How do I choose a pulley size?

Recommended Tools for Measuring Pulleys:

If you are looking for more details, kindly visit our website.

• Measuring tape, ruler, or calipers (for highest accuracy)

• String (only needed for flat idlers)

Measuring Pulley 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.

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 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.

To find a drive pulley based on the Outside Diameter, click FIND A DRIVE PULLEY and use the Outside Diameter filters on the left-hand side of the page to find the right size!

You can also find V-Groove Idler Pulleys the same way by clicking FIND A V-GROOVE IDLER PULLEY. 

NOTE: If you have a Flat Idler - you may want to locate the right size based on the Flat Diameter instead of the total Outside Diameter. If you have a flat idler, check out the next section!

Measuring Pulley Flat Diameter

Also Known As: belt diameter, face diameter

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

This is the diameter of the pulley where the belt rides. If your flat idler does not have flanges keeping the belt in place, then the Flat Diameter will be exactly the same as the Outside Diameter. 

However, if your pulley has flanges, your flat diameter will be smaller than your outside diameter. If your flat idler does have flanges, you can try and "guesstimate" the flat diameter by measuring the diameter the same as you would the outside diameter, just try to measure as if the pulley does not have flanges. 

 

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.

In this flat idler, the string measured 10 1/4''. Typing 10.25÷3.14 into a calculator gets you 3.26, so the flat diameter is about 3 1/4”.

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

Now that you have your idler pulleys Flat Diameter, use that information to filter to the right size Flat Diameter here: FIND A FLAT IDLER PULLEY                                                              

Measuring Pulley Width

Measure from side to side across where your belt sits, from outside edge to outside edge. The width dimensions on our site do not include the hub of your pulley.

Measuring Pulley 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.

Left: A ruler is used to measure the flat width. 

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

Far Right: A pulley being used as a backside idler. The flat side of the belt is making contact with the flat surface of the flat idler.

Measuring Inside Diameter of Pulley

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"

If you know your drive pulley is running on a 1" shaft, or that your idler pulley uses 3/8" bolts, then it's safe to assume that the inside diameter is the same as the shaft or bolt diameter. 

To find the pulley with the right inside diameter, click on either the drive pulley or idler pulley link below and then filter to the desired inside diameter on the left-hand side of the product page.

Note: Idler Pulleys often can have their inside diameters easily customized by selecting a bore adapter! For more information on this, contact us at [email protected] or 1-800-776-9315 or visit this helpful blog post: How to Use Multiple Bore Adapter Options

Measuring Pulley 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.

Measuring a Pulley's Keyway

A keyway is a notch made into the hub from the inside hole of a drive pulley and is where your key will be inserted to lock the pulley in place on the shaft. Most keyways are standardized to the shaft your drive pulley goes onto.

Standard Keyway Sizes:

Shaft Size/ Inside Diameter

Standard Keyway Size

1/2” (0.50)

1/8” (0.125)

5/8” (0.625)

If you want to learn more, please visit our website Joyroll.

3/16” (0.187)

3/4” (0.75)

7/8” (0.875)

1”

¼” (0.25)

1-1/8”

 

If you have your key on hand, measuring that is certainly the most precise option. Otherwise, a ruler works fine.

Measuring Pulley 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

0.3750

3L

0.4375

4L/A

0.5000

4L/A OR 5L/B

0.5625

5L/B

0.7810

C

 

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.

Measuring Pulley Overpin Diameter

This is yet another highly technical dimension that you will probably not need to find the correct pulley, but for reference, we've included information on this dimension for you:

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. 

How you can use this, though, is a tool to visualize how far into a groove your belt might 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:

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 pin 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.

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Enter any 3 Known Values to Calculate the 4th

Pulley 1

Pulley 2

Diam

inch

Diam

inch

RPM RPM Ratio:

If you know any 3 values (Pulley sizes or RPM) and need to calculate the 4th, enter the 3 known values and hit Calculate to find the missing value.

Decimal Fraction

For example, if Pulley 1 is 6" diameter, and spins at 1000 RPM, and you need to find Pulley 2 size to spin it at 500 RPM, enter Pulley 1 = 6, Pulley 1 RPM = 1000, Pulley 2 RPM = 500, and hit Calculate to find Pulley 2 diameter.

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