Quick answer: size a rubber grommet for an electrical box by the hole, not the conduit trade size. A 1/2" knockout is a 7/8" hole, so it takes a grommet with a 7/8" groove diameter. Then pick a groove width that matches the box wall and an inside diameter that fits your cable.
Knockout grommet size chart
| Knockout (trade size) | Hole diameter | Grommet to use | Inside diameters | Groove widths | From |
|---|---|---|---|---|---|
| 1/2" | 7/8" | 7/8" groove grommets | 9/16" to 3/4" | 1/16" to 3/8" | $0.37 |
| 3/4" | 1-1/8" (about 1.11") | We do not list a grommet for this hole size. | |||
| 1" | 1-3/8" | 1-3/8" groove grommets | 1" to 1-1/8" | 1/16" to 1/4" | $0.92 |
| 1-1/4" | 1-3/4" | 1-3/4" groove grommets | 1-3/8" to 1-1/2" | 1/16" to 3/8" | $0.92 |
| 1-1/2" | 2" | 2" groove grommets | 1-5/8" to 1-3/4" | 1/16" to 1/2" | $1.20 |
| 2" | 2-1/2" | 2-1/2" groove grommets | 2" to 2-1/4" | 1/4" to 3/8" | $1.40 |
Hole diameters are the common nominal knockout sizes. Punched and drilled holes vary, so measure yours with calipers before ordering. Prices are per grommet; quantity discounts apply automatically.
Drilled holes in boxes and panels
For a drilled 3/4" hole, the electrical box grommet fits panels up to 1/8" thick and passes up to a 1/2" bundle. Any other hole size from 3/16" to 4-3/16": use the rubber grommet size chart.
How to pick the groove width
The groove width must match the thickness of the box or panel wall. Measure it with calipers. Thin sheet-metal boxes and enclosures usually take a 1/16" groove; heavier enclosures and doubled panels need 1/8" or more. If the groove is too wide the grommet rattles; too narrow and it will not seat.
Grommet or cable connector?
A rubber grommet protects wire insulation from a sharp hole edge. It does not clamp the cable. Where your electrical code calls for a listed cable clamp or connector at a box entry, use that connector. Grommets suit enclosure pass-throughs, low-voltage and control wiring, and sealing open holes (use a blank grommet for a hole with nothing passing through).
FAQ
What size grommet fits a 1/2 inch knockout?
A 1/2" knockout is a 7/8" hole, so use a grommet with a 7/8" groove diameter. Ours have 9/16" to 3/4" inside diameters.
What size grommet fits a 1 inch knockout?
A 1" knockout is a 1-3/8" hole: use a 1-3/8" groove grommet, with a 1" or 1-1/8" inside diameter.
How do I measure a knockout hole?
Measure straight across the hole with calipers. Then measure the wall thickness for the groove width. See how to measure a rubber grommet.
Do you sell in bulk for panel shops?
Yes. Quantity discounts apply automatically at checkout, and larger orders can use our bulk pricing.