How to measure and specify v-belts?

Standards for v-belt profiles

Basically, there are these groups of v-belt and other belt profiles in use in Europe today:

  • conventional or classic v-belts (German “klassische Keilriemen”): standardized in DIN 2215 / ISO 4184; using one-letter profile names in different sizes (Z, A, B, C, D, E), in many cases the profile is also named by its width in millimeters (10, 13, 17, 22, 32, 40).
  • narrow-profile v-belts (German “Schmalkeilriemen”): standardized in DIN 7753 Part 1 / ISO 4184; using SP profile names in different sizes (SPZ, SPA, SPB, SPC). Probably, “SP” stands for German “Schmalprofil” (“narrow profile”).
  • high-performance narrow-profile v-belts, open shoulder, toothed (German: “Hochleistungs-Schmalkeilriemen – flankenoffen, formgezahnt”): standardized in Europe in DIN 7753 Part 1; using XP… profile names in different sizes (XPZ, XPA, XPB, XPC).
  • wide-profile v-belts (German “Breitkeilriemen”, “Variatorriemen”): standardized in DIN 7719 / ISO 1604.
  • flat belts (German “Flachriemen”): used in different applications such as tangential belts, folding and conveyor belts and machine belts [source]. There seems to be no widely accepted standardization in this area, they are named by the measures of their rectangular cross-section.
  • round belts (German “Rundriemen): mostly used for conveying and driving tasks in mechanical engineering.
  • ribbed v-belts (German “Rippenriemen”, “Mehrrippenriemen”): in cross-section similar to flat belts, just that the bottom looks like multiple little v-belts running in parallel. Used for example in several modern vehicles.

For a short description of the different types, see HUG-Technik on Keilriemen [German]; for a more detailed introduction, read on v-belts. For an overview about DIN and ISO standards relevant for v-belts and similar devices, see HUG-Technik on important standards for belts [German]. See also the English Wikipedia on mechanical belts and the German Wikipedia on v-belts; though both of them lack extensive information on belt specs yet.

The DIN and ISO standards cited above are used in Europe; in the US, the standard RMA/MPTA (and in the UK, BS 3790) specifies minimally different but overall compatible profiles with other profile names. [source]

Standards for v-belt length

Length conversion table. A very important tool to work with belt specs is a v-belt conversion table. That’s because the type of length for nominal length (the length to give when specifying a belt) is different for the different types of belts (for example, it is “inner length Li” for classic v-belts – also the only error in the v-belt  conversion table linked above).

Li, Lw, La. For every v-belt, one can give three lengths: inner length Li, effective length Lw and outer length La. These abbreviations are derived from German words “Innenlänge”, “Wirklänge” and “Außenlänge” respectively and might only be in use in Germany. Inner and outer length are the inner resp. outer circumference measure of the v-belt, without any linear tension on the belt and in circular shape. Effective length is a fictive median length of a v-belt that is the circumference at a certain depth of the belt profile. Namely, at a depth that has the “effective width” (German “Richtbreite”), for which see the corresponding column in the v-belt conversion table. Lw is used as the nominal length of belts with profiles SP*, XP* and X*. Synonymous to Lw, some manufacturers use Ld or Lp. [source]

Belt number. For classical v-belts, there is another identification system in addition to the “20 x 3500 Li” type: the belt number (German “Riemen-Nr.”). It consists of the normal profile size designation letter and a number that normally corresponds to the inner length in inches (rounded, where necessary); for example, “Z 22” or “D 150”. [source] A list of these numbers can be found on this v-belt index.

Measuring v-belt profiles

Real-life examples of measurements – all values below are measured with light touch of the caliper:

  • height of v-belts
    • nominal 10 mm; measured 9.3 – 9.7 mm (in convex curves, 9.8 mm)
  • width of v-belts
    • nominal 12.5 mm; measured 12.3 – 12.4 mm

Measuring v-belt lengths

Measuring v-belt length Lw. It is usually proposed to measure a v-belt by cutting it and nailing it flat to a board [source]. However, this is not what you want if you need to determine the size of a new v-belt that has lost its labelling. So here is a different procedure that I developed and tested successfully:

  1. Mark a line on your flat, hard floor by taping 2 measuring sticks to it, and also tape an end stop marker like a flat wood piece to the start of the first stick.
  2. Add a small cable tie around the v-belt to be measured and use that for marking the start and aligning it to the start marker you taped to the ground.
  3. Roll your v-belt on the ground along the measuring sticks until you went one full circle.
  4. Take the measure and interpret it as effective length Lw of the v-belt (which it is, approximately).

