Ever held a credit card and wondered how thick it actually is? It’s not just "thin." It’s a specific number. Most people guess wrong. They think in fractions or millimeters, but in the world of American machining and engineering, we live and die by the "thou." If you’ve ever fumbled with a pair of calipers, you’ve likely seen the decimal 0.100 pop up. That right there is 100 thousandths of an inch. It sounds tiny. It feels tiny. But in the world of high-precision manufacturing, 100 thousandths is actually a massive canyon.
Engineers call it "one hundred thou."
It’s the thickness of about 25 pages of a standard hardcover book stacked together. Or, if you’re at a bar, it’s roughly the thickness of two pennies glued to each other. When you’re working with a CNC mill or a lathe, being off by 100 thousandths isn't a "slight error." It’s a catastrophe. It’s the difference between a part that fits perfectly and a piece of scrap metal heading for the bin.
The math behind the 0.100 measurement
Let’s get the math out of the way because people get confused by the zeros.
Decimal notation is a bit of a trickster. You have a tenth of an inch (0.1), which is exactly the same as 100 thousandths (0.100). We add those extra zeros because, in a machine shop, we measure things to the third or even fourth decimal place. If you tell a machinist you need a gap of "point one," they might give you a side-eye. They want to know the tolerance. Are we talking exactly 0.100? Or 0.1005?
Converting this to the metric system—which most of the world uses—gives you roughly 2.54 millimeters.
Think about that. A single millimeter is already pretty small to the naked eye. Now you’ve got two and a half of them. It’s a beefy measurement when you’re talking about fuel injector pins or the gap in a spark plug, but it’s a sliver when you’re talking about the hull of a ship. Context is everything. In woodworking, 100 thousandths of an inch is a "heavy" 3/32 of an inch. A carpenter might call that "close enough." A NASA engineer would call that a mile.
📖 Related: Why Amazon Checkout Not Working Today Is Driving Everyone Crazy
Real world stuff that measures exactly 100 thousandths
You’d be surprised how often this specific thickness shows up in daily life. Most people don't carry a micrometer in their pocket, but they carry a wallet.
Standard credit cards are usually 0.030 inches thick. So, if you stack three credit cards, you’re sitting at 90 thousandths. Add a business card, and boom: you’re at 100 thousandths of an inch. It’s a tactile way to understand the scale.
- Sheet Metal: 12-gauge steel is often cited around 0.1046 inches. Close enough that many designers treat it as a 100-thou baseline for heavy-duty brackets.
- Washers: A standard thick flat washer in your junk drawer is often right in this neighborhood.
- Plectrums: Extra-heavy guitar picks (the ones that don't bend at all) often hover around the 1.5mm to 2.5mm range. A 2.5mm pick is exactly our target.
I once talked to a guy who specialized in engine rebuilding. He told me that if his head gasket was off by 100 thousandths, the compression ratio would drop so low the car wouldn't even start. It sounds like such a small amount of space, but in a combustion chamber, it's a vacuum.
Why "Thousandths" confuse everybody
The American customary system is weird. We use inches, but then we subdivide them into decimals instead of staying with fractions like 1/64 or 1/128. Why? Because fractions are a nightmare for calculators and digital readouts.
If you're looking at a blueprint, you’ll see "thou" used as the primary unit of communication. It bridges the gap between the "clunky" inch and the microscopic world. When someone says 100 thousandths of an inch, they are speaking a language of precision that dates back to the industrial revolution. Joseph Whitworth and others pushed for standardized measurements because, before that, a "bolt" in one factory wouldn't fit a "nut" from the factory next door.
Honestly, the decimal system won because it's just easier to add. Try adding 3/64 and 7/32 in your head while a machine is screaming at 5,000 RPM. You can't. But adding 0.046 and 0.218? Much simpler.
👉 See also: What Cloaking Actually Is and Why Google Still Hates It
The "Feel" of 100 Thousandths
If you want to look like a pro in a shop, learn the "feel."
Machinists use feeler gauges. They’re these little metal strips of known thicknesses. You slide them into gaps to check tolerances. When you slide a 0.100 gauge into a slot, there’s a specific kind of resistance. It’s called "wringing."
If the gap is exactly 100 thousandths, the gauge will slide in with a smooth, oily drag. If it's 101, it'll wiggle. If it's 99, it won't go in at all. Human fingers are surprisingly sensitive. We can often feel a difference of just one or two thousandths. So, feeling a gap of a full 100 thou feels like a massive opening. It’s funny how your perspective shifts once you start working with metal.
Misconceptions about precision
A common mistake is thinking that 0.1 is "basically zero."
It’s not.
In many high-tech fields, 0.100 is considered a "loose" tolerance. If you’re building a bridge, 100 thousandths is nothing. If you’re building a smartphone, 100 thousandths is the size of a whole internal component. Apple and Samsung fight over 0.001 inches of space inside a chassis to fit a slightly bigger battery. To them, 100 thousandths is a luxury they can't afford.
✨ Don't miss: The H.L. Hunley Civil War Submarine: What Really Happened to the Crew
Another big one: people confuse 100 thousandths with 100 microns.
Big mistake.
A micron (micrometer) is $1/1,000,000$ of a meter. There are 25,400 microns in an inch. So, 100 microns is actually about 0.0039 inches. That's about the thickness of a human hair. So, if you order a part and ask for 100 microns but meant 100 thousandths of an inch, your part is going to be about 25 times thinner than you expected. You’ll be looking at a piece of foil instead of a sturdy plate.
Common industry uses for 0.100"
- Electronics: Many through-hole components on a circuit board use a "100 mil" spacing. A "mil" is just another word for a thousandth. So, the pins on those classic black chips (DIP packages) are spaced exactly 100 thousandths apart. It's the standard grid for the entire hobbyist electronics world.
- 3D Printing: A common "thick" layer height is 0.3mm, but the actual walls of a print are often set in multiples that approach 2.5mm (roughly 100 thou) for structural integrity.
- Blade Grinding: Many heavy-duty survival knives are ground from stock that is 0.100 inches thick. It’s the "sweet spot" where the knife is strong enough not to snap but thin enough to actually slice an apple.
What you can do with this knowledge
Next time you’re DIY-ing a project or looking at a spec sheet, don't just gloss over the decimals.
If you see 0.100", grab a couple of pennies. Hold them together. That is your physical anchor. It helps you visualize the scale of what you’re building. If you’re 3D printing, try setting your wall thickness to 0.100 inches and see how much tougher the part feels compared to the default settings.
Actionable Steps for Precision Projects:
- Buy a digital caliper: Even a cheap $20 pair from a hardware store will help you see the world in "thou." Zero it out, then open it to 0.100. Look at the light passing through the gap. That’s your 100 thousandths.
- Check your spark plugs: If you work on older cars, your spark plug gap might be anywhere from 0.025 to 0.045. Knowing that 0.100 is more than double that gap helps you realize why precision matters in engines.
- Calibrate your eyes: Start guessing the thickness of cardboard, plastic lids, and glass. Measure them afterward. You’ll start to realize that 100 thousandths is a very common "industrial" thickness for consumer goods that need to feel "sturdy" but not "heavy."
Precision isn't just for scientists in white coats. It’s for anyone who wants their shelf to stay level or their door to stop squeaking. Understanding the scale of 100 thousandths of an inch is the first step toward moving from "that looks about right" to "that is exactly right."
Once you start seeing the world in thousandths, you can't un-see it. You'll notice the gap in your car's door panels. You'll feel the shim under a wobbly table. You'll finally understand why that "universal" bolt doesn't actually fit. It's all in the decimals.