{"id":101275,"date":"2026-10-07T09:17:15","date_gmt":"2026-10-07T09:17:15","guid":{"rendered":"https:\/\/dmfitcouk.onyx-sites.io\/?page_id=101275"},"modified":"2026-11-04T16:29:28","modified_gmt":"2026-11-04T16:29:28","slug":"thread-identification","status":"publish","type":"page","link":"https:\/\/dmfit.co.uk\/?page_id=101275","title":{"rendered":"Thread and tube size identification guide"},"content":{"rendered":"\t<section class=\"dmfit-threads\" aria-labelledby=\"dmfit-threads-title\">\n\t\t<header class=\"dmfit-threads__intro\">\n\t\t\t<p class=\"dmfit-threads__eyebrow\">DMfit Technical Support<\/p>\n\t\t\t<h2 id=\"dmfit-threads-title\">Identify tube sizes and threads<\/h2>\n\t\t\t<p>Use the tube outside diameter, thread diameter, thread pitch and thread form together. A single measurement rarely identifies a connection safely.<\/p>\n\t\t<\/header>\n\n\t\t<div class=\"dmfit-threads__callout\">\n\t\t\t<strong>Important: tube size and thread size are different.<\/strong>\n\t\t\t<span>A \u00bc\u2033 tube is 6.35 mm outside diameter. A \u00bc\u2033 pipe thread is a nominal pipe size and measures about 13\u201314 mm across the male thread.<\/span>\n\t\t<\/div>\n\n\t\t<section class=\"dmfit-threads__section\" aria-labelledby=\"dmfit-tube-title\">\n\t\t\t<h3 id=\"dmfit-tube-title\">1. Identify the tube size<\/h3>\n\t\t\t<p>Measure across the outside of the tube with callipers. Measure on a clean, round section, not over a cut end that has been crushed or burred. Tube size normally means its outside diameter (OD).<\/p>\n\t\t\t<div class=\"dmfit-threads__table-wrap\">\n\t\t\t\t<table>\n\t\t\t\t\t<caption>Common metric and imperial tube outside diameters<\/caption>\n\t\t\t\t\t<thead><tr><th scope=\"col\">Metric tube OD<\/th><th scope=\"col\">Nearest imperial size<\/th><th scope=\"col\">Exact imperial conversion<\/th><\/tr><\/thead>\n\t\t\t\t\t<tbody>\n\t\t\t\t\t\t<tr><td>4 mm<\/td><td>5\/32\u2033<\/td><td>3.97 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>6 mm<\/td><td>1\/4\u2033<\/td><td>6.35 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>8 mm<\/td><td>5\/16\u2033<\/td><td>7.94 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>10 mm<\/td><td>3\/8\u2033<\/td><td>9.53 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>12 mm<\/td><td>1\/2\u2033<\/td><td>12.70 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>15 mm<\/td><td>19\/32\u2033<\/td><td>15.08 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>16 mm<\/td><td>5\/8\u2033<\/td><td>15.88 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>18 mm<\/td><td>23\/32\u2033<\/td><td>18.26 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>22 mm<\/td><td>7\/8\u2033<\/td><td>22.23 mm<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>25 mm<\/td><td>1\u2033<\/td><td>25.40 mm<\/td><\/tr>\n\t\t\t\t\t<\/tbody>\n\t\t\t\t<\/table>\n\t\t\t<\/div>\n\t\t\t<p class=\"dmfit-threads__note\">Imperial conversions are exact inch-to-millimetre conversions, rounded to 2 decimal places. \u201cNearest imperial size\u201d is a comparison only: for example, 6 mm tube and \u00bc\u2033 tube are different sizes and are not interchangeable.<\/p>\n\t\t<\/section>\n\n\t\t<section class=\"dmfit-threads__section\" aria-labelledby=\"dmfit-thread-types-title\">\n\t\t\t<h3 id=\"dmfit-thread-types-title\">2. Recognise the thread type<\/h3>\n\t\t\t<div class=\"dmfit-threads__cards\">\n\t\t\t\t<article>\n\t\t\t\t\t<h4>BSPP \u2014 parallel<\/h4>\n\t\t\t\t\t<svg class=\"dmfit-threads__diagram\" viewBox=\"0 0 300 96\" role=\"img\" aria-labelledby=\"dmfit-bspp-diagram-title dmfit-bspp-diagram-desc\">\n\t\t\t\t\t\t<title id=\"dmfit-bspp-diagram-title\">BSPP parallel thread schematic<\/title><desc id=\"dmfit-bspp-diagram-desc\">The thread's outside edges stay parallel along its length.