How to measure a lawn mower belt — even if the old belt is broken

Discover how to measure a lawn mower belt when the original belt is broken. Use simple methods to determine the right belt length and size for your riding mower. 

Choosing a lawn mower belt may seem easy, but selecting an appropriate replacement is quite hard. Most of the time, mower owners take off the old belt and measure its length, to find a belt that is similar in size. However, one dimension rarely tells the full story, and this approach doesn’t often work properly.

A new belt may fit the mower perfectly but, later it may slip, come off, overheat or even break again. The reason is that the right mower belt is not defined by one measurement.

For the best choice of the right belt, you must consider not only its length but also its width, profile, thickness, design, purpose, and type of routing and positioning on pulleys and tensioner. 

In case the old belt is broken, stretched, or has been lost, the task becomes more complicated but not impossible.

This guide will help you to understand how to measure a lawn mower belt and how to avoid some mistakes in the process of finding a new one.

Safety First

Ensure that the mower is switched off before performing any work with the belt, deck, pulley system, or blades. The mower should not be moved until the movement has fully stopped.

Unplug the spark plug boot. In the case of a lawn tractor or a riding mower, it is important to take out the key to the mower and make sure that the PTO is switched off.

Don't work near the drive system with the ignition connected. Even if you are only looking at the belt or taking a measurement.

Which Belt You Need to Measure

There is more than one type of belt on riding lawnmowers and lawn tractors. The most used types of belts are as follows:

Deck belt – this belt is responsible for spinning blades on the lawn mower deck.

Drive belt – this one provides energy to the wheels or the transmission system.

Some model options may contain additional belts in their drive systems.

This point is important because belts can look alike but operate under different loads. Though a deck belt and a drive belt are similar in their measurements, they may have different functioning mechanisms. So if you want to measure mower belt size, make sure that you even know what a belt is for.

Identify the Mower Model Before Measurements

Before making any measurements, obtain the following information:

1. the mower model;

2.  the serial number, if required;

3.  the OEM part number from the old belt (if possible);

4.  the belt routing diagram;

5.  the parts schematic;

6.  the owner's manual.

Knowing all this information can help in getting more accurate results compared to measuring the old belt on its own. In use, a belt may be stretched, worn out at the sides, overheated, or deformed due to incorrect tensioning. It means that its current size may turn out to be completely different from the original size.

What To Check

When learning how to measure mower belt size, check these things:

How to Measure an Intact Belt

This is the basic method for how to measure lawn mower belt length when the old belt is still in one piece.

If the used lawn mower belt is still intact, you can measure it with a flexible tape measure.

Keep the belt on a flat surface without twisting and place the tape around the entire belt. Be careful while measuring the length since you should not stretch or pull the belt.

There are several options when it comes to describing the length of the belt. For instance, belt length can be described as:

• inside length

• outside length

• effective length

• pitch length

So, the length you will obtain while measuring the belt might differ from the one provided in the catalog.

In case you don’t have a flexible tape measure, you can utilize the mark-and-roll technique. You need to place a mark on the belt, place it on the tape measure at the zero-point, and start rolling the belt ensuring that the mark comes back to the zero point. Then you will have your approximate length of the belt.

How To Measure A Broken Lawn Mower Belt

When the belt breaks, save it for now. You should take each piece of the broken belt and lay it down on a flat surface. Arrange the broken ends of the belt together to form the belt. Next, measure the entire length of the belt.

Even if you measure it, sometimes it isn't sufficiently enough. The broken belt might have already gone through a series of changes in its shape and condition over time.

In fact, the broken belt might have already:

• lost some material through time;

• stretched;

•  worn down on the edges;

• overheated a lot;

• increased in size a lot

An example: if your broken belt has a measurement of around 94 inches in length, this will not guarantee that any 94-inch belt can be used.

UDC Parts expert tip: For example, your damaged belt is 94 inches long. First, find out the mower’s model number and the OEM part number. If the original belt measured 93.5 inches, it is possible that the one broke after stretching. Hence, a 94-inch new belt may cause slippage or even come off from the mower’s pulley. 

What If the Old Belt Is Missing?

When the old belt is absent, it's still possible to cut a string and measure its path. But that string should not stretch.

