Reading a Timing Belt's Markings: Pitch, Width, Profile and Length, and What to Measure When the Print Is Gone

A synchronous belt part number always encodes the same three facts, just in a different order depending on who made it: the tooth pitch (profile), the belt length (as pitch length or tooth count) and the belt width. Gates puts it plainly: width is "always shown as the last part of the size designation," and pitch length equals pitch multiplied by the number of teeth. An inch-series belt such as 210L100 reads as length code, pitch letter, width code; a metric HTD or GT belt such as 350-5M-15 or 1600-5MGT-25 reads as pitch length in millimeters, pitch code, width in millimeters.

If the print is gone, count the teeth, measure the distance across ten teeth and divide by ten to get the pitch, measure the width, and compare the tooth shape to a profile template. Length is teeth times pitch.

The interchange rule: match pitch and tooth profile, not just pitch. Trapezoidal, HTD and GT belts are different tooth shapes, and the only commonly cited crossover is specific and one-directional, covered below.

The pitch families and their pitches

Inch trapezoidal ("timing") belts come in six pitches. Gates lists MXL at 0.080 in (2.03 mm), XL at 0.200 in (5.08 mm), L at 0.375 in (9.53 mm), H at 0.500 in (12.70 mm), XH at 0.875 in (22.23 mm) and XXH at 1.250 in (31.75 mm). Optibelt's ZR series and Bando's catalog give the same values. In the United States these sections are covered by ARPM IP-24, "Specifications for Drives Using Synchronous Belts," which the association lists as covering MXL, XL, L, H, XH and XXH and reaffirmed in 2023. The international standard most buyers still quote, ISO 5296:2012, was withdrawn in January 2018; the current ISO document for imperial-pitch trapezoidal belts and pulleys is ISO 19347:2015, confirmed in 2021.

Metric trapezoidal belts are the T and AT families, almost always polyurethane. Optibelt's technical manual gives T2.5 = 2.5 mm, T5 = 5.0 mm, T10 = 10.0 mm, T20 = 20.0 mm, AT5 = 5.0 mm, AT10 = 10.0 mm and AT20 = 20.0 mm. The governing standard is ISO 17396, current edition ISO 17396:2024. AT is not simply a stronger T: Optibelt lists a T5 tooth width of 1.78 mm against 2.5 mm for AT5, and 3.48 mm for T10 against 5.0 mm for AT10, so the two do not share pulleys even at identical pitch.

Curvilinear (HTD) belts run 3M, 5M, 8M, 14M and 20M, meaning 3, 5, 8, 14 and 20 mm pitch. Optibelt confirms 3.0, 5.0, 8.0 and 14.0 mm for its OMEGA series. The international standard is ISO 13050:2022, "Metric pitch, curvilinear profile systems G, H, R and S"; the ARPM equivalent is IP-27 (2021). Continental's Synchroforce marking uses "H8M" to mean the H (HTD) profile at 8 mm pitch and states the H profile is compatible with ISO 13050 / IP-27.

Modified curvilinear belts are the GT family. Gates' PowerGrip GT3 is listed at 2, 3, 5, 8 and 14 mm (coded 2MGT through 14MGT), and Poly Chain GT Carbon at 5, 8, 14 and 19 mm.

How the code reads at each manufacturer

The same belt carries different strings at different makers because each uses its own field order and series name. Examples from the manufacturers' own pages:

• Gates PowerGrip Timing: 210L100 (length code, pitch letter, width code).
• Gates PowerGrip HTD: 350-5M-15, meaning 350 mm pitch length, 5M pitch, 15 mm width.
• Gates PowerGrip GT3: 1600-5MGT-25, which Gates describes as "5mm pitch, 1600mm pitch length, 25mm belt width."
• Gates Poly Chain GT Carbon: 8MGT-640-12, with pitch first, then length, then width. So the field order flips between two Gates product lines.
• Gates Synchro-Power (T profile): T5-270-8, pitch, length, width.
• Optibelt OMEGA: "optibelt OMEGA 150 3M 15," with 150 = pitch length in mm and 15 = width in mm.
• Optibelt ZR (inch series): width and length codes "were derived from the corresponding indications in inches"; width code 100 equals 25.4 mm.
• Continental Synchroforce: length in mm, then profile and pitch (for example "-H8M"), then width in mm, then a version suffix such as "-A".
• Bando inch series: "200 XH 10000" places the width code first (200 = 2.00 in), then type, then length in mm; "037 XL-1015" places width first, then type, then tooth count.