You could do two or three measurements and take the average, but this seems not necessary as this kind of measuring, properly executed, has repeatable results that are up to 1 mm exact. In contrast, measuring with a flexible measuring tape while holding the v-belt in your hands is not recommended, because the results are less exact (my experience: repeated measurement of a single belt resulted in 3505 mm Lw and 3520 mm Lw, while the result with the roll-on-floor technique was 3522 mm Lw). When measuring while holding the belt in your hand you have to take special care not to bend it during measuring, as bending will increase the measurement more towards La; for the above example, measuring with light bending increased the measurement to 3533 mm Lw while it should have been 3522 mm Lw).

It is said that the measure taken by this procedure is the “median length” of the v-belt [source], though “median” here it is probably not meant in the mathematical sense of “length at a profile height where half of the profile area is above and half is below that height”. In practice, these measurements are said to be a good enough approximation of the effective length Lw. While it is a good enough approximate, my tests suggest that the length measured this way also depends on the properties of the belt build-up, so is not always the average, median or whatever length that could be generically specified. The measurement is for “something between inner and outer length”; exactly which mostly depends on how the belt is built, namely, how far to the outside the pull-resistant strings are located. Because, these seem to work like a hinge when bending or unbending the belt, affecting the depth of the area on each side that gets either compressed (so, shortened) or pulled on (so, lengthened). These fibres are normally located right below the upper edge of the belt, so the measure is normally more towards the outer length than the Lw measure is.

Measuring v-belt length La. It is said that one can do that by placing the belt in circular shape on a flat surface and placing a flexible measuring tape around it [source]. However in practice, this is hard to do as you need some tension on the tape to not measure too much. It works well when the velt is still mounted, though.

Determining Lw without a belt. In case you have a device needing a belt but don’t know which one, place a rope around the belt path that has approx. the thickness of the pulley profile grooves. Mark the length of rope you need, take it out again and measure that length when laying it straight on flat ground. This is a  good approximate for the effective length Lw of a fitting belt. (As an alternative, there is a formula to calculate this from La or Li, and online calculators for that.)

Various experiences with measuring v-belts

  • At times, some v-belts seem to use the wrong signing schema. One belt had the classic profile, so the signing of “20 x 3550” was to be interpreted as Li = 3550 mm. However, all measurements turned out with Lw = 3522 mm, so an even larger measure of 3550 mm can only be La, not Li. The formula produces a result coherent with this: La = Lw + 31 mm = 3522 mm + 31 mm = 3553 mm. Another, quite old belt from Continental was SPA profile so should be labelled with Lw, yet the label said “12,5 x 1200 La“.
  • Amount of difference between measured flat length and Lw. In one case, a classic 20 x 2000 mm Li belt was measured with 2060 mm flat length. Lw according to the formula is Lw = Li + 48 mm = 2048 mm Lw. The difference of 12 mm is quite small and probably comes from the fact that the pull-resistant fibres are not exactly located at the diameter corresponding to Lw; see above.
  • Different formulas for converting to / from Lw. Interestingly, there seems to be at times a slight divergence of what formula is to be used for converting from effective length Lw to Li and La. For example, this v-belt conversion table and basically all other such conversion tables on the web state for a 17 mm wide classical v-belt: Lw = Li + 40. However, one 17 mm classical v-belt belt was found with an inscription saying “PETER-BTR 17 x 1320 Li / 1363 Lw”, corresponding to Lw = Li + 43.

Storing v-belts correctly

It is said that v-belts are normally built by manufacturers to reach a lab runtime of 25 000 hours [source]. If this is reached in practice also depends on proper storage conditions.

If properly stored, v-belt properties do not change for several years. However, most rubber-based products will deteriorate if improperly stored or handled (like being exposed to oxygen, ozone, extreme temperatures, light, humidity or various solvent agents). [source]

Therefore, the storage location should be dry and dust-free and must not contain chemicals, oils or solvents at the same time. V-belts should be stored without any force on them (pressure or pulling force) to avoid any permanent shape changes and other damages. This also means that they should only be stored in hanging condition if the hanger pin is at least ten times the height of the belt profile. Additional maintenance hints for rubber products are found in DIN 7716. [source]







25 responses to “How to measure and specify v-belts?”

