Timing Belt Length Calculator: Teeth, Pitch and Centres

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Reviewed by MLC Editorial Team | 2 October 2026

A timing belt (synchronous belt) is ordered by its pitch and its number of teeth, so the length has to come out as a whole number of teeth. Enter the belt pitch, the tooth count of both pulleys and the centre distance to get the belt pitch length, the tooth count, the next whole-tooth belt up and the centre distance that belt needs. This is a different calculation from a V-belt length, which is ordered by section and millimetre length instead.

Timing belt length calculator

Belt pitch

Custom pitch (mm)

Teeth on large pulley

Teeth on small pulley

Centre distance (mm)

Belt pitch length required

-- mm

Belt teeth: --

Next whole-tooth belt: --

A timing belt can only have a whole number of teeth, so the calculator rounds up to the next whole tooth and then gives the centre distance that belt needs. Stock belts are made in a limited set of tooth counts, so check the supplier list for the nearest length before fixing the centres.

Pitch diameter by tooth count

The pitch diameter of a timing pulley is the tooth count multiplied by the pitch, divided by pi. It is larger than the outside diameter you can measure with calipers, because the pitch line runs through the belt cords, not along the pulley tips. All values in millimetres.

Teeth	2 mm pitch	3 mm pitch	5 mm pitch	8 mm pitch
10	6.37	9.55	15.92	25.46
12	7.64	11.46	19.10	30.56
14	8.91	13.37	22.28	35.65
15	9.55	14.32	23.87	38.20
16	10.19	15.28	25.46	40.74
18	11.46	17.19	28.65	45.84
20	12.73	19.10	31.83	50.93
22	14.01	21.01	35.01	56.02
24	15.28	22.92	38.20	61.12
25	15.92	23.87	39.79	63.66
28	17.83	26.74	44.56	71.30
30	19.10	28.65	47.75	76.39
32	20.37	30.56	50.93	81.49
36	22.92	34.38	57.30	91.67
40	25.46	38.20	63.66	101.86
44	28.01	42.02	70.03	112.05
48	30.56	45.84	76.39	122.23
56	35.65	53.48	89.13	142.60
60	38.20	57.30	95.49	152.79
64	40.74	61.12	101.86	162.97
72	45.84	68.75	114.59	183.35

How the belt length is calculated

For an open drive on two pulleys the belt has two straight spans and two arcs. With pitch diameters D and d and centre distance C:

angle = asin((D - d) / 2C)

Length = 2C x cos(angle) + pi x (D + d) / 2 + angle x (D - d)

The angle is in radians. This is the exact length. The shorter formula often quoted, 2C + pi(D + d)/2 + (D - d) squared / 4C, is an approximation of it that drifts when the centres are close together and the pulleys are very different in size.

Worked example

A 5 mm pitch drive with a 40 tooth and a 20 tooth pulley at 200 mm centres. The pitch diameters are 63.66 mm and 31.83 mm. The exact belt length is 551.27 mm, which is 110.25 teeth, so the belt to order is 111 teeth at 555 mm pitch length. The approximate formula gives 551.27 mm for the same drive, a difference of 0.00 mm.

Choosing the centres

Design the drive around a belt that exists. Work out the length for your ideal centre distance, round to the nearest stocked tooth count, then set the centres from that belt. Leave adjustment in the motor mount or add an idler on the slack side, because a timing belt has to be fitted without being levered over the pulley flanges.

Check the teeth in mesh on the small pulley. A large ratio at short centres leaves only a few teeth carrying the load, and belt makers publish a minimum for each profile. If the figure is low, move the pulleys apart or use a larger small pulley.

Related standards

ISO 13050 - Synchronous belt drives: metric pitch, curvilinear profile systems G, H, R and S, belts and pulleys
ISO 5296 - Synchronous belt drives: belts with pitch codes MXL, XXL, XL, L, H, XH and XXH, metric and inch dimensions

References

ISO 13050:2022 - Synchronous belt drives, metric pitch curvilinear profiles

Every MLC reference tool in one place: engineering calculators and charts.

Frequently Asked Questions

How do I calculate timing belt length?

Work out each pulley pitch diameter as teeth x pitch / pi, then use the open-belt formula: length = 2C cos(a) + pi(D + d)/2 + a(D - d), where a = asin((D - d)/2C) in radians and C is the centre distance. Divide the length by the pitch to get the tooth count and round to a whole number.

How many teeth does my timing belt need?

Divide the belt pitch length by the pitch. The answer must be a whole number, so round to the nearest stocked tooth count and adjust the centre distance to suit. For example a 551.3 mm length at 5 mm pitch needs 111 teeth.

What is the pitch diameter of a timing pulley?

Tooth count multiplied by pitch, divided by pi. A 20 tooth 5 mm pitch pulley has a pitch diameter of 31.83 mm. The pitch diameter is larger than the measured outside diameter.

Can I use a V-belt calculator for a timing belt?

The length formula is the same, but a timing belt must have a whole number of teeth and its pulley diameters are fixed by tooth count and pitch. A V-belt calculator will not tell you the tooth count or the corrected centre distance.

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