How this calculation works
Why the knockout is the same for every type
The pipe ODs differ — EMT is thin wall, rigid is thick, PVC-coated rigid is thicker still. But the connector threads are standardized, so the hole in the box is the same regardless of which conduit lands in it. Punch to trade size and any connector of that size will fit.
That is why the knockout column has one value per row while the OD columns all differ.
What the numbers tell you on a job
EMT has the smallest OD of the metallic types, which is why it packs tightest in a rack and why it takes the smallest strut clamps.
IMC and rigid share the same OD. IMC has a thinner wall made of a stronger alloy, so the inside diameter is slightly larger and the pipe is meaningfully lighter. Same fittings, same clamps, less weight, more usable fill area.
PVC-coated rigid — Rob Roy or equivalent — adds about 40 mil of coating, so it runs oversize. Standard clamps and straps for the trade size are often too tight, and you need the coated hardware to match.
PVC Schedule 40 and Schedule 80 share an OD. The extra wall on Schedule 80 goes inward, which is why the two have different fill capacities despite looking identical from the outside. Schedule 80 is what you use above ground where the raceway is subject to physical damage.
Layout use
These numbers matter most on racks and through-penetrations. Six runs of 2 inch EMT on a rack is not 12 inches of pipe — it is about 13-1/8 inches of OD plus your clamp spacing. Working from trade size instead of OD is how racks end up an inch too short for the space.
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Frequently asked questions
Why is trade size not the actual size?
It is a legacy naming convention inherited from iron pipe sizing, where the number approximated the inside diameter of the wall thicknesses common at the time. Wall thicknesses changed; the names did not. Neither the inside nor the outside diameter of modern conduit matches its trade size.
Is the knockout the same for EMT and rigid?
Yes. Connector threads are standardized by trade size, so a 3/4 inch knockout accepts a 3/4 inch connector regardless of whether it is EMT, IMC, rigid or PVC. Only the pipe ODs differ.
Do Schedule 40 and Schedule 80 PVC use the same fittings?
Yes. They share an outside diameter, so couplings, connectors and clamps are interchangeable. The extra wall on Schedule 80 goes inward, which reduces the usable fill area — that is the only practical difference besides impact resistance.
Why does PVC-coated rigid need different clamps?
The coating adds roughly 40 thousandths per side, which puts the OD outside the range standard straps and clamps are cut for. Coated hardware is sized for it, and using it also avoids scraping through the coating, which would defeat the corrosion protection you paid for.
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