  1. Pierre

    My test on spz577 v-Belt.

    When I roll-up the belt on a flat table the measure is identical to the measure as been made on the entire length after cutting it (572 mm)

    But the measurement before being cut, over the entire circumference with a sewing thread or a rope is 590 mm.

    Multiple test, same results

  2. Pierre: yes, these results make sense. When measuring in circular shape, the outer portions of the belt are all tensioned a bit because the belt is bent. Which makes them longer. When measuring by roll-up, you measure each part of the belt in a moment where it is not tensioned (flat on ground), so a bit shorter than in circular shape.

    If the difference “La to Lw” you measured is too big however, it may be because measuring over the entire circumference is difficult except when the belt is still mounted: it needs some tension on the measuring tape to not measure too much.

    See also the last paragraph in section “Measuring vee-belt length Lw.” for some more details.

  3. Pierre

    I should specify that I perform the test by the two methods : with flat on the table and without flat with respecting the roundness as possible.
    And the result was as I said the same. 577 mm, in contrast to the rope which measured 590mm.
    Here is the question: What is the true length?
    I mean, the new belt I have just received, what measure I should used to check it.?

    Qc. Canada

  4. What means measuring “[on table] without flat with respecting the roundness as possible.”? When I do this, I roll the belt along on the table but in sections of 10-15 cm which I press flat on the table, then going to the next section. If so, then this is the length called Lw (“effective v-belt length”, but I have no idea what the proper English term is).

    Measuring with a rope around the complete circumference at once will give you La (“outer length” instead).

    Every v-belt has these three: effective length (Lw), outer length (La), inner length (Li). Which of these you need to check if your belt is the right one depends on which is given in the specs of your machine. If it states just a length there without indicating what length this is, it is possible to determine the missing type of length from the type of belt that is required. Here is an overview for European and US types [source]:

    nominal lenght is “effective length” Lw for V / N / VX / NX belts (USA standards RMA/MPTA) (source mentions outer length, which is wrong)
    nominal length is “inner length” Li for classical v-belts (source mentions effective length, which is wrong)
    nominal length is “effective length” Lw for ZX / AX / BX / CX profiles
    nominal lengthis “effective length” Lw for XPZ / XPA / XPB / XPC profiles
    nominal lenght is “effective length” Lw for SPZ / SPA / SPB / SPC profiles

    Or of course measuring the same length on the old belt, if available.

  5. Pierre

    I’m sorry for ambiguities.
    Just to say a flat surface.
    Rather than proceed by section, I rolled it as a perfect wheel being assured me that it is primarily the more circular as possible.
    In this way I would have expected that this measure coincides with the last measurement taken with the rope. (Sewing thread to maximize the reading)

    To my great surprise the belt length proceed as a perfect wheel (572mm) does not give the same measure as the rope. (590mm)
    Without forgetting that the measure once cut was identical to that of the wheel measurement by section as well of that wheel ”almost perfect.”

    Fact to note: This is a new belt.

    So if the manufacturer states that to the belt must be added to spz577 a length of 13 mm to give the result of 590 mm.
    Make me think that it is shorter than it should be.


  6. I’m sorry but I have no idea why measuring by rolling as a perfect wheel does not produce the same result as with the rope in your case. I did not have the issue in spite of all my confusion with belt measures so far …

    About the 572 mm, keep in mind that these measurements are only an approximate way to measure the effective length Lw (which is essentially an “imaginary” length at some depth in the v-belt profile). So it seems still possible that the SPZ577 labelling is right (Lw = 577 mm) and your measurement of the outside length with a rope accords with that (using the formula with 13 mm difference that you quoted, which is the right one).

    No warranties about all this … I’m not a professional in this field, I just had to sort and measure hundreds of old vee-belts at one time, so I did some research.

  7. Pierre


    Okay sir.
    Thank you for your explanations very interesting all the same.
    I will inform you if I get more details about the explanation.
    Pleasure to communicate with you,


  8. Belts are names as SPA/SPB what it stands for and what is its abbreviation. And also Lw, Pb, Pd what isthe meaning of it and what isthe use of it.

