You're probably standing in the middle of a Home Depot or looking at a messy blueprint, and you just need the number. No fluff. No math class lecture. Just the answer. Basically, 1 5/8 to decimal is 1.625.
That’s it. That is the number you need to punch into your calculator or your laser measurer.
But why does this specific fraction show up everywhere? If you've ever done woodworking, precision machining, or even followed a complex baking recipe from an old European cookbook, you’ve run into the "eighths" family. They are the backbone of the Imperial measurement system used in the United States. While the rest of the world has moved on to the logical, base-10 beauty of the metric system, we are still stuck trying to figure out how many points are in a sixteenth of an inch. It’s annoying. I get it. Honestly, it’s one of those things that should be simpler, yet here we are, staring at a tape measure wondering why 1.6 doesn't quite line up with that little tick mark past the inch and a half.
Getting the math right for 1 5/8 to decimal
Math doesn't have to be a headache. To turn 1 5/8 into a decimal, you just need to understand what the fraction is actually telling you. Think of it as two separate pieces. You have the whole number, which is 1. That stays exactly where it is, on the left side of the decimal point. Then you have the fraction, 5/8.
The fraction is just a division problem in disguise.
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Take 5 and divide it by 8. If you do that on your phone right now, you get 0.625. Now, just glue that back onto your whole number. You end up with 1.625. Simple.
Most people mess this up because they try to "eye-ball" it. They think, "Well, 1/2 is .50, so 5/8 must be around .6." Close, but "around .6" will ruin a cabinet door or make a bolt wiggle in a way that eventually snaps the head off. In the world of mechanical engineering, being off by 0.025—the difference between 1.6 and 1.625—is actually a huge deal. It’s the difference between a "press fit" and a "loose fit."
The breakdown of the eighths
If you’re working with tools, you’re going to see these numbers on repeat. It helps to have the mental map of how these decimals actually look.
- 1/8 is 0.125
- 2/8 (which is 1/4) is 0.25
- 3/8 is 0.375
- 4/8 (the famous 1/2) is 0.5
- 5/8 is 0.625
- 6/8 (3/4) is 0.75
- 7/8 is 0.875
See the pattern? Every time you add an eighth, you’re just adding 0.125 to the previous number. It’s a sequence. If you can memorize that 1/8 is 0.125, you can basically solve any measurement problem in your head while you're holding a saw. Or, you know, just keep a cheat sheet on your workbench. No shame in that.
Why 1.625 is the "magic number" in construction
Ever wonder why 1 5/8 is such a common thickness? Go to a lumber yard and look at a "2x4." You probably already know it isn't actually 2 inches by 4 inches. That would be too easy. After the wood is dried and planed down, a standard 2x4 is actually 1 1/2 inches by 3 1/2 inches.
Wait. Where does 1 5/8 come in?
Metal studs.
In commercial construction, the standard width for a non-structural metal stud is often 1 5/8 inches. When architects are drawing up plans in CAD (Computer-Aided Design) software, they don't type "1 5/8." The software wants decimals. So, they type 1.625. If the architect types 1.6 and the builder uses 1 5/8, over a long wall with thirty studs, that tiny error starts to stack up. Suddenly, the drywall at the end of the hall doesn't fit. This is what pros call "tolerance stack-up." It’s the silent killer of DIY projects.
Precision matters more than you think
Let's talk about the hobbyist. Say you're 3D printing a bracket to hold a pipe. The pipe is labeled 1 5/8 inches. If you design your 3D model with a hole that is exactly 1.6 inches wide, that pipe is never going into that hole. Not without a sledgehammer and a lot of swearing.
You need that extra 0.025.
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Actually, in 3D printing or machining, you usually want a "clearance fit." This means you’d actually want to make the hole slightly larger than 1.625. Maybe 1.65. But you can't even start that calculation if you don't know the base decimal of the fraction you're working with.
1 5/8 to decimal isn't just a school problem. It's a "making things work in the real world" problem.
Common mistakes when converting
One weird thing people do is they try to round to two decimal places too early. They see 1.625 and they think, "I'll just call it 1.63."
Don't do that.
Rounding is for the very end of the process. If you're doing a bunch of calculations—say, multiplying 1 5/8 by 12—and you use 1.63 instead of 1.625, you’re going to be off. It might only be a tiny bit, but accuracy is a habit. If you get lazy with the "small" fractions, the big projects will always feel slightly "off." You know that feeling when a drawer sticks or a shelf is just a tiny bit wobbly? That’s usually a rounding error in disguise.
Using a tape measure like a pro
Look at your tape measure. Between the 1-inch mark and the 2-inch mark, you see a bunch of lines of different lengths.
The longest line in the middle is 1/2 (1.5).
The next longest are the quarters (1/4 and 3/4).
Then come the eighths.
To find 1 5/8, you find the 1 1/2 mark (the big middle one) and jump one "eighth" tick mark forward. That is your 1.625.
If you're using a digital caliper, which is a tool every DIYer should own, it will give you the reading in decimals. If you clamp it down on a piece of metal and it says 1.625, you know you've got a perfect 1 5/8. If it says 1.61, you’re looking at something that’s been worn down or manufactured poorly.
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The decimal to fraction struggle
It goes both ways. Sometimes you have a digital reading and you need to know what socket wrench to grab. If your digital screen says 1.625, you aren't going to find a "1.625" wrench in your toolbox. You’re going to look for the 1 5/8.
But what if the number is weird? What if it says 1.635?
Then you have to decide if 1 5/8 is "close enough." In most home repairs, being within a few hundredths of an inch is fine. In automotive work? Probably not. If you're working on an engine and a part that should be 1.625 measures at 1.64, you might have a heat expansion issue or a part that is about to fail.
Tools that do the work for you
You don't have to be a math genius. There are plenty of conversion apps, but honestly, just typing "1 5/8 to decimal" into a search engine is what most of us do. Or, better yet, get a steel ruler that has decimals etched on one side and fractions on the other. Machinist scales often have "10ths" and "100ths" of an inch on one edge. It’s a lifesaver.
Practical Next Steps
Now that you know 1 5/8 to decimal is 1.625, here is how to actually use this information effectively in your workspace:
- Update your cut list: If you are using a digital saw or CNC machine, go through your notes and replace every instance of 1 5/8 with 1.625 immediately.
- Check your tools: If you're using digital calipers, make sure they are calibrated. Close them completely; they should read 0.000. If they don't, your 1.625 reading will be junk anyway.
- Memorize the "big three": * 3/8 = .375
- 5/8 = .625
- 7/8 = .875
Knowing these three will cover 90% of your fraction-to-decimal headaches.
- Account for the "Kerf": Remember that if you're cutting a piece of wood to 1.625 inches, the saw blade itself has a thickness (the kerf). Usually, a saw blade is about 1/8 inch (0.125) thick. You have to account for that loss of material if you're making multiple cuts from one board.
Understanding the relationship between these two ways of measuring is like learning a second language. Once it clicks, you stop seeing "hard math" and start seeing just another way to describe the same physical distance. Whether you’re building a deck, machining a custom car part, or just trying to finish a school project, 1.625 is the precise reality of 1 5/8. Use it, and your work will be better for it.