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The Starrett ProSite Protractor

I needed you 20 years ago

When I started building, 20-something years ago, we were lucky to have had a ‘chop saw’ on the job site. No air-nailers, certainly no cordless drills, no portable table saws, no Festool. And we certainly didn’t have a small indestructible “idiot proof” gadget that told us the exact miter for a particular corner, so that when we cut our molding, or even our fascia for decks, the joinery would be perfect.

Nope, back then, we’d cut a few scraps, then eyeball it, check it, go back and cut it again and again until the scraps looked good, and then we’d know the right miter angle and cut the final material. What a waste of time and materials. If we thought an outside miter looked like it was 45°, we’d cut the first piece 22 1/2—we’d gamble to see if the next mate was going to look good at 22 1/2. Talk about trial and error.

I can remember temporarily nailing fascia or trim in place, purposefully making it extra-long. Then I’d take scrap material and lay it on top of the final material intersecting the first piece. I’d draw two parallel lines on either side of the scrap and go back to the monster Hitachi 15-in. chop saw (with a broken or missing miter gauge, I might add) and do my darnedest to intersect my pencil marks through that little plastic viewfinder. If you weren’t cutting with the saw locked in a detent at 0° or 45° it was a real crapshoot.

Not knowing what miter you needed, a single cut or miter cut could be troublesome. Get two carpenters going about the problem of using a protractor with a miter saw and forget it! You may as well take the day off. It was like each of us spoke a different language. Single cut, miter cut, 45°, 90° (are you talking about the actual angle of the corner or the ‘miter’ setting on the saw to cut a piece to fit that angle?)…. You could never take two guys that hadn’t worked together and put them on a job. While both could be great carpenters, they wouldn’t understand what or how the other was describing what the other needed. Is your angle 90° or 45°? My saw doesn’t have a setting for 90° so it must be 45°, right?

Fast forward to the present day

Today, miter saws are more precise, miter gauges on some saws have 1° increments, we have laser cut readings, and micro-fine adjustment knobs so we can dial in an angle just right. Great, right? Well, how do you know if you need to cut a 45°, 46°, or even a 50° miter for a corner that may be a little out-of-square. Even better, how do you determine the required miter angles for crown molding when it has to run around an oddly shaped room with unusual corner angles? Use a protractor, right?  Wrong. They’ve been around for years. But they’ve never worked with a miter saw! I’m a carpenter, not a mathematician. My miter saw has a different numbering system than a standard protractor (well, not really, but that’s a different story—see “Miter Angles and Miter Saws”).

(Click any image to enlarge.)

A ‘special’ miter saw protractor—like the Starrett ProSite—is the answer for a mathematically challenged carpenter like myself. This gauge is easy to use, needs no batteries, has no LCD that can break, and isn’t finicky in extreme temperatures, like a lot the of equipment we lug around. I should say that although the gauge is certainly very handy, it only gives you the miter angles you need—not the bevel angles. If you cut crown nested in position you’re fine. If you like to cut crown on the flat, or your material is simply too big to fit nested on your saw, be sure to buy the Starrett 5-in-1 Tool—it has a crown chart on it, too (see photo, right). Of course, the chart is only good for two types of crown—38° spring angle and 45° spring angle; but for off-the-shelf moldings, that simple chart covers a lot of territory. For anything else (like custom moldings and high-end crown profiles, you’ll need to use a Bosch electronic miter finder, or a construction calculator, like a Construction Master Pro or Build Calc (a great app for iPhone users!).

But the Starrett protractor is a very-easy-to-use tool. It has two scales: The red miter cut arrow and outer scale are for cutting miter joints; the black “Single Cut” arrow and inner scale are for cutting a single angle. At first, the tool might be a little confusing, but once you get the hang of it, you’ll love it.

The easiest way to explain how it works is to set the Starrett for a 135° angle (the same angle in “Miter Angles and Miter Saws”). Most of us look at that angle and think it’s a 45°, but, really, that’s just the Single Cut. Look closer at the Starrett scale and you’ll see what I mean. The Single Cut arrow is pointing at 45°. The red Miter Cut arrow is pointing at 22 1/2°. Those are the actual angles on your miter saw. There’s no thinking or math involved.

Of course, if you want to know what the real angle is, you can turn the Starrett 5-and-1 Tool upside down and read the “protractor” angles.

That scale reads 135. The red metal tab and indicator reads just like a normal protractor.

Using the Starrett is simple. To measure an inside angle, open the unit like a boomerang, place it in the corner you wish to measure, and spread both sides until they make contact with the wall.
The device holds a measurement pretty well. There’s some tension built into the hinge-point which stops the legs from flopping after you’ve fit it to the wall.

In some of these photos, I’m using my original ProSite Protractor, not the newer 5-in-1 Tool. But the scales read the same. The Single Cut for this corner would be 20° on a miter saw, but for mitering two pieces, like the crown I’m installing, I have to set the saw at the Miter Cut, which is 35°.

Remember, if you’re cutting miters, you’ll want to read the outer scale with the red Miter arrow. That gauge gives you the angle to cut BOTH pieces so that you have a perfect miter. If the corner is out of square, the gauge will give you the correct miter for your miter saw.

I’m sure many of you have had the same experience I’ve had. I used to look at a miter that wasn’t closing up tight, and I’d guess that I needed to cut it 46°, but 46° on a miter saw is really 44°. I’ve wasted a lot of time and material trying to understand that!

The Starrett solves all those brain twisters. It is available in a large and small size, both plastic and metal. While I personally own both the large and small in metal, I’ve been thinking of getting several plastic versions for the crew. The scales on the metal version are on an adhesive-backed material—be careful not to wipe the unit down with certain chemicals, as it will take the printing off and leave the device useless.


