I need to buy an adapter to make my shop vac work with one of my hand tools. But as I'm shopping, I realize I'm not sure what exactly I need to buy. I'd like to understand how vacuum hose sizes are specified so that I can measure what I have.

For example: on my shop vac, the end of the hose measures 2-1/8"ID x 2-1/4"OD, and the end of the attachments measure 2-1/4"ID x 2-1/2"OD. What is the correct way to refer to the size of this hose?

  • Hah, welcome to my world. They just can't make something that a normal person can understand, can they! What you describe sounds like what is usually referred to as "D50". Which is funny because no visible part that you can measure is anywhere near something 50 (the ID is 53mm, and the OD is usually a cone going from 57 to 59mm -- respectively 2 1/8 and 2 1/4 inches). Presumably the D50 comes from the inner diameter of the spiral hose (which you can't see) having a nominal diameter of 50mm. Nominal is a posh word for "uh... approximately, or slightly bigger or smaller, or whatever".
    – Damon
    Feb 11 '16 at 9:45
  • 2
    Having bought parts that look like they might fit before, I can only recommend to go to a physical ( = not online) shop and take the part that you already have with you. Plug it in and see if it fits. No, buying "compatible" parts from the original manufacturer doesn't work either. I have a suction adapter for my plunge router which will neither fit the original 27mm hose, nor the original 36mm D50 adapter.
    – Damon
    Feb 11 '16 at 9:49
  • I've had this problem before too. Still don't know how to solve it.
    – grfrazee
    Feb 11 '16 at 14:30
  • 2
    Usually duct tape comes into play when things don't fit up.
    – grfrazee
    Feb 11 '16 at 14:41

Ridgid's Shopvac accessories come in 1-7/8" and 2-1/2" sizes. So from this we can deduce that Ridgid, at least, uses the outer diameter of the attachments. The sizes you describe match what Ridgid refers to as 2-1/2". That's just Ridgid, of course, but mostly I see this consistently. YMMV.

Some general strategies to help you:

  1. Take a quick look at the web site of the manufacturer for your equipment, or a vendor that sells accessories. Look at the size options they offer. That will give you the list of sizes they use to refer to your products.
  2. Take a ruler with you when shopping.
  3. If purchasing online don't be afraid to ask about the product (e.g. Amazon lets you post product questions, and for popular products they generally receive answers quickly; and most vendors have at least some form of product sales support).
  4. If purchasing online, you can often go back to the manufacturer's web site with a model number and find more detailed specifications on a product there.

There are also 1.5" fitting and 1.25" fittings (diameter where the male and female parts actually touch). 1.25" has been around for a long time, and is used on many different older vacuums.

2.25" and 2.5" are their larger cousins. If you buy an extension hose, often it will come with adapters for both sizes. Again, the 2.25" is the outer diameter of the male fitting where it inserts into the female hole of the same size.

I prefer to buy simple tapered fittings, rather than the newer lock type fittings, as they work with my box of older extra parts.

One ref is here: https://www.shopvac.com/us/en/accessory-guide

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