  9. I think “SP” means German “Schmalprofil” (“narrow profile”, also mentioned in the text), and the following letter (“A”, “B”, “C”, “Z” etc.) is just a symbol for a certain width.

    Lw means “effective length”, from German “Länge, Wirk-“. I also know of Ld and Lp, which are used by some manufacturers as a synonym for Lw, but I have not yet seen your Pb and Pd yet …

  10. Thanks for your ideas. You can also find the details on Drive Parts Direct, at the V & Wedge Belts. Drive Parts Direct aims to provide you with a convenient “one stop shop” for all your Power Transmission component requirements.

  11. Ralph Vladeck

    I have bought belts from some people who have organized their product line in a way that it is much easier to see the differences between models and it makes a big difference for me when buying belts for my machinery. You can check them our at

  12. salehi

    hello.i am mechanichal engineer.i want to test a v-belt for oil cooler fan that work in 6500 rpm.please help me to choose divice for test its.tension test and dinamic i can buy v-belt testers.thank yuo.

  13. hashir

    so what about this belt this is ‘le’ BELT – AV 13 X 780 LE
    I want to your assistance

  14. Jackson

    Why different suppliers produce belts have different La, though the markings on the belts (La, Li and inch size) are same, like A30 or whatever? how it happens, different standards or whatever? Please explain to me

  15. Thanks for sharing valuable information about the V belts. With this information, we can select V Belts as per our requirement. And thanks for sharing how to store V Belts for a long time. Keep sharing.

  16. qayyum dhale

    Thanks for sharing a valuable specifications.

  17. Jose

    I have a doubt about belts and it might sound ignorant but i’d like to know. Would Belt length affect RPM? I mean Would placing a longer belt on equipment that require a smaller size affect in anyway? I must certainly think it does..but can anyone explain?

  18. @Jose: No, belt length does not influence RPM. Only the diameter of the belt disks does.

    Think of a bicycle: it has small and big gears that determine the gear’s ratio, but the chain is only the link between the gears. When the circumference of the primary (powered) gear travels by 10 chain links, the secondary (driven) gear’s circumference also travels by 10 chain links, as the chain creates a fixed link between the two gears’ circumferences. Just how many RPM the 10 chain links per unit of time mean for each of the gear depends on the gear’s size – so gear size determines RPM.

    It’s just the same with v-belts and pulleys.

  19. elvis ngulube

    kindly give us a quote for :
    XPC 4000 v-Belts.
    QTY 5

  20. @elvis: Sorry, I don’t sell anything here. It’s just an article about v-belts. 🙂

  21. Andrew

    Very detailed explanation. Thanks!
    I’m trying to figure out the differences between XPZ and SPZ.
    Aside from cogged feature of XPZ, I see “high-performance” in your description. Does this mean XPZ is better than SPZ?

  22. @andrew: Honestly, I don’t really know. From dealing with all these belts, my guess is the following though: SPZ is an older type of belts. Then people discovered that by applying the fabric only on the outside and using a toothed profile, one can have a longer-lasting belt that also has more traction and does not develop cracks in the rubber on the inside easily. So the XPZ standard was developed accordingly. “High-performance” probably means that higher power transmission is possible with XPZ than with the same width of SPZ, but again, that’s just a guess.

    The exact reasons do not really matter in practice, except if you build your own machine and can choose what belt pulleys to use. Because to my knowledge, the two profiles are not interchangeable. So if your existing machine specifies that it needs SPZ, you can’t put in a XPZ belt and expect it to work.

  23. Anonymous

    Again, thank you!

  24. Thank you for your explanation on how to store V Belts for a long time. With this information, we can chooseV Belts
    as per our requirement. Keep sharing such articles.

  25. madhurapixgermany

    The particular producing method for the open-edged, shaped V-belts differs from that of the jacketed V-belts; these sq . measure belts with internal splines. The teeth facilitate to cut back the bending stress of the belt and permit it to work on plentiful smaller diameter pulleys. The increase in potency over the shrouded V-belts significantly reduces the necessity for multi-belt drives. Flankenoffene, formgezahnte Keilriemen Open-edged V-belts need the tension than jacketed V-belts so that they attain higher power transmitting.

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