<\/desc>\n\t\t\t\t\t\t<rect class=\"dmfit-threads__diagram-bg\" x=\"1\" y=\"1\" width=\"298\" height=\"94\" rx=\"8\"\/><path class=\"dmfit-threads__axis\" d=\"M20 48H280\"\/><path class=\"dmfit-threads__profile\" d=\"M22 26H40l9 20 9-20h22l9 20 9-20h22l9 20 9-20h22l9 20 9-20h22l9 20 9-20h22l9 20 9-20h22 M22 70H40l9-20 9 20h22l9-20 9 20h22l9-20 9 20h22l9-20 9 20h22l9-20 9 20h22l9-20 9 20h22\"\/><path class=\"dmfit-threads__dimension\" d=\"M28 17H272M28 79H272\"\/><text class=\"dmfit-threads__diagram-label\" x=\"150\" y=\"91\" text-anchor=\"middle\">constant outside diameter \u00b7 55\u00b0 form<\/text>\n\t\t\t\t\t<\/svg>\n\t\t\t\t\t<p>Also called G thread. The thread sides stay parallel. It commonly seals with an O-ring, bonded washer or gasket at the face.<\/p>\n\t\t\t\t<\/article>\n\t\t\t\t<article>\n\t\t\t\t\t<h4>BSPT \u2014 tapered<\/h4>\n\t\t\t\t\t<svg class=\"dmfit-threads__diagram\" viewBox=\"0 0 300 96\" role=\"img\" aria-labelledby=\"dmfit-bspt-diagram-title dmfit-bspt-diagram-desc\">\n\t\t\t\t\t\t<title id=\"dmfit-bspt-diagram-title\">BSPT tapered thread schematic<\/title><desc id=\"dmfit-bspt-diagram-desc\">The thread becomes narrower towards its tip and uses a 55 degree BSP profile.<\/desc>\n\t\t\t\t\t\t<rect class=\"dmfit-threads__diagram-bg\" x=\"1\" y=\"1\" width=\"298\" height=\"94\" rx=\"8\"\/><path class=\"dmfit-threads__axis\" d=\"M20 48H280\"\/><path class=\"dmfit-threads__profile\" d=\"M22 18l28 2 9 18 9-17 22 2 9 17 9-16 22 2 9 16 9-15 22 2 9 15 9-14 22 2 9 14 9-13 22 2 9 13 9-12 22 2 M22 78l28-2 9-18 9 17 22-2 9-17 9 16 22-2 9-16 9 15 22-2 9-15 9 14 22-2 9-14 9 13 22-2 9-13 9 12 22-2\"\/><path class=\"dmfit-threads__dimension\" d=\"M28 12L272 35M28 84L272 61\"\/><text class=\"dmfit-threads__diagram-label\" x=\"150\" y=\"93\" text-anchor=\"middle\">tapered \u00b7 55\u00b0 form<\/text>\n\t\t\t\t\t<\/svg>\n\t\t\t\t\t<p>Also called R thread for an external taper. The thread narrows towards its end. BSP uses a 55\u00b0 thread form.<\/p>\n\t\t\t\t<\/article>\n\t\t\t\t<article>\n\t\t\t\t\t<h4>NPT \u2014 tapered<\/h4>\n\t\t\t\t\t<svg class=\"dmfit-threads__diagram\" viewBox=\"0 0 300 96\" role=\"img\" aria-labelledby=\"dmfit-npt-diagram-title dmfit-npt-diagram-desc\">\n\t\t\t\t\t\t<title id=\"dmfit-npt-diagram-title\">NPT tapered thread schematic<\/title><desc id=\"dmfit-npt-diagram-desc\">The thread becomes narrower towards its tip and uses a 60 degree profile.<\/desc>\n\t\t\t\t\t\t<rect class=\"dmfit-threads__diagram-bg\" x=\"1\" y=\"1\" width=\"298\" height=\"94\" rx=\"8\"\/><path class=\"dmfit-threads__axis\" d=\"M20 48H280\"\/><path class=\"dmfit-threads__profile\" d=\"M22 18l28 2 10 17 10-16 22 2 10 16 10-15 22 2 10 15 10-14 22 2 10 14 10-13 22 2 10 13 10-12 22 2 10 12 10-11 22 2 M22 78l28-2 10-17 10 16 22-2 10-16 10 15 22-2 10-15 10 14 22-2 10-14 10 13 22-2 10-13 10 12 22-2 10-12 10 11 22-2\"\/><path class=\"dmfit-threads__dimension\" d=\"M28 12L272 35M28 84L272 61\"\/><text class=\"dmfit-threads__diagram-label\" x=\"150\" y=\"93\" text-anchor=\"middle\">tapered \u00b7 60\u00b0 form<\/text>\n\t\t\t\t\t<\/svg>\n\t\t\t\t\t<p>Common on North American equipment. NPT uses a 60\u00b0 thread form and a different pitch\/profile from BSP; the same nominal size does not make them interchangeable.