <unitThree>s, etc (including immediately/eventually back to X′ <unitOne>s, where X′≠X, and it may even be doubtful if X′≈X to a useful degree).

Conversely, where the conversion ratio is exact and rational (especially decimally rational, yet may have a number of significant digits actual precision may get lost by there being an unknown/unstated/misstated exactitude (and the inverse of any rational number is often (decimally!-)irrational/even more unweildy).

This is not to say that 192g of water (especially if overprecise, for the circumstances) cannot be usefully summarised as "about 0.2L", even without transitioning through non-metric scales or convolutions. (How many pounds weight? ...that means a given amount of pounds force, in a given situation. ...that could be expressed as newtons. Which, in a particular setup conveys a given pressure as of <blah> atmospheres... The number of moles of a gaseous substance that would apply that same degree of pressure at a given temperature is... and eventually that's then related back again to a volume of water under STP.) But you really need to know where (and how much of) the fuzziness crept in along the way.

The usual culprit is when information is translated between audiences. A scientific paper mentions a phenomenon as a large number of kilogrammes (SI), maybe already with an 'errorbar'/rounding to that. Some more public article relates that as "N [thousand/million/etc, additionally rounded?] kg" but also in (rounded) tons. Long or short tons, maybe. Perhaps tonnes, for the least problematic conversion! That, though, then gets taken up and reprinted elsewhere again, intermediate journalist/subeditor now reporting the ton(ne)s, but explaining how many pounds-weight that is (or numbers of elephants/jumbo-jets/Sydney Harbour Bridges that is, by some look-up value that might itself be a vague average or estimate), more or less. Picked up by someone who likes the latter value, but feels the need to state (their own calculation of) what the SI-compatible units would be... YGTI.

That was, of course, the whole point of 2585 and 3065, only relevant to this comic's side-conversation about units. To which I'd add, maybe you can more easily read ⅛th-inches from an 'imperial' measuring tape, than the equivant not-exactly-3-centimetres from a metric one, but ⅕th-cm is easier than whatever that is in inches (2½-sixteenths, so five-thirty-twoths? ...just cross-comparing on my own measuring tape, which is top-half (feet-and-)inches with sixteenths, bottom half centimetres with tenths, in the British style). So it largely depends upon what scale works best/good enough for your use, and that you're used to. I, personally, still think of my own bodyweight in 'stone(s)', but am no longer quick enough to do the factor-14 conversion to relate that to what's often stated in US-lbs (rather than st+lbs, or "Xnand a half stone"-ish, how we use them here) in common US usage. Nor have I ever really dwelt upon my weight in kg. So, unlike my height in feet-and-inches also being known in its reasonable dqyivalent of centimetres(/metres-point-two-decimals, give or take how unruly/untrimmed my hair currently is), I'd have to always do/accept a conversion (or just read off what the scales tell me, though I rarely bother to find out for my own 'fun'). Yet I usually bake/etc in grammes (or kilogrammes, as necessary), or perhaps ounces (three ounces of chocolate chips in one of my otherwise gramme-measured recipies, with perhaps 250g of this and 175g of that, for the rest of the mixture) just out of long-standing habit and the divisions being handier (ok, so basically that's basically 85g, but aiming for a 1 oz division (and slightly going over, if strictly necessary ;) ) is better than aiming half way between two 10nbsp;g ones on a scale that only otherwise has 25 g 'intersticial' graduations.

Additionally: has anyone else appreciated that the 'hook head' of a metal tape-measure is loosely riveted to the steel tape, such that when measuring the external distance (the hook 'hooking' over one end of the long item, or the edge of a windowsill/frame/etc, in tension) it makes an accurate internal measure of the tape-end, but when measuring an internal distance (pushed up against the internal corner of the wall, or whatever, in slight compression) the end pushes into the tape so your 'zero' is the external limit of the hook-bend? Not precisely equivalent, especial on cheap and/or worn retracting tapes where the hook has lateral wobble/angling to it, but clearly intentional to lessen the worry about the hook-metal's thickness (16th″? Half-mil? Where's my micrometer, so that I may measure my measuring tape's actual dimensions?!), even if it's usually barely significant a distinction when used over (in my current case) a 9′+ tape (at least 2.80 m, but I feel its spring complaining, so not trying for the full three metres, or possibly ten feet or more, just for fun!). And the tape-body is marked with graduations, too ("METRICmeters" [sic] on one side, "Non-METRICinches"(!) on the other) so I can add to the 'visible tape' the correct suplementary length of 'unexposed' tape still within the casing. 82.132.238.27 11:58, 9 September 2025 (UTC)

Re "Additionally: has anyone else appreciated…," people on the (notional) tape measure forums can't stop talking about this! With stories about the children or apprentices who "fix" the problem leading to non-stopped cursing by the storyteller, &c, &c. Was that supposed to be a rhetorical question? Not sure why this is topical here, though. JohnHawkinson (talk) 13:09, 9 September 2025 (UTC)

More notes on nominal wood sizing - The nominal lengths of framing lumber are not adjusted, so a nominal 2"x4"x8' board is actually 1.5"x3.5"x8'. In fact they are often slightly oversize to allow for cutting to exact length. However, the nominal length and width for sheet goods like plywood are normally accurate although thickness may or may not be, with softwood sheet goods usually being around 1/4" thinner than stated and hardwood typically being accurate. 2600:1700:b39:3010:30e0:301c:3150:8abf (talk) 18:17, 9 September 2025 (please sign your comments with ~~~~)

I tried to pay for some gasoline with dimensional currency, and the Secret Service has just pulled me over. :( These Are Not The Comments You Are Looking For (talk) 00:17, 14 September 2025 (UTC)
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