You’re staring at a standard plastic ruler, and honestly, it’s a bit of a nightmare. You need exactly 1.57 inches on a ruler, but the little black lines aren't cooperating. Most rulers in the US are divided into sixteenths or eighths. They aren't designed for decimals. If you’re working on a DIY project or a 3D printing calibration, that tiny gap between "close enough" and "perfect" feels like a mile.
Measuring isn't just about looking at a stick. It's about math.
Most people give up and just aim for "somewhere past an inch and a half." But if you're building a mechanical part or trying to fit a 40mm pipe (which is almost exactly this size) into a bracket, "somewhere" doesn't cut it. 1.57 inches is a specific, weird number. It’s the decimal equivalent of certain metric-to-imperial conversions that pop up constantly in engineering and crafting.
Why 1.57 inches on a ruler is so frustrating
Standard rulers are fractional. They speak in halves, quarters, eighths, and sixteenths. Decimals like .57 don't live on that scale naturally. To find 1.57 inches on a ruler, you have to perform a sort of mental gymnastics to translate a base-10 decimal into a base-16 fraction.
Let’s look at the math. An inch is usually split into 16 marks. Each mark represents 0.0625 inches. If you do the division, you'll realize that 1.5 inches is 1 and 8/16. To get to 1.57, you need to add a little bit more. Specifically, you need about another 1.1 sixteenths of an inch.
Basically, 1.57 inches is just a hair past the 1 and 9/16 mark.
It’s tiny. We are talking about a difference of 0.0075 inches if you stop at the 9/16 line. For most people hanging a picture frame, that’s invisible. For a machinist? That’s a massive failure. This is why the tool you choose matters more than the eyes you're using to squint at the wood.
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Translating the decimal to a fraction you can actually see
If you are using a standard ruler, you need to know where 1.57 inches sits relative to the marks you can actually see.
- 1 1/2 inches is 1.50. You’re too short here.
- 1 9/16 inches is 1.5625. This is incredibly close.
- 1 5/8 inches is 1.625. Now you’ve gone too far.
So, for all practical purposes, if you are using a basic hardware store ruler, 1.57 inches on a ruler is located right on the 1 and 9/16 inch mark, plus the thickness of a human hair. Seriously. A human hair is roughly 0.003 inches thick. Two hairs' width past that 9/16 mark and you are basically at 1.57.
The Metric Connection: Why this number keeps coming up
Why are you even looking for 1.57 inches anyway?
Usually, it's because of the metric system. 40 millimeters is a standard size for pipes, electrical conduits, and camera lens diameters. When you convert 40mm to inches, you get exactly 1.5748 inches. Most people just round that down to 1.57.
If you are working with international plans, you'll see this everywhere. You might be following a European woodworking plan but using an American tape measure. It’s annoying. It’s the friction of two different worlds colliding on your workbench.
Honestly, if you find yourself needing to measure 1.57 inches frequently, stop using an imperial ruler. Switch to a metric ruler and look for the 40mm line. It’s much easier to hit a solid line on a metric scale than it is to guess the "phantom" space between sixteenths on an imperial one.
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Tools that actually work for decimal measurements
A wooden ruler is a suggestion, not a precision instrument. If you need 1.57 inches for something that actually matters—like a 3D print or a metal fitting—you need a dial caliper or a digital caliper.
Calipers allow you to toggle between inches, decimals, and millimeters with a single button. You slide the jaws open until the screen reads 1.570. It takes the guesswork out of it.
Machinists like those at Starrett or Mitutoyo have spent decades perfecting these tools because they know human eyes are liars. We see what we want to see. When you look at a ruler, your angle of vision (called parallax error) can make you think you’re at 1.57 when you’re actually at 1.55.
Dealing with the "Close Enough" factor
Let's talk about tolerance.
In construction, "close enough" is usually 1/8th of an inch. In fine cabinetry, it might be 1/32nd. At 1.57 inches, you are working in the realm of 1/64ths or smaller.
If you are sewing, 1.57 inches is basically 1 and 9/16. Just cut the fabric. The tension of the thread will change the measurement more than your eyes will.
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If you are working with PVC pipe, 1.57 inches is the outer diameter of some 1.25-inch Schedule 40 pipes (approximately). If the pipe doesn't fit the hole, you don't need a better ruler; you need a deburring tool.
How to mark 1.57 inches without losing your mind
If you must use a standard ruler, here is the "expert" way to mark it.
Don't use a blunt pencil. A standard #2 pencil has a lead width of about 0.04 inches. That’s almost the entire difference between 1.5 and 1.57! Use a marking knife or a 0.3mm mechanical pencil.
Find the 1.5-inch mark. Count one sixteenth past it. That is 1.5625. Put your marking tool right on the outside edge of that 1/16th mark, away from the 1.5 mark. That "outside edge" is essentially 1.57 inches.
Actionable steps for precision
If you're stuck and need to get this right now, follow these steps:
- Check your ruler's start point. Many rulers have a bit of "dead space" before the zero mark. Don't measure from the physical end of the plastic; measure from the first line.
- Convert to metric if possible. Find 40mm. It is the most accurate way to find 1.57 inches on a ruler without doing math in your head.
- Use the 9/16 mark. If you're using imperial, find 1 and 9/16 inches. It is 1.5625. This is only 0.0075 inches away from your target. For 99% of home projects, this is identical.
- Invest in a digital caliper. If this is for a hobby like reloading ammunition or 3D printing, a $20 digital caliper from a hardware store will save you hours of frustration.
- Watch for Parallax. Look straight down at the ruler. If you look from an angle, the marks will appear to shift, and you’ll miss the 1.57 mark entirely.
Accuracy isn't about having the best eyes. It's about knowing the limitations of your tools and choosing the right one for the job. 1.57 inches is a specific, narrow target, but once you understand it’s just 40mm or a "heavy" 1 9/16, it becomes a lot less intimidating.