Inch and 5/8: The Ultimate Guide for DIYers and Mechanics

Inch and 5/8? Your eyes glaze over. You glance at your tape measure, a sea of tiny black lines staring back, and a wave of uncertainty washes over you. Where exactly is that? Is it the line after one and a half? Before it? It’s one of those measurements that feels both incredibly specific and frustratingly vague.

This seemingly random fraction is a common sticking point for everyone from weekend woodworkers to home mechanics. It sits in that awkward space between more familiar measurements like 1/2″ and 3/4″. But here’s the good news: understanding and mastering this measurement is easier than you think.

This is your ultimate guide to the world of 1 5/8 inches. We’re going to break it down into simple, visual terms, convert it to metric without the headache, and show you exactly where you’ll find it in the real world. By the time you’re done here, you’ll be able to spot an inch and 5/8 from a mile away.

Breaking It Down: What Does “Inch and 5/8” Actually Mean?

Let’s start by demystifying this number. The “inch” part is easy, but the “5/8” is where people get tripped up.

Think of a standard American inch on a ruler or tape measure. That one inch is typically divided into 16 equal parts, marked by lines of varying lengths. The fraction “5/8” is simply telling you how many of those parts to count.

But wait, the fraction is over 8, not 16? Correct. Fractions are all about equivalents.

• 1/2 inch is the same as 4/8 or 8/16.
• 5/8 inch is the same as 10/16.

So, when a guide asks for 1 5/8″, it’s asking for one full inch, plus ten-sixteenths of the next inch.

How to Find 1 5/8″ on a Tape Measure (The Easy Way)

Forget the math for a second. Let’s get visual. Pull out a tape measure and find the “1” mark.

1. Look between the 1-inch mark and the 2-inch mark.
2. Find the longest line between them. That’s your halfway point: 1 1/2 inches.
3. Remember that 1/2 is the same as 4/8. We’re looking for 5/8.
4. So, you just need to go one more eighth-of-an-inch mark past the halfway point.
5. Find the 1 1/2″ line. The very next medium-length line after it is your target: 1 5/8 inches.

It’s always the first eighth-inch mark after the half-inch mark. Once you see it that way, you can’t unsee it. It stops being a random fraction and becomes a clear, physical location on your tape.

The Global Standard: Converting an Inch and 5/8 to MM

In today’s global marketplace, working exclusively in inches is a luxury we rarely have. Whether you’re ordering a specialty part from a European website, working on an imported car, or following a design plan from overseas, you’re going to need to speak metric. So, let’s tackle the big conversion: inch and 5/8 to mm.

The formula is your golden rule: 1 inch = 25.4 millimeters (mm).

To convert 1 5/8 inches, we first need to turn that clunky fraction into a simple decimal.

• Step 1: Convert the fraction to a decimal.
  Divide the top number (numerator) by the bottom number (denominator): 5 ÷ 8 = 0.625.
• Step 2: Add the whole number.
  So, “an inch and 5/8” is 1.625 inches in decimal form.
• Step 3: Multiply by the magic number.
  Take your decimal value and multiply it by 25.4.
  1.625 × 25.4 = 41.275 mm.

So, there’s your precise answer: 1 5/8 inches is exactly 41.275 mm.

For most practical purposes, you can round that to 41.3 mm. If you’re drilling a hole or looking for a component, this level of precision is often crucial. Being off by even half a millimeter can mean the difference between a perfect fit and a costly mistake.

Real-World Sightings: Where Does 1 5/8″ Show Up?

This isn’t just a theoretical number; it’s a workhorse measurement in dozens of industries. Knowing where to expect it can save you a lot of headaches.

In the Garage: Automotive Applications

If you work on cars, trucks, or heavy equipment, you’ll run into 1 5/8″ all the time. It’s a common size for:

• Large Axle Nuts: On some trucks and SUVs, this is the size needed to remove the large nut holding the CV axle in place.
• Trailer Hitch Balls: The shank on many heavy-duty trailer hitch balls requires a 1 5/8″ wrench or socket to tighten the nut properly.
• Pitman Arm Nuts: In the steering systems of older or larger vehicles, this size is frequently used.
• Industrial Fasteners: On farm equipment or construction machinery, large bolts and nuts of this size are standard.

I saw a comment on a forum that summed up the experience perfectly:

“Spent my whole Saturday trying to swap the hitch on my truck. Dug through three toolboxes before I realized the nut was 1 5/8″. None of my sets went that high. A 2-minute job turned into a 2-hour trip to the auto parts store. Never again.”

This is a classic example. It’s a size that’s just large enough to not be included in standard home toolkits.

Around the House: Construction and DIY

In the world of construction, 1 5/8″ is a key dimension, especially for interior work.

