CALCULATOR · FORMULAS & WORKED EXAMPLES

Conduit Trade Size to Metric Converter

Convert either nominal label

Trade size (inch label)Metric designator

Trade size 1/2 ↔ metric designator 16.

Identification labels only; neither is a measured inside diameter.

Complete conversion table with area references

Nominal trade-size ↔ metric-designator mapping, with supported size pages
Trade size (inch label)	Metric designator	Existing conduit area / fill pages
1/2	16	EMT 1/2 · PVC 1/2 · RMC 1/2
3/4	21	EMT 3/4 · PVC 3/4 · RMC 3/4
1	27	EMT 1 · PVC 1 · RMC 1
1-1/4	35	EMT 1-1/4 · PVC 1-1/4 · RMC 1-1/4
1-1/2	41	EMT 1-1/2 · PVC 1-1/2 · RMC 1-1/2
2	53	EMT 2 · PVC 2 · RMC 2
2-1/2	63	EMT 2-1/2 · PVC 2-1/2 · RMC 2-1/2
3	78	EMT 3 · PVC 3 · RMC 3
3-1/2	91	EMT 3-1/2 · PVC 3-1/2 · RMC 3-1/2
4	103	EMT 4 · PVC 4 · RMC 4
5	129	PVC 5 · RMC 5
6	155	PVC 6 · RMC 6

Nominal labels versus physical dimensions

Trade size 1-1/4 corresponds to metric designator 35; trade size 2-1/2 corresponds to 63. These are exact lookup pairs. A measured 35 mm bore does not automatically establish trade size 1-1/4.

For fill calculations, select the conduit type and its stored interior area. EMT, IMC, RMC, PVC Schedule 40, PVC Schedule 80, FMC, LFNC-B and ENT have separate area tables. The same trade-size label is not a guarantee of equal usable area.

Worked examples

Trade to metric: look up 3/4 in the left column → metric designator 21. Do not calculate 0.75 × 25.4 = 19.05 mm and use that as the designation.

Metric to trade: look up metric designator 103 → trade size 4. Then use the specific conduit family’s area table for fill; 103 is not a measured bore.

Frequently asked questions

Does 1/2-inch conduit have a 16 mm inside diameter?

No. Trade size 1/2 and metric designator 16 identify a nominal size. Neither label specifies a measured inside or outside diameter. Actual interior area depends on conduit type and construction.

Can I multiply trade size by 25.4 to find the metric designator?

No. Multiplication by 25.4 converts a physical inch measurement into millimeters. A nominal designation is a table lookup: trade size 1 maps to 27, not 25.4.

Why are there no EMT links for trade sizes 5 and 6?

This site’s EMT dataset stops at trade size 4. The PVC and rigid conduit datasets include 5 and 6. A nominal size mapping does not establish that every conduit type is available in that size.

Methodology & sources

Reference: NEC Table 300.1(C). The 12 pairs below and the identification-only note appear in NFPA public development material, Table 300.1(C). That document is not represented as a newly acquired final adopted edition. This converter covers only the requested 1/2 through 6 trade sizes.

Linked interior areas reuse the existing site dataset attributed to Chapter 9 Table 4. Their original final-edition cell verification remains incomplete; see data-source limitations. The nominal mapping does not certify those area cells.

Hommer Zhao · Updated 2026-10-18