{
 "version": "2026-10-20",
 "license": "Dimension sheets: CC BY 4.0. Standard and published-size files: CC BY 4.0. Xem Studio's own designs (files_license = Xem Studio Design License): free to use and sell things from, not to redistribute as files — https://xemstudio.com/license/. Other publishers' files: their own license, linked beside them.",
 "slug": "conduit-knockout-diameters-nema-bulletin-71",
 "name": "Conduit knockout diameters (NEMA Bulletin 71): 1/2 to 6 in trade sizes, minimum, nominal and maximum, with bushing and locknut diameters + hole gauge plates",
 "kind": "standard",
 "category": "Tri-clamp & pipe",
 "group": "Pipe, tube & threads",
 "year": null,
 "question": "What size hole is a 1/2 inch or 3/4 inch conduit knockout, and how big are 1-1/4, 1-1/2 and 2 inch knockouts?",
 "tags": [
  "knockout",
  "conduit",
  "electrical box",
  "enclosure",
  "nema bulletin 71",
  "panel hole",
  "locknut",
  "bushing"
 ],
 "aliases": [
  "1/2 knockout size",
  "3/4 knockout hole diameter",
  "conduit knockout size chart",
  "electrical knockout sizes",
  "1 inch knockout diameter",
  "1-1/4 knockout size",
  "knockout hole size for 1/2 inch conduit",
  "7/8 knockout 1/2 conduit",
  "1-1/8 knockout 3/4 conduit",
  "knockout diameter chart mm",
  "nema bulletin 71",
  "knockout punch sizes",
  "2 inch conduit knockout size",
  "knockout hole gauge stl",
  "conduit hole size for panel",
  "electrical box knockout dimensions"
 ],
 "overall_mm": {
  "length": 298.5,
  "width": 78.7,
  "height": 3
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": false,
 "free_files": 3,
 "url": "https://xemstudio.com/objects/conduit-knockout-diameters-nema-bulletin-71/",
 "sheet_txt": "https://xemstudio.com/objects/conduit-knockout-diameters-nema-bulletin-71/sheet.txt",
 "sheet": {
  "basis": "NEMA Engineering Bulletin No. 71 (reaffirmed 2011), Tables 71-1A and 71-1B, read twice; three Xem-drawn hole gauge plates at the nominal diameters",
  "overall_mm": {
   "length": 298.5,
   "width": 78.7,
   "height": 3
  },
  "notes": [
   "Knockout diameters for single or concentric knockouts in enclosures (outlet boxes, panels, cabinets), by conduit trade size, from NEMA Engineering Bulletin No. 71 (issued June 1965, revised 9 Sept 2004, reaffirmed 2011), Table 71-1A. The text of the table was read twice and agreed on every row. The figures exclude any projection of breakout ears or tabs (note B).",
   "Table 71-1B in the same bulletin gives the diameter of the flange of a metallic bushing and of the locknut covering diameter for a nonmetallic bushing (Dim. B), which sets how far the fitting overlaps the hole: bushing and locknut overlap minimums are listed per size there (for 1/2 in: 0.033 in for the metallic bushing and 0.001 in for the nonmetallic locknut covering; 3/4 in nonmetallic: 0.000). A change in the knockout diameter, bushing flange, locknut or thread dimension can make an incompatible assembly (note A); dimensions for 1/2 through 1-1/4 are from UL 514 Table 20.2 (note E).",
   "Trade size is a name, not a dimension: 1/2 in conduit is 0.840 in OD (rigid) or 0.706 in OD (EMT), and the knockout is a larger hole (0.875 in nominal) so the fitting's thread or body passes. Conduit and EMT outside diameters are on the pages emt-conduit and rigid-imc-emt-conduit-sizes-to-4-in; the metric designator column is the identification number for the same sizes (not a millimetre size).",
   "Files: three hole gauge plates (1/2 to 2 in; 2-1/2 to 4 in; 5 and 6 in) with a through hole at each NEMA nominal knockout diameter (inches × 25.4: 22.225 mm for 1/2 in and so on, not the rounded mm column). Use them to check a punch, a step drill or a template. Plate thickness 3 mm, 8 mm between holes and 3 mm corner radii are Xem's choices. Hole diameters are exact; open them by your process's allowance if the gauge must pass a fitting. First plate 298.5 × 78.7 mm.",
   "Not covered: the outline of breakout ears and tabs, concentric-knockout ring diameters, and the thread major diameters (Table 71-1C) of rigid conduit and IMC (the bulletin gives them; not copied)."