<\/p>\n\t\t\t\t<\/article>\n\t\t\t\t<article>\n\t\t\t\t\t<h4>Metric \u2014 usually parallel<\/h4>\n\t\t\t\t\t<svg class=\"dmfit-threads__diagram\" viewBox=\"0 0 300 96\" role=\"img\" aria-labelledby=\"dmfit-metric-diagram-title dmfit-metric-diagram-desc\">\n\t\t\t\t\t\t<title id=\"dmfit-metric-diagram-title\">Metric parallel thread schematic<\/title><desc id=\"dmfit-metric-diagram-desc\">The thread's outside edges stay parallel and its size is specified by diameter and pitch in millimetres.<\/desc>\n\t\t\t\t\t\t<rect class=\"dmfit-threads__diagram-bg\" x=\"1\" y=\"1\" width=\"298\" height=\"94\" rx=\"8\"\/><path class=\"dmfit-threads__axis\" d=\"M20 48H280\"\/><path class=\"dmfit-threads__profile\" d=\"M22 26H40l11 20 11-20h20l11 20 11-20h20l11 20 11-20h20l11 20 11-20h20l11 20 11-20h20l11 20 11-20h22 M22 70H40l11-20 11 20h20l11-20 11 20h20l11-20 11 20h20l11-20 11 20h20l11-20 11 20h20l11-20 11 20h22\"\/><path class=\"dmfit-threads__dimension\" d=\"M28 17H272M28 79H272\"\/><text class=\"dmfit-threads__diagram-label\" x=\"150\" y=\"91\" text-anchor=\"middle\">parallel \u00b7 size in mm \u00d7 pitch<\/text>\n\t\t\t\t\t<\/svg>\n\t\t\t\t\t<p>Marked with M, the thread\u2019s outside diameter in millimetres, and its pitch. For example, M10 \u00d7 1 has a nominal 10 mm diameter and 1 mm crest-to-crest pitch.<\/p>\n\t\t\t\t<\/article>\n\t\t\t<\/div>\n\t\t\t<p class=\"dmfit-threads__note\">The drawings are simplified side-profile schematics, not to scale. Use thread gauges and the fitting specification to confirm a match.<\/p>\n\t\t<\/section>\n\n\t\t<section class=\"dmfit-threads__section\" aria-labelledby=\"dmfit-measure-title\">\n\t\t\t<h3 id=\"dmfit-measure-title\">3. Measure male and female threads<\/h3>\n\t\t\t<div class=\"dmfit-threads__measure-grid\">\n\t\t\t\t<article>\n\t\t\t\t\t<h4>Male thread<\/h4>\n\t\t\t\t\t<ol>\n\t\t\t\t\t\t<li>Use callipers across the widest points of the thread crests to measure the outside diameter (OD).<\/li>\n\t\t\t\t\t\t<li>Check the OD near the first and last full thread. A clear change suggests a tapered thread; similar readings suggest a parallel thread.<\/li>\n\t\t\t\t\t\t<li>Use a thread-pitch gauge to match the crest spacing. Record threads per inch (TPI) for pipe threads or millimetres per pitch for metric.<\/li>\n\t\t\t\t\t\t<li>Compare the diameter, pitch and profile with a thread chart or known fitting.<\/li>\n\t\t\t\t\t<\/ol>\n\t\t\t\t<\/article>\n\t\t\t\t<article>\n\t\t\t\t\t<h4>Female thread<\/h4>\n\t\t\t\t\t<ol>\n\t\t\t\t\t\t<li>Measure the opening with the inside jaws of callipers. This is an approximate clue, not the female thread\u2019s full major diameter.<\/li>\n\t\t\t\t\t\t<li>Check pitch with a thread-pitch gauge, keeping the gauge square to the thread. Use a suitable thread plug gauge where available.<\/li>\n\t\t\t\t\t\t<li>Look for parallel or tapered sides and note any shoulder, seal face or O-ring seat.<\/li>\n\t\t\t\t\t\t<li>Confirm against the equipment maker\u2019s drawing or thread specification before ordering.<\/li>\n\t\t\t\t\t<\/ol>\n\t\t\t\t<\/article>\n\t\t\t<\/div>\n\t\t\t<p class=\"dmfit-threads__note\">On female threads, the calliper reading is smaller than the matching male thread OD, so do not identify the thread from the opening measurement alone.