Width codes on inch belts are hundredths of an inch. Bando's table pairs 025 with 6.4 mm, 037 with 9.5 mm, 050 with 12.7 mm, 075 with 19.1 mm, 100 with 25.4 mm, 150 with 38.1 mm, 200 with 50.8 mm and 300 with 76.2 mm. Metric belts state width directly in millimeters.

Why profiles do not interchange, even at the same pitch

Trapezoidal, HTD and GT teeth are different shapes, and pulleys are cut for one shape. Gates is explicit about its small-pitch GT belts: "PowerGrip GT3 belts must run in PowerGrip GT2 sprockets," and "PowerGrip GT3 belts are not compatible with PowerGrip HTD sprockets, or any other sprockets available on the market." That statement sits in the manual covering 2, 3 and 5 mm GT3 belts.

The one commonly stated exception is the larger pitches. Gates' GT3 product page states: "8MGT, 14MGT pitch: perfect fit on HTD profile pulleys," and describes GT3 as a way "to upgrade existing HTD belt drive applications." Gates' GT2 Twin Power page repeats "Perfect fit on HTD profile pulleys" for the 8 mm double-sided belt. Gates publishes no statement on the reverse case, an HTD belt in a GT2 sprocket; do not assume it works. Optibelt makes a parallel claim for its own curvilinear belts: "All timing belts with the Optibelt profile can be used in optibelt ZRS HTD or RPP timing belt pulleys."

Metric and inch families also do not cross. 5M is 5.00 mm; XL is 5.08 mm. Over 100 teeth that is 8 mm of accumulated error plus a different tooth shape. Likewise 8M (8.00 mm) sits between XL (5.08 mm) and L (9.53 mm) and matches neither, and H at 12.70 mm has no metric twin. Near 5 mm, tooth shape, not pitch alone, separates 5M, T5, AT5 and XL.

Measuring a belt with no readable print

1. Count the teeth. Mark one tooth with a paint pen and count around once.
2. Measure pitch. Lay a ruler or caliper across ten tooth centers (or ten tooth gaps) on the flattened belt and divide by ten. Gates defines pitch as the distance between two adjacent tooth centers on the pitch line, so measuring on a flat section keeps the reading close to that line. Ten-tooth spans put 5.00 mm versus 5.08 mm at 50.0 versus 50.8 mm, which a caliper resolves.
3. Measure width across the back of the belt, away from frayed edges.
4. Compare tooth shape. Trapezoidal teeth have straight flanks and a flat crest; HTD teeth are rounded; GT teeth are a deeper modified curve. Hold the belt against a known belt of each family or a printed profile template, or photograph the teeth edge-on beside a scale.
5. Compute length: teeth times pitch. For example, Gates' TP1040-8MGT-85 lists 130 teeth at 8 mm pitch and a 1040 mm pitch length.
6. Cross-check against the pulleys. Count pulley grooves and measure the outside diameter; the pulley's groove count and pitch must match the belt. If the belt is too worn to measure confidently, the pulleys usually are not.
7. Record whether the belt is single or double sided (Gates' Twin Power and Optibelt's D-series designate double-sided belts), and note the body material (rubber versus polyurethane) and any visible cord type.

What to send a supplier

Photos of the tooth side, the back, and any trace of print; tooth count; measured pitch over ten teeth and the single-tooth value; width; single or double sided, and the body material (rubber or polyurethane); both pulley groove counts and outside diameters; machine make and model; and the duty (motor power, speed, hours per day, and anything unusual such as oil, heat or washdown). With that list a supplier can identify the family, confirm the profile and propose a cross-reference or a compatible alternative without a return trip to the machine. The method is the same one we use for an inductive sensor with no part number and for a process pressure gauge; for where belt drives sit in the wider cost picture, see our motion control piece.