  ],
  "table": {
   "columns": [
    "trade size (in)",
    "metric designator",
    "knockout min (in)",
    "knockout nominal (in)",
    "knockout max (in)",
    "knockout min / nominal / max (mm, as printed)",
    "metallic bushing flange Ø B (in)",
    "nonmetallic locknut covering Ø B (in)"
   ],
   "rows": [
    [
     "1/2",
     "16",
     "0.859",
     "0.875",
     "0.906",
     "21.82 / 22.23 / 23.01",
     "1.00",
     "0.937"
    ],
    [
     "3/4",
     "21",
     "1.094",
     "1.109",
     "1.141",
     "27.79 / 28.17 / 28.98",
     "1.23",
     "1.189"
    ],
    [
     "1",
     "27",
     "1.359",
     "1.375",
     "1.406",
     "34.52 / 34.93 / 35.71",
     "1.54",
     "1.485"
    ],
    [
     "1-1/4",
     "35",
     "1.719",
     "1.734",
     "1.766",
     "43.66 / 44.04 / 44.86",
     "1.92",
     "1.859"
    ],
    [
     "1-1/2",
     "41",
     "1.958",
     "1.984",
     "2.016",
     "49.73 / 50.39 / 51.21",
     "2.18",
     "2.109"
    ],
    [
     "2",
     "53",
     "2.433",
     "2.469",
     "2.500",
     "61.80 / 62.71 / 63.50",
     "2.68",
     "2.635"
    ],
    [
     "2-1/2",
     "63",
     "2.938",
     "2.969",
     "3.000",
     "74.63 / 75.41 / 76.20",
     "3.20",
     "3.156"
    ],
    [
     "3",
     "78",
     "3.563",
     "3.594",
     "3.625",
     "90.50 / 91.29 / 92.08",
     "3.83",
     "3.812"
    ],
    [
     "3-1/2",
     "91",
     "4.063",
     "4.125",
     "4.156",
     "103.20 / 104.78 / 105.56",
     "4.40",
     "4.343"
    ],
    [
     "4",
     "103",
     "4.563",
     "4.641",
     "4.672",
     "115.90 / 117.88 / 118.67",
     "4.94",
     "4.890"
    ],
    [
     "5",
     "129",
     "5.625",
     "5.719",
     "5.750",
     "142.88 / 145.26 / 146.05",
     "6.05",
     "6.062"
    ],
    [
     "6",
     "155",
     "6.700",
     "6.813",
     "6.844",
     "170.18 / 173.05 / 173.84",
     "7.20",
     "7.110"
    ]
   ]
  }
 },
 "sheet_text": "Conduit knockout diameters (NEMA Bulletin 71): 1/2 to 6 in trade sizes, minimum, nominal and maximum, with bushing and locknut diameters + hole gauge plates — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/conduit-knockout-diameters-nema-bulletin-71/  (sheet CC BY 4.0)\nBasis: NEMA Engineering Bulletin No. 71 (reaffirmed 2011), Tables 71-1A and 71-1B, read twice; three Xem-drawn hole gauge plates at the nominal diameters\nCatalog version: 2026-10-20\n\nOverall (mm): length 298.5, width 78.7, height 3.0\n- Knockout diameters for single or concentric knockouts in enclosures (outlet boxes, panels, cabinets), by conduit trade size, from NEMA Engineering Bulletin No. 71 (issued June 1965, revised 9 Sept 2004, reaffirmed 2011), Table 71-1A. The text of the table was read twice and agreed on every row. The figures exclude any projection of breakout ears or tabs (note B).\n- Table 71-1B in the same bulletin gives the diameter of the flange of a metallic bushing and of the locknut covering diameter for a nonmetallic bushing (Dim. B), which sets how far the fitting overlaps the hole: bushing and locknut overlap minimums are listed per size there (for 1/2 in: 0.033 in for the metallic bushing and 0.001 in for the nonmetallic locknut covering; 3/4 in nonmetallic: 0.000). A change in the knockout diameter, bushing flange, locknut or thread dimension can make an incompatible assembly (note A); dimensions for 1/2 through 1-1/4 are from UL 514 Table 20.2 (note E).\n- Trade size is a name, not a dimension: 1/2 in conduit is 0.840 in OD (rigid) or 0.706 in OD (EMT), and the knockout is a larger hole (0.875 in nominal) so the fitting's thread or body passes. Conduit and EMT outside diameters are on the pages emt-conduit and rigid-imc-emt-conduit-sizes-to-4-in; the metric designator column is the identification number for the same sizes (not a millimetre size).\n- Files: three hole gauge plates (1/2 to 2 in; 2-1/2 to 4 in; 5 and 6 in) with a through hole at each NEMA nominal knockout diameter (inches × 25.4: 22.225 mm for 1/2 in and so on, not the rounded mm column). Use them to check a punch, a step drill or a template. Plate thickness 3 mm, 8 mm between holes and 3 mm corner radii are Xem's choices. Hole diameters are exact; open them by your process's allowance if the gauge must pass a fitting. First plate 298.5 × 78.7 mm.