Sealing a hole in sheet metal, a pipe end, or an unused knockout? This rubber plug size guide shows you how to measure so your plug seats tight the first time.
The 3 Measurements That Matter
- Hole diameter — measure the actual opening you are plugging, at its narrowest point. For tapered or punched holes, measure the smaller face.
- Stem diameter — the widest part of the plug body that enters the hole. For a compression fit, the stem should measure equal to or up to 1/16" larger than the hole so the rubber compresses and grips.
- Stem length — must be longer than your material thickness so the plug fully seats. Extra length behind the panel is fine.
Which Style of Plug Do You Need?
| Plug style | Best for | Shop |
|---|---|---|
| Line-sealing / stem plugs | Sealing pipe ends, tube IDs, drilled holes, pressure-testing lines | Rubber Line Seal Plugs |
| Push-in bumper plugs | Sheet metal holes that need a finished button face — doubles as a rubber foot | Push-In Rubber Plugs |
| Blank / closed grommets | Lining and sealing a panel hole with nothing passing through | Closed-Hole Grommets |
| Tapered stoppers | Variable hole sizes, lab and drain applications — one plug covers a size range | Rubber Stoppers |
Common Rubber Plug Sizes
| Hole size | Recommended plug stem |
|---|---|
| 1/4" | 1/4" – 5/16" stem |
| 3/8" | 3/8" – 7/16" stem |
| 1/2" | 1/2" – 9/16" stem (see our 1/2" stem line seal plug) |
| 5/8" | 5/8" – 11/16" stem |
| 3/4" | 3/4" – 13/16" stem |
| 1" | 1" – 1-1/16" stem |
Pro tips: Rubber compresses — when between sizes, size up for a watertight fit. For outdoor or engine-bay holes, pick EPDM or Buna-N compounds (see our rubber material guide). And if your hole needs a wire or hose to pass through rather than a full seal, you want a grommet instead — use the grommet size chart.