Place it exactly in the belt path:

• around drive pulleys;

• over spindle pulleys;

• over idler pulleys;

• over tensioner pulleys;

•  and thus, through other pulleys of the whole belt system

It's only necessary to mark the intersection point of the string. After that, the length of the string can be easily measured.

Note that this method will only work if the routing of the string is correct. If it is wrong due to a mistake around an idler pulley, the length measured will also be wrong. Remember that the measured length will not equal the length of the belt, and pulley diameter alone cannot be used to calculate the correct belt size.

Routing Can Change Everything

A common mistake people make is that they assume the belt only travels around all the pulleys in a circular manner. However, there are instances where the routes can be more complex, especially with a mower deck.

A mower deck typically contains a drive pulley, spindle pulleys, idler pulleys, and a tensioner.

In some instances, some of the pulleys will touch the V side of the belt. Others will touch the smooth back side of the belt. Therefore, if you go wrong in routing the belt, you will end up with the wrong length, tension, and overall working of the belt.

Before you use the string method to measure, ensure that you have the proper routing diagram. The diagram can be found in places like under the hood of the machine, on the mower deck or the belt system, the operating manual, or the parts diagram.

How to Measure Belt Width

A caliper is the best instrument for measuring width. The biggest section of the V-belt should be measured.

Pick an area that is not excessively worn out. Because of the friction between the belt and the pulley’s sides, it is normal for the old belt to appear narrower. If only the worn-out part is measured, you might think you need a narrower belt, even though the specification of the original design does not require it.

The width of the V-belt is significant because it must fit in a specific way inside the pulley. If the V-belt is too narrow, it will sit too low. If it is too wide, it will occupy a space that is too high.

Why the Right Length May Still Be Wrong

A belt that is appropriate for length may also fail in that the width, contour, or construction is wrong. A poor match can put the belt too deep in the pulley, cause it to ride too high, slip under load, overheat, become worn, and fall off the pulley or affect its operation with the tensioner.

There is also an inherent factor about construction. Many deck belts must deal with large changes in load while the blades are active or cutting tall grass. Most use reinforced cords, like aramid cords, for better strength.

Other types of belts, such as drive belts, may have a different type of construction in a different location in the mower itself.

UDC Parts expert tip:  Make sure to check the pulley groove before deciding on what to replace. The right belt should fit well in the groove without going overly deep or high. If the old belt did either of those, it is advisable to verify the OEM profile instead of basing an order on measured length only.

Don't Copy the Wear of the Old Belt

Sometimes an old belt begins to slip, jump out of place, or lose its tension. The user takes off the old belt, measures it, and buys the same unadjusted length.

However, if the old belt was stretched, the measurement could replicate the issue. The old belt’s worn length could be one reason why it didn't work well.

So instead of just relying on the older belt, always check the OEM specification.

Check the Mower If Belt Keeps Breaking

A new belt should not be replacing faulty components. If you have a belt that broke unexpectedly or a new belt that broke shortly thereafter, inspect pulleys and idlers, spindle bearings and tensioner, tension spring, belt guides, alignment, and routing.

A loose pulley, stuck bearing, bent guide and misrouted belt may cause damage to an otherwise good belt.

If two belts break one after another, don't just replace the belts.  It is time to check the components. The problem can be in a pulley, bearing, guide or tension.

Steps To Choose a New Belt

The most reliable method is to:

1. Find the exact model of the mower.

2. Check the belt number from the manufacturer.

3. Check if it is the deck or drive belt.

4. Measure the belt length if the old belt is available.

5. If the belt is broken, measure the pieces as an estimation.

6. If the belt is missing completely, it can be done by measuring with the assistance of a piece of string and the routing diagram.

7. Measure the width.

8. Look at the profile and thickness.

9. Compare the type of construction and the tensile strength of the belt.

10. Check the pulleys, tensioner, guides, and bearings before putting it in place.

Do Not Choose a Belt by Length Alone

If you want to know the right size of a lawn mower belt, it's all about measuring it. However, measuring is not enough; you should take other important details into consideration as well. Among other things, you should find out the mower model you own, its OEM number, belt type, how the belt is routed, what the pulley specifications are, and belt profile.
To be considered a correct belt, it should fit into the pulley assembly properly. The belt should not only be the right size but also work in the right way.