\n- Not covered: the outline of breakout ears and tabs, concentric-knockout ring diameters, and the thread major diameters (Table 71-1C) of rigid conduit and IMC (the bulletin gives them; not copied).\n\ntrade size (in)  metric designator  knockout min (in)  knockout nominal (in)  knockout max (in)  knockout min / nominal / max (mm, as printed)  metallic bushing flange Ø B (in)  nonmetallic locknut covering Ø B (in)\n1/2  16  0.859  0.875  0.906  21.82 / 22.23 / 23.01  1.00  0.937\n3/4  21  1.094  1.109  1.141  27.79 / 28.17 / 28.98  1.23  1.189\n1  27  1.359  1.375  1.406  34.52 / 34.93 / 35.71  1.54  1.485\n1-1/4  35  1.719  1.734  1.766  43.66 / 44.04 / 44.86  1.92  1.859\n1-1/2  41  1.958  1.984  2.016  49.73 / 50.39 / 51.21  2.18  2.109\n2  53  2.433  2.469  2.500  61.80 / 62.71 / 63.50  2.68  2.635\n2-1/2  63  2.938  2.969  3.000  74.63 / 75.41 / 76.20  3.20  3.156\n3  78  3.563  3.594  3.625  90.50 / 91.29 / 92.08  3.83  3.812\n3-1/2  91  4.063  4.125  4.156  103.20 / 104.78 / 105.56  4.40  4.343\n4  103  4.563  4.641  4.672  115.90 / 117.88 / 118.67  4.94  4.890\n5  129  5.625  5.719  5.750  142.88 / 145.26 / 146.05  6.05  6.062\n6  155  6.700  6.813  6.844  170.18 / 173.05 / 173.84  7.20  7.110\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- NEMA: Engineering Department Bulletin No. 71, Knockout Diameters and Fitting Dimensions to Assure Mechanical and Electrical Continuity (Tables 71-1A, 71-1B; reaffirmed 2011) — web — NEMA publication; dimensions are facts — https://www.nema.org/Technical/Documents/Bull_71_reaffirmed_12_15_11.pdf\n- McMaster-Carr: Knockout bushings (per-part CAD; the standard body publishes no CAD) (free STEP/IGES per part, for your own design use; no redistribution) — STEP, IGES, PDF — McMaster-Carr site terms: personal design use, no redistribution [verify license at link] — https://www.mcmaster.com/knockout-bushings\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 3 parts): https://xemstudio.com/files/hardware/conduit-knockout-diameters-nema-bulletin-71/index.json\nAPI: https://xemstudio.com/api/geometry?object=conduit-knockout-diameters-nema-bulletin-71\n",
 "sources": [
  {
   "publisher": "NEMA",
   "title": "Engineering Department Bulletin No. 71, Knockout Diameters and Fitting Dimensions to Assure Mechanical and Electrical Continuity (Tables 71-1A, 71-1B; reaffirmed 2011)",
   "url": "https://www.nema.org/Technical/Documents/Bull_71_reaffirmed_12_15_11.pdf",
   "formats": [
    "web"
   ],
   "license": "NEMA publication; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "McMaster-Carr",
   "title": "Knockout bushings (per-part CAD; the standard body publishes no CAD) (free STEP/IGES per part, for your own design use; no redistribution)",
   "url": "https://www.mcmaster.com/knockout-bushings",
   "formats": [
    "STEP",
    "IGES",
    "PDF"
   ],
   "license": "McMaster-Carr site terms: personal design use, no redistribution",
   "license_verified": false,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "emt-conduit",
  "rigid-imc-emt-conduit-sizes-to-4-in",
  "device-boxes-us",
  "metal-boxes-us",
  "ceiling-boxes-us",
  "panel-cutouts",
  "cable-glands-metric-thread",
  "cable-glands-pg-thread",
  "rubber-grommets-panel-hole"
 ],
 "fits": [],
 "dimensions": "exact (nominal, no tolerance added)",
 "dimension_rule": "All geometry on Xem Studio is exact (nominal, true size) with no tolerance added. Where a part meets another part, the object lists its mating features (feature, exact size, what it mates with, fit class: press, slide, run, socket, clamp or designed). Apply your process's allowance to those features only; 'designed' gaps are part of the design and stay as drawn. Typical accuracy by process and ISO 286 fits: https://xemstudio.com/tolerances/",
 "mating": [
  {
   "feature": "1/2 in trade-size knockout",
   "exact": "Ø22.225 nominal (21.82 to 23.01 printed); metallic bushing flange Ø25.4 (1.00 in)",
   "mates": "1/2 in conduit fitting: its thread or body passes (RMC OD 21.34, EMT OD 17.93), locknut or bushing larger than the hole",
