You’re standing in the middle of a hardware store aisle, or maybe you’re staring at a blueprint for a new kitchen island, and there it is: that little single tick mark. It looks like an apostrophe. Or maybe a prime symbol if we’re being technical. You pause. Does 3' mean inches or feet?
It happens to the best of us. Honestly, even seasoned pros in the construction industry sometimes have a "brain fart" moment when staring at a messy set of handwritten notes. If you’re looking for the short, quick answer so you can get back to your project, here it is.
3' means 3 feet. If you see two marks—3"—that means 3 inches.
It’s a tiny distinction with massive consequences. If you mix these up while ordering a custom rug or cutting a piece of expensive crown molding, you’re going to have a very bad, very expensive afternoon.
The Logic Behind the Symbols
Why do we do this to ourselves? Why not just write "ft" or "in"?
Most of this comes down to old-school drafting. Back before everyone had a computer to render high-definition 3D models, architects and engineers had to hand-draw every single line. Efficiency was king. Writing out "feet" and "inches" hundreds of times on a single sheet of vellum was a waste of time.
The single prime symbol ($'$) for feet and the double prime symbol ($''$) for inches became the universal shorthand. It’s snappy. It’s clean. But it’s also easy to misread if the person writing it has messy handwriting.
Think of it this way: a foot is a larger unit of measurement than an inch. However, the symbol for the larger unit is the smaller one (the single mark). That’s usually where people get tripped up. You’d think the bigger unit would get the double marks, right? Nope. Logic doesn’t always win in historical notation.
Does 3' Mean Inches or Feet in Different Industries?
While the standard is almost always feet, the context of where you see the symbol matters.
Take the maritime world. If you're looking at a nautical chart, those marks might indicate minutes and seconds of arc rather than physical distance on a ruler. In geography, $45^\circ$ $30'$ means 45 degrees and 30 minutes. It’s a measure of an angle, not how long your boat is.
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But for 99% of people reading this, you’re probably dealing with one of these three scenarios:
- Home Improvement: You’re measuring a window or a door frame.
- Online Shopping: You’re trying to figure out if that "3' shelf" will fit in your bathroom.
- Height: Someone told you they are 5'11".
In all these cases, that single mark always—always—means feet.
If you’re browsing a site like Wayfair or Amazon, sellers often get lazy. They might list a product as 3'. If it’s a rug, it’s 3 feet. If it’s a "3' screws" listing, they’ve made a typo, and they definitely mean 3 inches. Common sense has to play a role here. Nobody is using 3-foot-long screws to hang a picture frame unless they are trying to pin their house to the neighbor's foundation.
The Cost of Getting it Wrong
Let’s talk about real-world blunders. There is a famous story—likely apocryphal but rooted in truth—about a stage designer for a rock band who received dimensions for a "Stonehenge" prop. The band wanted it 20 feet tall. The designer saw 20" on a napkin and built a miniature model that fit on a coffee table.
This isn't just a movie trope.
In 1999, NASA lost the Mars Climate Orbiter because one team used metric units (Newtons) and another used English units (pound-force). While that's a metric-versus-imperial disaster, the same level of catastrophe happens on construction sites when someone reads 3' as 3" or vice versa.
Imagine you’re pouring a concrete slab. You need the depth to be 3'. That is a massive, structural foundation. If the guy on the mixer thinks you meant 3", your building is going to sink into the dirt before the roof is even on.
Why We Still Use Imperial Anyway
Most of the world has moved on to the metric system. It’s cleaner. Everything is base-10. It makes sense. But in the United States, Liberia, and Myanmar, we’re still clinging to the system of King’s feet and barleycorns.
Because of this, we are stuck with the prime and double prime symbols. If you go to Canada or the UK, you might see a mix. You might see a piece of timber labeled in millimeters but sold by the "two-by-four" name. It’s a mess.
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A Quick Cheat Sheet for Your Brain
If you find yourself constantly forgetting, try these mental hooks:
- The "Inch" has two syllables: In-ches. It gets two marks ($''$).
- The "Foot" has one syllable: Foot. It gets one mark ($'$).
- Alphabetical order: "F" (Foot) comes before "I" (Inch). "1" (the single mark) comes before "2" (the double mark).
It’s silly, but it works.
I once worked with a carpenter who told me he remembered it by thinking of the marks as "little legs." A foot is a whole limb, so it just needs one solid kickstand. An inch is so small it needs two marks to keep it from falling over. Hey, whatever keeps you from ruining a $400 slab of marble.
When 3' Might Not Be Exactly 3 Feet
Here is where things get really annoying.
In the lumber world, a "two-by-four" isn't actually two inches by four inches. It’s roughly 1.5" by 3.5". This is because the wood is "surfaced" or planed down after it’s cut.
However, when it comes to length—the 3' part—that is usually accurate. If you buy a 3' board, it should be 36 inches long.
But wait.
If you are looking at architectural drawings, sometimes the "3'" refers to a center-line measurement. This means the 3 feet starts from the middle of one stud to the middle of the next. If you measure from the outside edge, you’ll be off by an inch and a half. Always check the "lead lines" on a drawing to see exactly where that 3' starts and stops.
The Typo Factor
We live in an era of digital shorthand. People are fast and loose with their keyboards.
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If you see someone write "I'm 6'' tall," they are telling you they are six inches tall. Unless they are a very articulate garden gnome, they made a typo. They meant 6'.
Conversely, if a recipe tells you to cut a piece of parchment paper 12' long, please don't unroll twelve feet of paper for a batch of cookies. They meant 12".
Context is your best friend.
How to Write it Corrected
If you want to be the person who never gets misunderstood, stop using the symbols.
I know, I know. It’s not as "cool" or "pro-looking." But writing "3 ft" or "3 in" eliminates 100% of the ambiguity. In professional contracts or high-stakes engineering notes, writing out the unit is often mandatory to prevent lawsuits.
If you must use the symbols, ensure they are clear. A slanted apostrophe can sometimes look like a stray pen mark. A vertical prime symbol is the standard.
Practical Steps for Your Next Project
Don't let a tiny tick mark ruin your work. If you're currently staring at a measurement and wondering "does 3' mean inches or feet," follow these steps:
- Check the surrounding numbers. Are other measurements in the same range? If everything else is in the 10-50 range and you see a 3', it’s almost certainly feet. If everything else is small (like 0.5 or 1.2), that 3' might be a typo for 3".
- Use a tape measure for a reality check. Pull the tape to 3 feet (36 inches). Then look at the object you’re measuring. Does 3 feet look "right"? Or does 3 inches look "right"? Usually, the physical scale of the object makes the answer obvious.
- Confirm with the source. If you’re looking at a quote from a contractor and it says "Build 3' wall," ask them to clarify if they mean 3 feet high or 3 feet long. Never assume.
- Mark your own work clearly. If you are writing down dimensions for someone else, use "ft" and "in." It takes two extra seconds and saves hours of potential headache.
You’re now equipped to handle the weird quirks of the Imperial measurement shorthand. Just remember: one mark for the big guy (feet), two marks for the little guy (inches).
Go double-check that measurement one more time before you make the cut. You’ll thank yourself later.