12 Responses to “The Starrett ProSite Protractor”

  1. Keith Mathewson

    Nice tool but I think I’ll stick with a Stanley 30, they’ve been around for decades.

  2. Dave

    Awesome intro to a new (for me) tool. I’ve cut those angles wrong dozens of times. Thanks!

  3. James Bodine

    Thank you for the informative article. One might consider installing a zero clearance fence on the Kapex. Thanks for clarifying how to use the Starrett protractor.

  4. s. donato

    I love that tool. i have been using the older model for years and didn’t realize at first why i liked mine better then all the others that are on a jobsite. this explained a lot – i just got used to mine working correctly ;-) and everyone else’s just giving you angles that i had to do math to make it work.

    perfect timing to publish this since the other article about how the numbers on the mitersaw are wrong…. well i guess they are only wrong if you are not using the right tool.

  5. MechanismBusboy

    As a building teacher, I always try to teach my carpentry students how to avoid making mistakes in calculation. There are ways to set your chop saw for a basic miter cut without cutting scrap guides or using any numbers whatsoever. Let me give it a try here:

    Set a sliding T-bevel square to your inside or outside angle. Trace this angle onto a board or edge of your chopsaw table. Take a scribe or compass and draw an arc from the angle’s vertex (point of origin) This will be at the very edge of the board — the arc can be small or large, whatever is comfortable as long as it crosses the angle line you’ve drawn and cuts the edge of the board or table too. Move the point of your scribe to this intersection where the arc cuts the angle. Draw a small quick circle that looks to be more than half the angle–take a guess. Now move your compass, without changing the setting, to the other arc intersection (this one is at the edge of the board or table where your first arc leaves or crosses) and draw another circle the same size. The two circles will overlap in two spots that represent the exact bisection of your angle. You can connect them if you like — I simply reset my sliding t-bevel to cross these points, lock it, and then use it to set my miter right on the saw in front of me. This sounds complicated, but in reality takes about fifteen seconds, and is absolutely accurate (and won’t set you back $100). Look ma, no numbers.


    • Clay

      Scott, can you send a picture of that method to me? I like to have multiple options.


      frijoli@triad DOT rr DOT com

    • Remko

      Hey Scott
      Your info sounds amazing but I can’t picture it.
      Can I please have an image like John asked for
      Globetrotterschip at gmail dot com
      Thanks heaps

    • Mike V.

      I know this comment is years old, but I just found this article and tried this arc method. It works wonderfully! See the attached picture for some clarification.
      First, you measure the corner with your t-bevel. In this example we’re dealing with an acute angle, so an outside corner. Imagine a baseboard on an outside corner of a wall. Because of the buildup of drywall mud and corner bead, the outside corners are almost always acute.
      Register your t-bevel on an edge of something flat that you can draw on. In the picture I just used a 2×6 scrap.
      I chose to draw a line parallel to the board edge just to make it easier to use the compass for drawing the arcs.
      Now draw the line along your t-bevel such that it goes across the parallel line and is maybe 4 to 6 inches long. The length is arbitrary. In this example the acute angle of our outside corner is on the viewer’s left.
      The intersection of these 2 lines is called point A.
      Now set your compass pivot point on point A and draw an arc so that it covers the entire angle that we’re working on. Again the size of the radius is arbitrary, but it should be big enough to intersect the line you drew against the t-bevel and the parallel line on the board edge.
      In the picture we now have points B and C.
      Next set your compass pivot on point B and size the radius anywhere between points A and B. Larger will be more accurate.
      Draw this arc so that it intersects line A B and the original arc.
      Now, without altering the compass, set the pivot on point C and make another arc. Draw it such that in intersects the previous arc in two places. You should now see a “football” shape.
      Finally you can use your t-bevel to connect the two ends of the “football” shape and point A, represented in the picture with a dashed line. This dashed line perfectly bisects the original acute angle. Now that the t-bevel is set for the bisecting line, you can use it to set your miter saw and the miter should be perfect, or very very close!
      I like this method. There’s no numbers at all and it’s quick and easy once you do it a few times. The longer your lines and bigger your arcs should make it more accurate. And make sure your pencil is nice and sharp!

  6. John

    Example: a inside corner for baseboards measures 94 degrees so a baseboard needs to be cut with 2 47 degree angles. If the saw is set at 47 degrees the miter will leave a piece of baseboard with a 43 degree angle. Therefore, if I am correct the miter saw really needs to be set at the inverse of the angles (180 – 94)=(86/2)=43.
    Is this correct?

  7. Fernando

    Hi, 46º angle on a miter saw is 44º, sorry i dont understand that, can you explain it ? thanks in advance

    • zack

      Your miter saw is referenced against the fence. So the angle between the fence and the 0 position is actually 90 degrees. This is why you can use a speed square to make sure 0 on your gauge is perfectly 90 degrees as the blade comes down.

      That said, you should look at the 0 on your miter saw as if it was 90 degrees, not 0. Now if you pretend 0 is actually 90, move the saw to the left and count backwards from 90. Instead of 15 degrees labeled on the saw, it would be 75. Instead of 20, it would be 70. As you count backwards, when you reach 45, it’s actually correct. 45 is just half of the 90.

      So if you keep moving the saw to the left past the 45 mark, it becomes 44, 43, 42, until you run out of space. And obviously if you move the saw back toward the 0 position, you’re counting 45, 46, 47, 48 back up to 90, which is the 0 mark.

      Therefore, if you put the miter saw gauge on 46, it’s actually 44 if you use the 90 degree-count-backward technique.

      If you move the saw to the right, you do the exact same thing, start at 90 and count backwards until you hit 45, which is spot on, then keep counting down beyond it.

      Some people put tape over the miter saw markings to label the correct angles, or sharpie might do.


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