You're probably staring at a tape measure or a blueprint right now, feeling that specific kind of frustration that only the Imperial system can provide. Fractional inches are a headache. Seriously. If you are trying to figure out exactly what 5 5/8 in mm comes out to, you aren't just looking for a number; you're likely trying to make sure a bolt fits, a shelf sits level, or a 3D print doesn't fail three hours into the job.
The short answer? It is 142.875 mm.
But wait. If you just round that to 143 mm and call it a day, you might be fine. Or you might ruin your project. It depends entirely on whether you’re building a birdhouse or machining a custom car part.
The Math Behind 5 5/8 in mm
Math is annoying, but let's break this down so you never have to Google it again. The international standard—set way back in 1959—defines one inch as exactly 25.4 millimeters. No more, no less.
To get to the bottom of 5 5/8 inches, we have to turn that fraction into a decimal first. Most people remember that 1/2 is 0.5, but 5/8? It’s basically five divided by eight, which gives you 0.625. So, your total measurement in decimal form is 5.625 inches.
Now, grab a calculator (or just trust me).
$5.625 \times 25.4 = 142.875$
There it is. 142.875 millimeters.
In a workshop setting, we often truncate this. If you are using a standard metric ruler, you'll struggle to see anything smaller than a half-millimeter. Honestly, for most home DIY tasks, calling it 143 mm is "good enough." But "good enough" is how you end up with wobbly furniture or stripped threads.
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Why 142.875 mm is a Weird Number for Manufacturing
If you’re working in a machine shop or doing precision engineering, that ".875" at the end is a nightmare. Most metric-based designs prefer even numbers or increments of 0.5 mm. When you convert 5 5/8 in mm, you’re stepping out of the clean, logical world of metric and dragging the "messiness" of Imperial with you.
Think about wood expansion. If you are cutting a piece of oak to 142.875 mm, the moisture in the air could change that measurement by a full millimeter by tomorrow morning. On the flip side, if you are working with cold-rolled steel, 0.875 mm is a massive gap. It's the difference between a "press fit" and a piece of metal that just falls out of the hole.
I’ve seen people try to buy 143 mm bolts to replace a 5 5/8 inch specialized fastener. Don't do that. Even though the numbers are close, the thread pitch is totally different. Metric and Imperial are like two people speaking different languages; they might agree on the length of a sandwich, but they’ll never agree on how to build the plate it sits on.
Real-World Tools and Their Limits
Most digital calipers you buy at a hardware store will toggle between inches and millimeters with a single button. It’s a lifesaver. However, keep in mind that "accuracy" and "resolution" aren't the same thing. Your calipers might show 142.88 mm (rounding up that last digit), but that doesn't mean the tool is actually accurate to that level.
Check the specs on your tool. A standard set of Mitutoyo or Starrett calipers—the gold standard for most pros—will usually be accurate to within 0.02 mm. Cheaper "no-name" brands might claim three decimal places but actually fluctuate by 0.1 mm if you squeeze the jaws too hard.
When measuring for 5 5/8 in mm, consistency is your best friend. Measure three times. If you get 142.8, 142.9, and 142.7, just take the average and move on.
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The Tolerance Factor
In the world of professional fabrication, we talk about "tolerances."
If your blueprint says 5 5/8" +/- 1/16", you have a huge margin of error. 1/16th of an inch is about 1.5 mm. That's a canyon! You could literally cut your piece to 141 mm or 144 mm and still be "within spec."
But if you’re doing engine work or aerospace components? You might see a tolerance of +/- .005 inches. That’s roughly 0.12 mm. Suddenly, that 142.875 mm measurement needs to be dead-on.
Common Places You’ll See This Measurement
You might run into 5 5/8 inches more often than you think. It's a common size for:
- Certain types of recessed lighting cans (downlights).
- The diameter of specific plumbing flanges.
- The width of some crown molding profiles.
- Mid-sized speaker drivers (sometimes marketed as 5.5 inches, but actually measuring 5 5/8 to the mounting holes).
If you are trying to fit a European-made LED housing into a hole cut for an American 5 5/8" fixture, you're going to have a bad time if you don't convert accurately. The European fixture will likely be exactly 140 mm or 145 mm. Neither will fit perfectly without adjustment.
Precision Hacks for the Workshop
If you don't have a digital converter handy, here’s how I usually handle this on the fly.
I know that 5 inches is roughly 127 mm.
I know that 5/8 is a bit more than half an inch (which is 12.7 mm).
Actually, 5/8 is exactly 15.875 mm.
Add 127 and 15.875, and you get your 142.875.
Doing it in chunks makes it harder to make a massive mistake. If you ended up with 160 mm or 110 mm, you'd know immediately that your math went sideways.
Always use a dedicated conversion chart if you’re doing a whole project. Switching back and forth in your head is a recipe for a "measure twice, cut once, cry anyway" situation.
Actionable Next Steps
- Check your blueprints: Determine if the project was originally designed in Metric or Imperial. If it was designed in Metric and someone converted it to 5 5/8", go back to the original Metric number. It's likely a round number like 140 mm or 143 mm.
- Zero your calipers: Before measuring for 142.875 mm, close your calipers and hit the zero button. It sounds obvious, but dust or a tiny metal chip in the jaws can throw you off by 0.5 mm easily.
- Write it down: Never trust your memory for the decimal points. Write 142.88 mm on a piece of masking tape and stick it to your workpiece.
- Account for the blade: If you are sawing, remember the "kerf"—the width of the blade. A standard saw blade is about 3 mm thick. If you don't account for that, your 142.875 mm piece will end up being roughly 139 mm.
Converting 5 5/8 in mm is straightforward math, but applying it requires a bit of common sense regarding your material and your tools. Stick to the 142.875 figure for your planning, and round only when the physical reality of your project allows for it.