Measuring Electrical Conduit Sizes at the Job Site


Electricians typically run multiple cables through a single raceway, so it's vital that they know how much space is available inside the duct. That isn't always apparent at a glance.

Electrical conduit dimensions come in three categories: the trade size, the metric designator, and the nominal internal diameter. None of these is guaranteed to precisely match the inner diameter of a pipe! 

Even worse, bundles of conduit can arrive on-site unlabeled, and it isn't easy to tell the difference between 1.25- and 1.5-inch conduit with the naked eye.

Electrical Conduit Dimensions for EMT

Electrical metallic tubing is one of — if not the — most common types of raceway electricians work with. Here are the official dimensions, trade sizes, and metric designators for EMT according to NFPA 70, the National Electrical Code®.

Trade Size Metric Designator Inner Diameter (Inches) Inner Diameter (Millimeters)
½ 16 0.622 15.8
¾ 21 0.894 20.9
1 27 1.049 26.6
1 ¼  35 1.380 35.1
1 ½ 41 1.610 40.9
2 53 2.067 52.5
2 ½ 63 2.731 69.4
3 78 3.356 85.2
3 ½ 91 3.834 97.4
4 103 4.334 110.1
5 129 5.073 128.9
6 155 6.093 154.8

When you’re dealing with electrical conduit, dimensions are harder to discern than you might think. Even the NEC’s internal diameters are listed as “nominal,” meaning these numbers are there to identify rough sizes, not provide a specific, accurate measurement.  

In fact, products from different manufacturers may have subtle differences in size, even from one section of conduit to the next. Any variation in inner diameter can cause problems during joining, or when planning how many cables you can pull within a single run.

So, given all of these factors, what's the ideal way to measure a length of raceway before installing it?

Determining True Electrical Conduit Dimensions

Measuring Electrical Conduit Sizes at the Job Site The best way to figure out electrical conduit dimensions is to measure the internal diameter of your raceway. That's why we recommend keeping an Electrical Conduit Gauge on hand at every work site.

These simple triangular measuring tools accurately gauge the inner wall-to-wall diameter of any piece of conduit. While the all-steel Electrical Conduit Gauge from BHS was designed to measure the inner diameter of steel and aluminum conduit, when used correctly it's just as effective for PVC and flexible conduit.

The Electrical Conduit Gauge accurately measures tubing from 0.5 inches to 4 inches in diameter. Measurements are laser-engraved into corrosion resistant stainless steel, providing clear trade size recognition for conduit even in harsh industrial environments, without fear of obscuring markings with decades of hard use. There's even a handle 0.5-inch hole at the base for simple storage by hanging.

Throw an Electrical Conduit Gauge into your kit alongside your conduit cutter and a bending tool, and you'll never have to guess about electrical conduit dimensions. That keeps you ready to install complex runs wherever the job requires it. 

References:

"Conduits Sizes." EngineeringToolBox. The Engineering ToolBox, n.d. Web. 22 June 2017.

http://www.ecmag.com/section/your-business/cool-tools-conduit-benders

Eby, Mike. "The Basics of Steel Conduit." ECMWeb. Penton, 1 Aug. 2004. Web. 22 June, 2017.

"Guidelines for Installing Steel Conduit/Tubing." SteelTubeInstitute. Steel Tube Institute of North America, n.d. PDF. 22 June 2017.

Table 4 - Dimensions and Percent Area of Conduit and Tubing.” National Electrical Code. NFPA, 2023. 6 Jan 2025.