(0) Item

Need Help? Call us at (800) 776-9315

Measure Your Pulley

Information on how to measure the most important dimensions of your pulley, so you can find the best replacement possible. Not sure what a particular term means? That's here, too.

Recommended Tools:

  • Measuring tape, ruler, or calipers(for highest accuracy)
  • String (only needed for flat idlers)

Outside Diameter

Also Known As: OD, length, breadth

Place your pulley face down on a flat surface, then put your ruler or caliper on top, measure from outside edge to outside edge across the circle of the pulley.

Measure Pulley Outside Diameter

The above pulley has an outside diameter of 4.5”

Check carefully to make sure you are measuring across the center. One trick is to to hold one side of your ruler still and rotate the outside edge around - the outside diameter is always going to be the longest dimension you measure.

Flat Diameter

Also Known As: belt diameter, face diameter

You only need to get this measurement if you are using a flat idler or a rope idler pulley.

This is the diameter of the pulley where the belt rides. For some flat idlers, you can use the same process as the outside diameter, just move the edge of your ruler in a little bit to "guesstimate." If your pulley has flanges, your flat diameter will be smaller than your outside diameter.

flat diameter estimate
Looks like this one's about 5"

To be more precise, you can wrap a string tightly around the flat of the pulley where your belt rides. Mark the string where they overlap, then measure its length. Divide that number by 3.14.

measure the flat diameter of your flat idler pulley using a string

In this flat idler, the string measured 9 ¾”. Typing 9.75÷3.14 into a calculator gets you 3.1, so the flat diameter is just about 3”.

No calculator on hand? You can use your browser’s search bar!                                                                                        



Measure from side to side across where your belt sits, from outside edge to outside edge. The width will not include the hub of your pulley.

measuring v groove idler with ruler

Flat Width

Also Known As: belt width

This dimension is only needed for measuring flat idlers and drives.

Calipers work best for measuring this dimension.

If you don’t have calipers, you will need to measure the width between flanges from inside edge to inside edge using a ruler and eyeballing it. If your pulley has flanges, the flat width will always be narrower than the overall width. Some flat idlers have a wide angle on their flanges – this helps your belt last longer, but does make it even trickier to measure accurately.

Is this being used as a backside idler? Try also checking the widest part of your belt. If it’s a 5L or B belt, for example, then the flat of your pulley will need to be about 7/8” or bigger.

measuring the flat width of a flat idler pulley using a ruler, calipers, or your belt

Even inexpensive calipers can make measuring your parts so much easier.

Inside Diameter

Also Known As: Bore, center hole, bolt size, shaft size.                                                                                         

Measure across the center hole of your pulley. Do not include any keyway in your measurement.

This pulley has an inside diameter of 3/4"

Measuring the inside diameter of drive pulley incorrectly through the keyway
Whoops, these calipers are actually measuring the keyway height instead of the inside diameter.

If you know your drive pulley is running on a 1" shaft, or that your idler pulley uses 3/8" bolts, then you don't need to measure the inside diameter on the pulley itself.

Frontside and Backside Offset:

The offset is the distance from the centerline of the pulley to the outside edge of the bore. On a V-groove drive for instance, the centerline or “middle” of a pulley would be the lowest point of the V, and you would measure from there to the edge of the hub. Generally the “frontside offset” is the longer of these dimensions, while the “backside offset” is the shorter.

This is tricky to get if you can’t cut your pulley in half, so the good news is you probably won’t need it.


Most keyways are standardized to the shaft your drive pulley goes onto:

Shaft Size/ Inside Diameter

Standard Keyway Size

1/2” (0.50)

1/8” (0.125)

5/8” (0.625)

3/16” (0.187)

3/4” (0.75)

7/8” (0.875)


¼” (0.25)



If you have your key on hand, measuring that is certainly the most precise option.

Otherwise, a ruler works fine.

Pin Diameter

In the pulley world, a “pin” is a precisely-cut cylinder that links to the engineering standards of a specific belt type. During the design process, pulleys are “married” to a pin size which will be used in design processes for new equipment.

Did any of that make sense? No? No problem, because all most of us will need to know is the following:

Pin Diameter

Belt Type the Pin is used for






4L/A OR 5L/B






IMPORTANT: Having a pin diameter of 0.5625 does not mean that a 4L belt will not work, it just means that the pulley was not necessarily designed with that belt type in the mind of the engineer. Check in the “Belts that fit” section to see if your belt is listed on the pulley you are considering.

Overpin Diameter

If you place two pins on either side of the grooves on a v-belt pulley and measure the distance, you will get the Overpin diameter. It is yet another highly technical dimension that you will probably not need today.

How you can use this, though, is a tool to visualize how far into a groove your belt might to ride into the v-belt pulley you’re considering.

For instance, if the Outside Diameter is 7”, but the Overpin Diameter shows 7.35” then the belt is riding at the top of that pulley. Look at another 7” pulley, and the overpin shows 6.5. Now we’re looking at a belt that rides pretty deep into that groove.  

Another way to consider this is by looking at different belts inside the same groove:

comparing different belts inside the same v groove pulley

The diameter of the belt in the groove is also known as the "pitch diameter"

In a 7" pulley with this kind of groove, a 3L belt has a pitch diameter of 6", while a 5L belt is at 6.8". If your original 3L belt was riding higher up in the groove of your original 7" pulley, then using a pulley with a 0.5625" pin will give your belt some slack that you might not want.


Note: Some places use "stickout" as a synonym of Offset, we find that to be less useful.


How far does your bore “stick out” from the edge of your pulley? Place the pulley face down, with the longest part of the bore resting on the table.

One of the easiest things to measure, so long as your ruler's marks go all the way to the edge instead of stopping early.

Place your pulley, longer part of the bore pointed down, and measure from the table to outside edge of the pulley’s flange.

Measuring stickouts of different pulleys
The pulley all the way to the right has a "negative stickout" and won't be appropriate for certain applications.

The stickout is particularly useful for when you are bolting an idler up and don’t want the flanges to come into contact with anything. 


Measure from one end of your bore through to the other side. If you only have a thick ruler, try using a toothpick or pencil.

When you have a negative stickout, you can't just put your pulley down on a table and successfully obtain a throughbore, so make sure to check the back side.

Did we miss anything? Still confused? Email us at sales@phoenix-mfg.com, our team of experts is happy to help!