What is Li in belt measurements?

Li usually means inside length, measured along the inner side of the belt.

What is La in belt measurements?

La usually means outside length, measured around the outer side of the belt.

What is Lp in belt measurements?

Lp usually means pitch length, which is the reference line used for belt sizing.

Are belt length conversions exact?

No. Belt length conversions are approximate and should always be checked against the printed marking and actual belt section.

Tools

Belt Length Converter: Inch, MM, Li, La & Lp

Use this belt-specific helper to estimate Li, La, and Lp from a printed length in inches or millimeters. It is built for practical belt ordering, not general-purpose unit conversion.

Length value Unit Length type Belt section

Selected section dimensions

Top width--

Height--

Pitch width--

Override length factor manually

Use this to fine-tune the selected section factors without changing the rest of the logic.

Enter the printed belt length or measured length. The tool will estimate related belt-length types based on the selected section.

Belt length conversions are approximate. Li, Lp/Ld, and La can vary by belt standard, brand, and factory. For factory orders, always confirm the printed belt marking and whether the size is inner length, pitch/datum length, or outer length.

Standard section values are based on manufacturer catalogue belt length factors. Banded 2RA/3RA/4RA and 2RB/3RB/4RB/5RB values are practical Pakistani-market approximations and should be confirmed with the supplier before production.

Results

The output below updates instantly as you change the inputs.

Input Length (in)

Input Length (mm)

Inner Length (Li)

Pitch or Datum Length (Lp/Ld)

Outer Length (La)

Conversion note

Enter a length to see an estimate.

What the length types mean

Li is the inner length. It is measured along the inner side of the belt and is often used when the inside reference is available.

La is the outer length. It is measured around the outer side of the belt and can differ from the inner length by belt section.

Lp / Ld is the pitch or datum length. It is the working reference used to compare many belt specifications.

Banded Belts (2RA, 2RB, etc.) are made by joining multiple A or B ribs side-by-side. The per-rib profile matches the base section, but the total width varies.

Diagram showing Li, La and Lp (datum) length measurements on a V-belt. — Figure 1. Illustration showing the difference between Li (inside length), La (outside length), and Lp/Ld (pitch/datum length) measurements on a V-belt.

Standard section offsets used here

These are conservative reference differences used by the calculator. They are not factory standards or guaranteed purchase values.

Section	Top width	Height	Pitch width	Li to Lp	Lp to La	Li to La
8	8 mm	5 mm	6.7 mm	19 mm	12 mm	31 mm
Z	10 mm	6 mm	8.5 mm	22 mm	16 mm	38 mm
A	13 mm	8 mm	11 mm	36 mm	14 mm	50 mm
2RA	26 mm	9.5 mm	22 mm	45 mm	18 mm	63 mm
3RA	39 mm	9.5 mm	33 mm	45 mm	18 mm	63 mm
4RA	52 mm	9.5 mm	44 mm	45 mm	18 mm	63 mm
B	17 mm	11 mm	14 mm	43 mm	26 mm	69 mm
2RB	34 mm	13 mm	28 mm	49 mm	33 mm	82 mm
3RB	51 mm	13 mm	42 mm	49 mm	33 mm	82 mm
4RB	68 mm	13 mm	56 mm	49 mm	33 mm	82 mm
5RB	85 mm	13 mm	70 mm	49 mm	33 mm	82 mm
20	20 mm	13 mm	17 mm	48 mm	31 mm	79 mm
C	22 mm	14 mm	19 mm	56 mm	32 mm	88 mm
25	25 mm	16 mm	21 mm	61 mm	39 mm	100 mm
D	32 mm	19 mm	27 mm	79 mm	40 mm	119 mm
E	38 mm	23 mm	32 mm	92 mm	53 mm	145 mm
SPZ	10 mm	8 mm	8.5 mm	37 mm	13 mm	50 mm
SPA	13 mm	10 mm	11 mm	45 mm	18 mm	63 mm
SPB	17 mm	14 mm	14 mm	60 mm	28 mm	88 mm
19	19 mm	15 mm	16 mm	69 mm	25 mm	94 mm
SPC	22 mm	18 mm	19 mm	83 mm	30 mm	113 mm
3V	9.7 mm	8 mm	9.7 mm	37 mm	13 mm	50 mm
5V	15.8 mm	14 mm	15.8 mm	60 mm	25 mm	85 mm
8V	25.4 mm	23 mm	25.4 mm	92 mm	53 mm	145 mm
3L	9.7 mm	5.6 mm	8.5 mm	22 mm	16 mm	38 mm
4L	12.7 mm	7.9 mm	11 mm	36 mm	14 mm	50 mm
5L	16.7 mm	9.7 mm	14 mm	43 mm	26 mm	69 mm

The tool uses the table above as verified catalogue-style reference data; it does not treat the values as universal exact values.

Why belt lengths differ

Different belt constructions measure from different reference lines. That is why the same belt can be listed with Li, La, or Lp values that are not identical.

Use inches when the belt marking is written in imperial values. Use mm when the printed size or supplier note uses metric values.

Always verify the section and printed marking before ordering. The wrong section can produce a misleading conversion even if the length looks close.

Common mistakes when ordering belts

• Treating Li, La, and Lp as the same measurement.
• Assuming a metric value can be swapped directly across belt sections.
• Ignoring the section when the printed length looks close.
• Ordering from a calculator result without checking the old belt marking.

Where this helps most

Use this tool when checking Li, La and Lp belt size difference guidance before placing an order.

It is also useful when comparing belts in V belts, industrial belts, and automotive belts.

If the printed marking is unclear, contact GM Traders before buying.

Contact GM Traders for belt availability

Share the belt section, the printed marking, and the measurement you have. GM Traders can help confirm whether the belt is available and whether the size reference looks practical for your application.

Contact GM Traders Browse Products

Frequently asked questions

Why is the manual override field included?

It lets GMT tune real-world section differences later without replacing the entire tool logic.

Can this replace belt marking verification?

No. It is only a practical estimate. Always confirm the printed marking and the actual belt section before ordering.

Does the tool store any customer data?

No. It runs entirely in the browser with no backend, database, or API.

Ready to request belt supply?

Send the belt category, quantity, and city, and GM Traders will respond with the next step.

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