Rubber Band Sizes Explained: Numbers, Inches, mm and How to Measure
If you are ordering rubber bands for a production line, the first problem is rarely price — it is that "size 32" from one supplier arrives as something different from "size 32" from another. This guide explains the three numbering systems in use today, shows you how to measure a band you already buy, and tells you which numbers to put on a purchase order so the second shipment matches the first.
The Three Systems You Will Meet
1. The RPI number (1 to 117)
The traditional American system numbers bands from #1 to #117. The number encodes two things at once: the flat length and the width. It does not encode thickness or material, which is where most confusion starts.
2. Inches (length × width)
A #32 is usually written as 3-1/2" × 1/8". Read that carefully: 3-1/2" is the flat length — the band laid out straight, not the diameter of the loop. Width is 1/8". Two buyers can look at the same band and disagree about its "size" simply because one measured flat and the other measured across the loop.
3. Millimetres (loop diameter)
Most Asian and European manufacturers — including our factory in Dongguan — quote bands by loop diameter in millimetres, because that is the number a machine specification sheet actually uses. Our stock range runs from 20 mm to 70 mm loop diameter, with wall thicknesses from 0.8 mm to 3 mm.
Common Sizes and What They Are For
| Size | Flat length | Width | Typical use |
|---|---|---|---|
| #8 | 7/8" | 1/16" | Bundling thin cables, hair ties |
| #12 | 1-3/4" | 1/16" | Stationery, small parts |
| #16 | 2-1/2" | 1/16" | General office filing |
| #32 | 3-1/2" | 1/8" | Produce, cartons, warehouse |
| #64 | 3-1/2" | 1/4" | Heavy bundles, thick paper |
| #84 | 5-1/2" | 3/8" | Pallets, large boxes |
| #105 | 8" | 5/8" | Oversize loads, moving |
For reference: loop diameter ≈ flat length ÷ 1.57. A #32 band (3.5" flat) therefore has a loop diameter of roughly 2.2" — or about 56 mm. That single conversion causes more wrong orders than any other mistake.
How to Measure a Band You Already Use
Step 1 — Measure the flat length
Lay the band on a table and press it flat without stretching it. Measure from one fold to the other. A relaxed band only — a stretched measurement will understate the size you need.
Step 2 — Measure the width
Measure across the band's flat face, not the edge. Width controls how the band distributes load: a 1/8" band cuts into soft produce, a 1/4" band spreads the same tension over twice the contact area.
Step 3 — Measure the wall thickness
Use a caliper, not a ruler. Thickness is what determines how many times the band can stretch before it takes a set. Our standard office wall is 1.1 mm; heavy-duty lines run 1.6 mm to 3 mm.
Step 4 — Count the stretch you actually apply
Measure the band relaxed, then measure it again stretched around the item it holds. The ratio between the two is your working elongation. Stay under 2.5× for long service life; repeated stretch beyond 3× is the single most common reason bands fail early.
Why the Same Size Arrives Different
Three reasons, all of them fixable on a purchase order:
- Tolerance. Cheap bands are cut to ±2 mm. An order that repeats a month later can drift enough to change how the band feeds on automated equipment. We hold ±0.5 mm on cut length.
- Compound. A band made from filled compound feels identical on arrival and loses 40% of its recovery in three months. Ask for the material grade, not just the size.
- Cure. Under-cured rubber is soft and stretches easily — which feels like "better" in a hand sample and fails within weeks in service.
Frequently Asked Questions
What is a #32 rubber band in mm?
3-1/2" flat length and 1/8" width. Converted: approximately 56 mm loop diameter and 3.2 mm width. If your drawing calls for 56 mm, ask for 56 mm and confirm flat length in writing — the two numbers should agree.
How do I convert flat length to loop diameter?
Multiply flat length by 2 for the circumference, then divide by π. For a 3-1/2" band: 3.5 × 2 = 7"; 7 ÷ 3.14 = 2.23" — or 56.6 mm. Use 1.57 as the shortcut divisor: flat length ÷ 1.57 ≈ diameter.
What is the most common rubber band size sold?
For office work it is #32 (3-1/2" × 1/8"). For industrial bundling and produce it is #64 (3-1/2" × 1/4"). In our own catalogue the equivalent metric sizes are 32 mm and 50 mm diameter.
Does thickness matter more than width?
They solve different problems. Thickness controls service life and how much tension the band holds; width controls contact pressure. If bands are snapping, increase thickness. If they are crushing or marking the product, increase width.
What size rubber band holds a pallet?
A typical 1,200 × 800 mm pallet load needs a loop of 2,400 mm or more, which is outside stock ranges and is normally a custom mould. For standard pallet work buyers usually move to a 8"–12" wide oversize band, or a #105 (8" × 5/8") for lighter cartons.
Can I get any size between the standard numbers?
Yes — cut-to-size. We hold tooling across 20 mm to 70 mm in 1 mm steps and can cut between them. Minimum order depends on whether the mould already exists; if it does, a custom size costs the same per kilo as a stock size.
What to Put on Your Purchase Order
Five numbers prevent almost every re-order problem: loop diameter (mm), width (mm), wall thickness (mm), material grade, and cut tolerance. Add the working elongation if the band runs on automated equipment.
Tell us the five numbers — or send one band you are happy with and the size you wish it were — and we will measure it, confirm the compound, and return a sample with the specification written on it before you commit to a bulk order.
Browse our stock sizes on the regular size rubber bands page, or see the oversize range on large size rubber bands. For a size between our standard numbers, start with custom rubber bands.