<\/p>\n\t\t<\/section>\n\n\t\t<section class=\"dmfit-threads__section\" aria-labelledby=\"dmfit-reference-title\">\n\t\t\t<h3 id=\"dmfit-reference-title\">4. Common pipe thread reference<\/h3>\n\t\t\t<p>These are approximate male major diameters for quick identification. Pipe sizes are nominal: the fraction in the thread name is not the measured thread diameter.<\/p>\n\t\t\t<div class=\"dmfit-threads__table-wrap\">\n\t\t\t\t<table>\n\t\t\t\t\t<caption>Common BSP and NPT male thread sizes<\/caption>\n\t\t\t\t\t<thead><tr><th scope=\"col\">Nominal size<\/th><th scope=\"col\">BSP male OD<\/th><th scope=\"col\">BSP pitch<\/th><th scope=\"col\">NPT male OD<\/th><th scope=\"col\">NPT pitch<\/th><\/tr><\/thead>\n\t\t\t\t\t<tbody>\n\t\t\t\t\t\t<tr><td>1\/8\u2033<\/td><td>9.73 mm<\/td><td>28 TPI<\/td><td>10.29 mm<\/td><td>27 TPI<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>1\/4\u2033<\/td><td>13.16 mm<\/td><td>19 TPI<\/td><td>13.72 mm<\/td><td>18 TPI<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>3\/8\u2033<\/td><td>16.66 mm<\/td><td>19 TPI<\/td><td>17.15 mm<\/td><td>18 TPI<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>1\/2\u2033<\/td><td>20.96 mm<\/td><td>14 TPI<\/td><td>21.34 mm<\/td><td>14 TPI<\/td><\/tr>\n\t\t\t\t\t\t<tr><td>3\/4\u2033<\/td><td>26.44 mm<\/td><td>14 TPI<\/td><td>26.67 mm<\/td><td>14 TPI<\/td><\/tr>\n\t\t\t\t\t<\/tbody>\n\t\t\t\t<\/table>\n\t\t\t<\/div>\n\t\t\t<p class=\"dmfit-threads__note\">BSP taper and parallel versions share the basic diameter and pitch for a given nominal size; the taper and sealing method differ. The values above are identification guides, not manufacturing tolerances.<\/p>\n\t\t<\/section>\n\n\t\t<aside class=\"dmfit-threads__warning\">\n\t\t\t<strong>Do not force a thread to fit.<\/strong> BSP and NPT can look similar and may start to engage, but have different thread angles and pitches. Stop if it binds, rocks or cross-threads. Verify the thread and sealing method from the component specification.\n\t\t<\/aside>\n\n\t\t<div class=\"dmfit-threads__sources\">\n\t\t\t<strong>Further reference<\/strong>\n\t\t\t<ul>\n\t\t\t\t<li><a href=\"https:\/\/www.parker.com\/content\/dam\/Parker-com\/Literature\/Instrumentation-Products-Division\/Installation-Instructions\/Parker_Tube_Fitting_Installation_Manual.pdf\" target=\"_blank\" rel=\"noopener\">Parker Tube Fitting Installation Manual \u2014 Thread Identification<\/a><\/li>\n\t\t\t\t<li><a href=\"https:\/\/www.parker.com\/content\/dam\/Parker-com\/Literature\/LPCE\/product-experience\/_common\/Fitting-Threads.pdf\" target=\"_blank\" rel=\"noopener\">Parker Fitting Threads \u2014 BSP, NPT and metric overview<\/a><\/li>\n\t\t\t\t<li><a href=\"https:\/\/safst.swagelok.com\/en\/resources\/weekly-column\/post-1-3\" target=\"_blank\" rel=\"noopener\">Swagelok \u2014 The Virtue of Thread Identification<\/a><\/li>\n\t\t\t<\/ul>\n\t\t\t<p>This guide is an initial aid only. Confirm the fitting thread, seal and application with the equipment manufacturer before assembly.<\/p>\n\t\t<\/div>\n\t<\/section>\n\t\n","protected":false},"excerpt":{"rendered":"","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_jetpack_hide_featured_image":false,"_jetpack_featured_image_duplicate_dismissed":0,"footnotes":""},"class_list":["post-101275","page","type-page","status-publish","hentry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.6 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>BSP, NPT &amp; Tube Size Identification Guide | DMfit UK<\/title>\n<meta name=\"description\" content=\"Identify BSPP, BSPT, NPT and metric threads. 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