# Sticky  Belt Length Calculator



## Bob Driver

You come across people looking for the size/length of a missing/broken belt all the time on this forum. I just shared this link on a post, but thought other people besides that poster might find this handy. I've found the calculator in "Method # 3" to be so accurate, I have this link bookmarked on my shop laptop. Pay attention to the * notes at the bottom and drawing to the right if you have questions and this calculator will be almost spot on..... 

Pogobill --- Judging from the # of times this problem comes up, this may be worthy of a "Sticky Note" somewhere.

Belt Length Calculator


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## fuddy1952

Years ago I had to measure belts all the time for different things. I just wrapped a string around where it was supposed to go marking where string overlapped. That was length.


Sent from my SM-S205DL using Tapatalk


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## Bob Driver

fuddy1952 said:


> Years ago I had to measure belts all the time for different things. I just wrapped a string around where it was supposed to go marking where string overlapped. That was length.
> 
> 
> Sent from my SM-S205DL using Tapatalk


Only problem with a "string" (construction string) is you're measuring the inside dimension at the pulley "root" and v-belts aren't supposed to touch the "root" of the pulley. They grip at the "V" on the sides, not at the bottom flat of the belt. Belts where the universal belt # starts with "A" or B" are measured on the *inside flat* of the belt. *Outside flat* length of a belt is listed as "4L" or "5L". The longer the belt, the farther your measurement is off with a string. 1/2" or 5/8" rope is more accurate because they add in the "cross-section" of the belt you're running to the length dimension. 

On a 5 1/2" diameter pulley, a *5L belt* will transmit roughly 2 1/2 horsepower. Using a *B section* belt on the same 5 1/2” diameter pulley you can transmit up to roughly 7 horsepower, which is close to three times the horsepower of the 5L belt. On big V-twin engines (20+ HP, with 5/8" belts), I always run B Section belts, they last 2 times as long as 5L belts, even in if the 5L belt is kevlar


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## fuddy1952

Bob Driver said:


> Only problem with a "string" (construction string) is you're measuring the inside dimension at the pulley "root" and v-belts aren't supposed to touch the "root" of the pulley. They grip at the "V" on the sides, not at the bottom flat of the belt. Belts where the universal belt # starts with "A" or B" are measured on the *inside flat* of the belt. *Outside flat* length of a belt is listed as "4L" or "5L". The longer the belt, the farther your measurement is off with a string. 1/2" or 5/8" rope is more accurate because they add in the "cross-section" of the belt you're running to the length dimension.
> 
> On a 5 1/2" diameter pulley, a *5L belt* will transmit roughly 2 1/2 horsepower. Using a *B section* belt on the same 5 1/2” diameter pulley you can transmit up to roughly 7 horsepower, which is close to three times the horsepower of the 5L belt. On big V-twin engines (20+ HP, with 5/8" belts), I always run B Section belts, they last 2 times as long as 5L belts, even in if the 5L belt is kevlar


You're correct. What I was working with were small belts. Knowing that other information and factoring it in looks like would work.

Sent from my SM-S205DL using Tapatalk


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## mikegazaille

Bob Driver said:


> You come across people looking for the size/length of a missing/broken belt all the time on this forum. I just shared this link on a post, but thought other people besides that poster might find this handy. I've found the calculator in "Method # 3" to be so accurate, I have this link bookmarked on my shop laptop. Pay attention to the * notes at the bottom and drawing to the right if you have questions and this calculator will be almost spot on.....
> 
> Pogobill --- Judging from the # of times this problem comes up, this may be worthy of a "Sticky Note" somewhere.
> 
> Belt Length Calculator


If belt is broken I measure the back side(or flat side), if no belt is available I use a fabric tape measure to go around pulleys


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## Bob Driver

mikegazaille said:


> If belt is broken I measure the back side(or flat side), if no belt is available I use a fabric tape measure to go around pulleys


Not all V-belt part #'s use the outside dimension as the length. On a 1/2" belt, the inside dimension is 2" shorter than the outside. On a 5/8" belt, the inside dimension is 3" shorter than the outside. 

Belt part #'s that start with just a plain "A"(1/2"), or "B"(5/8"), are INSIDE dimension. Belt part #'s that start with "4L" (1/2"), or 5L (5/8") are OUTSIDE dimension. Generally speaking, belts that contain a "X", are OUTSIDE dimension also. Metric belts (start with SP) are usually outside dimension, but not always, especially if they are made by our friends in China....... 

That confusion is why I bought a belt gauge years ago and use either a 1/2", or 5/8" rope for the measurement if the belt is missing. A flat tape measure is a sure fire way to get the measurement wrong, especially on a mower deck belt with 3 spindles, two idlers, and the drive pulley......


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