   "fit": "designed",
   "files": "knockout-gauge-1-2"
  },
  {
   "feature": "3/4 in trade-size knockout",
   "exact": "Ø28.17 nominal (27.79 to 28.98 printed); metallic bushing flange Ø31.24 (1.23 in)",
   "mates": "3/4 in conduit fitting (RMC OD 26.67, EMT OD 23.42)",
   "fit": "designed",
   "files": "knockout-gauge-1-2"
  },
  {
   "feature": "1 in trade-size knockout",
   "exact": "Ø34.925 nominal (34.52 to 35.71 printed); metallic bushing flange Ø39.12 (1.54 in)",
   "mates": "1 in conduit fitting (RMC OD 33.40, EMT OD 29.54)",
   "fit": "designed",
   "files": "knockout-gauge-1-2"
  },
  {
   "feature": "2 in trade-size knockout",
   "exact": "Ø62.71 nominal (61.80 to 63.50 printed); metallic bushing flange Ø68.07 (2.68 in)",
   "mates": "2 in conduit fitting (RMC OD 60.33, EMT OD 55.80)",
   "fit": "designed",
   "files": "knockout-gauge-1-2"
  }
 ],
 "fit_classes": {
  "press": "stays put: pin, tube, bearing or magnet held by interference. ISO 286 equivalent: H7/p6 (locational interference) or tighter.",
  "slide": "assembles by hand and is held by a grub, pinch or clamp: shaft in a hub, rod in a socket, spigot in a bore. ISO 286 equivalent: H7/g6 or H7/h6.",
  "run": "turns or slides in use: pin in a link, rod through a guide. ISO 286 equivalent: H8/f7 (close running).",
  "socket": "large sliding fit: syringe barrel, sleeve, bobbin in a bore. ISO 286 equivalent: H9/d9 (free running) or looser.",
  "clamp": "split part that closes on the mating part with screws: make it at size.",
  "designed": "a gap that is part of the design (piston fit option, displacer gap, air gap): do not change."
 },
 "faq": [
  {
   "q": "What size is a 1/2 inch conduit knockout?",
   "a": "0.875 in nominal (22.23 mm), 0.859 to 0.906 in (21.82 to 23.01 mm), per NEMA Bulletin 71."
  },
  {
   "q": "What size is a 3/4 inch conduit knockout?",
   "a": "1.109 in nominal (28.17 mm), 1.094 to 1.141 in."
  },
  {
   "q": "What size is a 1 inch conduit knockout?",
   "a": "1.375 in nominal (34.93 mm), 1.359 to 1.406 in."
  },
  {
   "q": "What size is a 2 inch conduit knockout?",
   "a": "2.469 in nominal (62.71 mm), 2.433 to 2.500 in."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "knockout-gauge-1-2-to-2in",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/conduit-knockout-diameters-nema-bulletin-71/knockout-gauge-1-2-to-2in.step",
   "stl": "https://xemstudio.com/files/hardware/conduit-knockout-diameters-nema-bulletin-71/knockout-gauge-1-2-to-2in.stl",
   "note": "knockout hole gauge plate, 298.5 × 78.7 × 3 mm, 6 through holes at the NEMA Bulletin 71 nominal knockout diameters for the 1/2 to 2 in trade sizes (22.225, 28.169, 34.925, 44.044, 50.394, 62.713 mm), smallest first, 8 mm between hole edges; plate thickness, spacing and corner radius are Xem choices"
  },
  {
   "file": "knockout-gauge-2-1-2-to-4in",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/conduit-knockout-diameters-nema-bulletin-71/knockout-gauge-2-1-2-to-4in.step",
   "stl": "https://xemstudio.com/files/hardware/conduit-knockout-diameters-nema-bulletin-71/knockout-gauge-2-1-2-to-4in.stl",
   "note": "knockout hole gauge plate, 429.4 × 133.9 × 3 mm, 4 through holes at the NEMA Bulletin 71 nominal knockout diameters for the 2-1/2 to 4 in trade sizes (75.413, 91.288, 104.775, 117.881 mm), smallest first, 8 mm between hole edges; plate thickness, spacing and corner radius are Xem choices"
  },
  {
   "file": "knockout-gauge-5-to-6in",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/conduit-knockout-diameters-nema-bulletin-71/knockout-gauge-5-to-6in.step",
   "stl": "https://xemstudio.com/files/hardware/conduit-knockout-diameters-nema-bulletin-71/knockout-gauge-5-to-6in.stl",
   "note": "knockout hole gauge plate, 342.3 × 189.1 × 3 mm, 2 through holes at the NEMA Bulletin 71 nominal knockout diameters for the 5 and 6 in trade sizes (145.263, 173.050 mm), smallest first, 8 mm between hole edges; plate thickness, spacing and corner radius are Xem choices"
  }
 ],
 "markdown": "https://xemstudio.com/objects/conduit-knockout-diameters-nema-bulletin-71/index.md"
}