Ever stared at a tiny piece of hardware or a watch case and wondered exactly how big it is? You aren't alone. It’s a classic headache for anyone living in the US or UK who has to deal with the metric system on the fly. Basically, 20 mm is roughly 0.787 inches.
It sounds simple. Just a number, right?
But honestly, that decimal is where things get messy. If you're building a deck, choosing a watch strap, or even checking the size of a medical device, "roughly" sometimes isn't good enough. You've probably noticed that in the hardware store, nobody talks in decimals like 0.787. They talk in fractions. If you're looking for the closest fractional equivalent to 20 mm, you’re looking at 25/32 of an inch.
It’s just shy of 13/16. That tiny difference matters.
The Math Behind 20 mm in Inches
Let’s get the technical stuff out of the way first. To convert millimeters to inches, the international standard—set way back in 1959—is that exactly 25.4 millimeters equals 1 inch.
So, you take your 20 and you divide it.
$$20 / 25.4 = 0.78740157...$$
Most people just round that to 0.79. It’s easier. If you’re a machinist working with high-precision CNC equipment, you’re looking at those extra decimal places. If you’re just trying to figure out if a bolt fits, 0.79 is your friend.
Why do we even have two systems? It’s a historical fluke. The US stuck with the British Imperial system even after the British themselves started moving toward metric. Now, we live in this weird hybrid world where your car engine is metric but your house plumbing is in inches. It's annoying. It’s confusing. But it’s the reality of modern DIY and manufacturing.
Real World Context: Watches and Jewelry
If you’ve ever bought a luxury watch or a replacement strap, you’ve seen the number 20 mm everywhere. It is, without a doubt, the most common lug width for men’s watches.
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The Rolex Submariner? Historically 20 mm. The Omega Speedmaster? 20 mm.
If you try to buy a strap in inches, you’re going to have a bad time. A 3/4 inch strap is about 19.05 mm. If you put that on a 20 mm watch, you’ll see a tiny, ugly gap between the strap and the watch lug. It looks cheap. It rattles. On the flip side, a 13/16 inch strap is about 20.6 mm. You’ll be jamming that leather in there, scuffing the sides, and probably breaking a spring bar in the process.
This is one of those rare cases in the US where the metric measurement is the "true" measurement. Even American watch enthusiasts talk in millimeters. It’s the industry language.
Household Items That Are Exactly 20 mm
Sometimes you just need a visual. You don't have a ruler handy, but you have random junk in your pockets or kitchen drawers.
A standard US nickel is actually quite close, but not quite there. A nickel is 21.21 mm in diameter. So, 20 mm is just a hair smaller than a nickel.
Think about a standard AA battery. The diameter of a AA battery is roughly 14 mm. So, 20 mm is about one and a half times the width of that battery.
If you look at a standard wedding band, a "wide" band is often 8 mm. Imagine two and a half of those stacked together. That’s your 20 mm. It’s a substantial size for jewelry but small for construction.
Why the Fractional Conversion is a Trap
Here is something nobody talks about: the "nominal" size.
In plumbing and construction, a 3/4 inch pipe isn't actually 0.75 inches on the outside. It’s a name. If you measure 20 mm and think, "Oh, that’s basically 3/4 of an inch," and you go buy a 3/4 inch fitting for a 20 mm metric pipe, it won’t fit.
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Metric and Imperial threads don't play nice.
A 20 mm bolt has a different thread pitch than a 3/4 inch bolt. If you force one into the other, you’ll strip the threads and end up with a permanent, expensive mistake. I’ve seen people do this with bike parts and car engines. They think, "Close enough." It’s never close enough.
The Precision Factor: When 0.787 Isn't 0.79
In the world of 3D printing or engineering, we talk about "tolerances."
If you are 3D printing a part that needs to slide into a 20 mm hole, you have to account for the "squish" of the plastic. If you convert your 20 mm design to inches using a simplified 0.79, you’ve already added a massive error.
0.003 inches might not seem like much. It's about the thickness of a human hair. But in a ball bearing or a piston, 0.003 inches is the difference between a smooth machine and a pile of scrap metal.
For most of us? We just want to know if the 20 mm IKEA screw is the same as the one we lost. (Spoiler: It’s not. Buy the metric ones).
Comparing 20 mm to Other Common Sizes
To get a better sense of scale, it helps to look at the neighbors.
- 10 mm: About the width of a fingernail.
- 15 mm: About the diameter of a dime.
- 20 mm: Just smaller than a nickel.
- 25 mm: Almost exactly one inch (25.4 mm).
When you see it laid out like that, you realize 20 mm is a bit of an "in-between" size. It's too big to be tiny, but too small to be significant. It’s the size of a large marble. It's the width of a standard postage stamp (usually around 20 mm to 25 mm).
Practical Tips for Working with 20 mm
If you are currently staring at a project that requires a 20 mm measurement and you only have an Imperial ruler, stop. Just stop.
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Don't try to eyeball 25/32 of an inch. You'll miss.
Instead, flip that ruler over. Almost every tape measure and ruler sold in the last twenty years has both inches and centimeters. 20 mm is simply 2 centimeters. It’s the second big line after the zero.
If your ruler doesn't have metric, use your phone. Every smartphone has a "Measure" app now. It uses augmented reality to give you a surprisingly accurate reading in whatever units you want. It’s way better than trying to do long division in your head while standing in the middle of a Home Depot.
A Note for the Hikers and Photographers
If you’re into photography, you know 20 mm well. It’s a classic wide-angle focal length.
On a full-frame camera, a 20 mm lens gives you a huge field of view. It’s great for landscapes. But notice that even in the US, lenses are always metric. We don't call it a "0.78 inch lens." That sounds ridiculous.
Similarly, in hiking, 20 mm often refers to the width of webbing or straps on a backpack. Most buckles are standardized. If you break a 20 mm buckle on your Osprey pack, a 3/4 inch replacement buckle from a local craft store might work, but it will probably slip under heavy loads.
The strap is 20 mm (0.787"). The buckle is 19.05 mm (0.75"). That 1 mm difference means the teeth on the buckle won't bite into the fabric correctly.
Actionable Next Steps
Don't let the conversion trip you up. Here is exactly what you should do if you’re dealing with a 20 mm requirement:
- Check the Tolerance: If this is for a watch strap or a craft project, 0.79 inches is fine. If this is for an engine or a pipe, stop and buy a metric tool.
- Get a Digital Caliper: If you do any kind of DIY, spend twenty bucks on a digital caliper. It toggles between mm and inches with one button. It’s the only way to be 100% sure.
- Buy Metric for Metric: If you’re replacing a 20 mm fastener, don't buy the "closest" inch equivalent. Go to the specialty drawer at the hardware store and find the M20 (though that's huge) or the specific 20 mm length bolt you need.
- Memorize the "Nickel" Rule: If you just need a quick visual, remember that 20 mm is slightly smaller than a US nickel.
Understanding 20 mm in inches is less about the math and more about the context. Whether you're a horologist, a mechanic, or just someone trying to put together furniture, knowing that 20 mm sits right at that awkward 0.787-inch mark saves you a lot of frustration and stripped screws.