If you’re staring at a tape measure or a set of calipers and trying to figure out how to punch 1 5/8 in decimal into a spreadsheet or a CNC machine, you probably just want the number. It’s 1.625.
There. That’s the answer.
But honestly, why does this specific fraction trip everyone up? It’s not just you. I’ve seen seasoned carpenters and professional machinists pause for a second when they hit the eighths. It’s that weird middle ground. It isn't as intuitive as a half ($0.5$) or a quarter ($0.25$), but it shows up everywhere—from the thickness of a heavy-duty bolt to the precise depth of a shelf groove.
The Math Behind 1 5/8 in Decimal
Let’s break it down so you never have to Google this again. When you have a mixed number like $1\ 5/8$, you’re looking at two distinct parts: a whole number and a fraction. The "1" stays exactly where it is, on the left side of the decimal point. The "5/8" part is what needs the work.
Basically, a fraction is just a division problem that hasn't been finished yet. To turn 5/8 into a decimal, you take 5 and divide it by 8.
8 doesn't go into 5. So, you add a decimal and some zeros. 8 goes into 50 six times (that’s 48), with 2 left over. 8 goes into 20 twice (that’s 16), with 4 left over. Finally, 8 goes into 40 exactly five times.
$5 \div 8 = 0.625$.
Now, just stick that back onto your whole number. $1 + 0.625 = 1.625$.
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It's clean. It's precise. Unlike 1/3, which turns into a repeating nightmare of $0.333...$, the eighths are "terminating decimals." They have a hard stop. That’s why they are so beloved in American manufacturing and construction; they provide a high level of precision without the infinite baggage of other fractions.
Why Precision Matters in the Real World
You might think, "Is $1.62$ close enough?"
Actually, no. Not in most trades.
If you are working in a machine shop using a mill or a lathe, that extra five-thousandths of an inch ($0.005$) is the difference between a part that slides in perfectly and one that requires a sledgehammer. Engineers at companies like Boeing or Caterpillar live and die by these three decimal places. If you tell a CNC programmer that 1 5/8 is 1.6, you're going to end up with a very expensive piece of scrap metal.
In woodworking, it’s a bit more forgiving, but even then, precision matters. Think about a 1 5/8 inch hole saw. If you’re trying to fit a piece of 1.625-inch conduit through a hole you’ve rounded down to 1.6 inches, you’re going to be sanding for an hour.
Common Pitfalls and Memory Tricks
People often confuse 5/8 with 3/4 or 5/6.
Here is a quick way to keep it straight in your head:
Think about a dollar. Most of us are great at "quarter math."
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- 1/4 is 0.25 (a quarter)
- 1/2 is 0.50 (two quarters)
- 3/4 is 0.75 (three quarters)
Now, realize that 5/8 sits exactly halfway between 1/2 ($0.50$) and 3/4 ($0.75$).
Halfway between 50 and 75 is 62.5.
So, 1 5/8 in decimal is 1.625.
I’ve found that visualizing a ruler helps more than memorizing a chart. Look at the marks. You have your half-inch mark. Then you have the 5/8 mark right past it. It’s the first "big" tick mark after you cross the halfway point.
Why We Still Use This System
We live in a world that is largely metric, yet the U.S. Customary System hangs on. Why?
Part of it is legacy infrastructure. Millions of miles of pipes, billions of bolts, and almost every residential home in America are built using these fractions. When you go to Home Depot, you aren't looking for a 41.275mm pipe. You’re looking for a 1 5/8 inch pipe.
Converting 1 5/8 in decimal is essentially a translation service between the old world of physical tools and the new world of digital design. CAD software like AutoCAD or SolidWorks usually defaults to decimals. If you're drawing a part based on a physical measurement you took with a tape measure, you have to speak the software's language.
A Quick Cheat Sheet for the "Eighths"
Since you're already here looking at 1 5/8, you’ll probably run into its siblings sooner or later.
- 1/8 = 0.125
- 1/4 (2/8) = 0.25
- 3/8 = 0.375
- 1/2 (4/8) = 0.5
- 5/8 = 0.625
- 3/4 (6/8) = 0.75
- 7/8 = 0.875
Notice the pattern? Every time you add an eighth, you’re just adding 0.125.
$0.5 + 0.125 = 0.625$.
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The Industry Standard for 1 5/8
Interestingly, 1 5/8 is a "standard" size for several specific things.
In the construction of metal stud walls, the 1 5/8 inch stud is a common width for narrow partitions or furring strips. If you're calculating the total thickness of a wall for an architectural drawing, you can't just write "1 5/8." The software wants 1.625.
If you get it wrong, your floor plan measurements will be off. Over a 50-foot run of wall, those tiny errors compound. Suddenly, your hallway is two inches narrower than it's supposed to be, and the building inspector is shaking his head.
The "Close Enough" Trap
There is a weird habit in some DIY circles to round 1 5/8 to 1.6 or even 1.63.
Don't do it.
Rounding to 1.6 is a 1.5% error. In engineering, that’s massive. Rounding to 1.63 is closer, but it’s still "dirty" math. If you’re using a digital caliper—those silver tools with the LCD screen—it will show you 1.625 (or maybe 1.6255 depending on the temperature of the metal). Trust the tool.
Actionable Steps for Your Project
If you are currently working on a project that involves 1 5/8 in decimal, here is how to ensure you don't mess up the execution:
- Calibrate your tools: If you’re using digital calipers, "zero" them out before measuring your 1 5/8 piece. Dust or a tiny metal chip can throw you off by 0.01 inches.
- Set your software precision: If you are using Excel or a CAD program, ensure the cells or units are set to at least three decimal places. If it's set to two, it will round 1.625 to 1.63, and you'll lose your precision.
- The "V" Trick: When marking 1 5/8 on a piece of wood, don't just draw a line. Draw a "V" (a crow's foot) where the point of the V is exactly at the 1.625 mark. It’s much more accurate than a thick pencil lead line.
- Verify the Material: Remember that a "1 5/8" piece of lumber (like a 2x4 actually being 1.5x3.5) isn't always 1.625 inches. Always measure the actual object before you rely on the decimal conversion for your calculations.
Knowing that 1 5/8 in decimal is 1.625 is the first step. Applying it with the understanding that those three decimal places represent real-world physical space is what makes you an expert.
Whether you're building a deck, machining a custom car part, or just helping a kid with their math homework, keep the 0.625 number in your back pocket. It's one of those "golden ratios" of the imperial system that separates the pros from the amateurs.