• Steel Studs: This is the standard width for many light-gauge steel studs used in commercial and residential framing. If you’re mounting something heavy on a wall in an office building, you need to know this.
• Hole Saws: 1 5/8″ is a very common hole saw size, perfect for running conduit or small plumbing pipes through studs.
• Sink Drains: The diameter of a standard bathroom sink drain tailpiece is often 1 1/4″ or 1 1/2″, but the hole cut in the vanity countertop might be closer to 1 5/8″ to provide a little wiggle room.

In the Workshop: Woodworking and Crafts

Even in finer crafts, this measurement appears.

• Picture Framing: Certain styles of wide moulding for large art pieces can be 1 5/8″ wide.
• Custom Cabinetry: The “stile” or “rail” (the vertical and horizontal parts of a cabinet door frame) is often a custom width, and 1 5/8″ provides a classic, sturdy look that isn’t too bulky.

A Game of “Close Enough”: Comparing 1 5/8″ to 3/4″

Context is everything when it comes to measurements. To better understand 1 5/8″, it helps to compare it to another ubiquitous fraction: 3/4″. People often wonder if they can substitute one for another, or if two of one equal another.

Let’s be clear: they are very different sizes.

• 1 5/8″ = 1.625 inches
• 3/4″ = 0.75 inches

As you can see, 1 5/8″ is more than double the size of 3/4″. The more interesting comparison is between 5/8″ and 3/4″.

• 5/8″ = 0.625 inches
• 3/4″ (which is 6/8″) = 0.75 inches

The difference between them is just 1/8 of an inch. That might not sound like much, but in mechanical applications, it’s a canyon. Trying to use a 3/4″ wrench on a 5/8″ bolt will result in a stripped bolt head and a busted knuckle.

Converting 3/4 Inch to MM

Just like we did for our primary keyword, let’s run the conversion for 3/4 inch to see how it stacks up in metric.

• Step 1: Convert to a decimal.
  3 ÷ 4 = 0.75 inches.
• Step 2: Multiply by 25.4.
  0.75 × 25.4 = 19.05 mm.

So, a 3/4 inch measurement is equivalent to 19.05 mm. This is a critical number to know for one of the most common tool-related questions in any workshop.

The Toolbox Conundrum: Imperial vs. Metric Sockets

This leads us directly to the toolbox. You have a bolt, and you need a socket. Is it Imperial (SAE) or Metric?

Let’s talk about the 3/4 inch to mm socket dilemma. The exact conversion for a 3/4″ fastener is 19.05 mm. If you look in a metric socket set, you won’t find a 19.05 mm socket. You’ll find a 19 mm socket.

So, can you use a 19 mm socket on a 3/4″ bolt?

Yes, almost always. The difference is a mere 0.05 mm, which is about the thickness of a human hair. The 19 mm socket will fit snugly over a 3/4″ bolt head and will work perfectly fine 99% of the time. It’s one of the few “close enough is good enough” exceptions that mechanics universally accept.

What About a Socket for 1 5/8 Inches?

Now let’s apply that logic to our main size.

• 1 5/8 inches = 41.275 mm

The closest standard metric socket sizes are 41 mm or 42 mm.

• Will a 41 mm socket work? Maybe, but it’s risky. You’d be trying to force a 41 mm socket over a 41.275 mm fastener. If the tolerances are loose, it might go. More likely, it won’t fit, or you’ll have to hammer it on, which is a terrible idea.
• Will a 42 mm socket work? It will fit, but it will be loose. The gap of over 0.7 mm is significant. While it might work for a low-torque application, applying any real force will likely cause the socket to slip and “round off” the corners of the nut.

The Verdict: For a 1 5/8″ fastener, you really need the correct 1 5/8″ socket. This is not a size where you can easily substitute a metric equivalent. It highlights a key principle of tool use: the bigger the fastener, the more important an exact fit becomes.

How to Measure 1 5/8 Inches with Confidence

Knowing the theory is one thing; executing it is another. Let’s cover the best tools for the job.

1. The Analog Tape Measure: This is your first line of defense. As we covered, find 1 1/2″ and go to the next 1/8″ mark. Practice finding it quickly. Open and close your tape measure, spotting 1 5/8″, 2 5/8″, 3 5/8″, and so on. Build that muscle memory.
2. The Digital Caliper: For precision work, nothing beats a digital caliper. You can clamp it onto an object, and it will give you the measurement instantly in decimal inches (1.625) or millimeters (41.275) with the push of a button. It removes all guesswork and is an essential tool for any serious maker.
3. The Combination Square: In woodworking, a combination square is perfect for marking a 1 5/8″ line accurately. You slide the ruler part to the 1 5/8″ mark, lock it down, and you have a perfect physical guide to scribe a line.

The key is to use the right tool for the level of precision you need. Eyeballing it might be fine for a garden post, but it’s a recipe for disaster when machining a part.

Frequently Asked Questions (FAQ)

Conclusion

inch and 5/8 is no longer a mystery. It’s not some random, arbitrary line on your tape measure. It’s 1.625 inches. It’s 41.275 millimeters. It’s the standard for steel studs, the key to removing an axle nut, and the perfect width for a custom cabinet frame.

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