{"id":1770,"date":"2026-08-26T06:44:32","date_gmt":"2026-08-26T06:44:32","guid":{"rendered":"https:\/\/www.thepipingmart.com\/blog\/?p=1770"},"modified":"2026-08-26T06:51:25","modified_gmt":"2026-08-26T06:51:25","slug":"what-are-the-different-pipe-size-notations","status":"publish","type":"post","link":"https:\/\/www.thepipingmart.com\/blog\/what-are-the-different-pipe-size-notations\/","title":{"rendered":"What Are The Different Pipe Size Notations?"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">Pipe size notation trips up more procurement teams than any other line on a purchase order. A single fitting can carry three different labels at once: NPS 4, DN 100, or NB 100, and none of these numbers match the pipe&#8217;s actual outside or inside diameter. That&#8217;s because Nominal Pipe Size (NPS), Nominal Diameter (DN), and Nominal Bore (NB) are dimensionless reference values, not physical measurements. Engineers working across ASME- and ISO-based projects need to read pipe size notations correctly before ordering flanges, valves, or fittings. Get the conversion wrong, and the parts won&#8217;t line up at the flange face. This page breaks down each system and lays out the differences between NPS, DN, and NB.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Introduction to Pipe Size Notations<\/strong><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Pipe sizing didn&#8217;t begin with one clean system. Early piping standards grew out of iron pipe manufacturing in the 1800s, where wall thickness and bore varied by manufacturer and by region. NPS shaped North America as a way to order pipe without listing the exact OD and ID for every schedule combination. DN was later standardised under ISO 6708, providing European and international projects with a metric-based pipe size designation. NB developed separately across the UK and former Commonwealth countries, often used loosely to mean either of the other two, depending on the plant. All three notations still exist because no single global standard ever replaced the others.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Nominal Pipe Size (NPS)<\/strong><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">NPS stands for &#8216;Nominal Pipe Size&#8217; and governs piping built to ASME B36.10M for carbon steel and ASME B36.19M for stainless steel. The number itself, \u00bd, 2, or 12, does not equal the outside diameter for most sizes. Below NPS 14, the actual OD runs larger than the number suggests. NPS 2 carries an OD of 2.375 inches, not 2.000 inches. At NPS 14 and above, the OD in inches matches the NPS value. Wall thickness gets set separately, through schedule numbers such as Schedule 40 or Schedule 80, referenced in our pipe schedule guide.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Nominal Diameter (DN)<\/strong><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">DN, short for nominal diameter (DN), is the metric counterpart defined under ISO 6708 and used across EN piping standards. DN 15 lines up roughly with NPS \u00bd, and DN 100 with NPS 4. The number is loosely tied to millimetres, but it isn&#8217;t an exact bore measurement either. Two pipes, both labelled DN 100, can carry different actual OD values depending on whether one follows ASME dimensions and the other follows EN dimensions. Specifying DN alone, without naming the governing standard, leaves room for a mismatch once fittings reach the site. Our flange dimension standards page covers this topic in more detail.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Nominal Bore (NB)<\/strong><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">NB, or nominal bore, shows up mostly in Indian, UK, and Commonwealth piping documentation. It isn&#8217;t a separate dimensional standard the way NPS and DN are. Instead, NB borrows its numbers from whichever system the region defaults to, inches in older UK practice and millimetres in most current Indian projects. NB \u00bd usually lines up with NPS \u00bd. NB 50 usually lines up with DN 50. Because NB usage varies by region and project convention, the referenced standard matters more than the NB designation alone.&nbsp;<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Difference Between NPS, DN, and NB<\/strong><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The table below sets the differences between NPS, DN, and NB side by side, covering definition, units, governing standards, and how each figure relates to the pipe&#8217;s real dimensions.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong>Aspect<\/strong><\/td><td><strong>NPS (Nominal Pipe Size)<\/strong><\/td><td><strong>DN (Diameter Nominal)<\/strong><\/td><td><strong>NB (Nominal Bore)<\/strong><\/td><\/tr><tr><td>Definition<\/td><td>Inch-based, dimensionless designation used in North American practice<\/td><td>Metric, dimensionless designation used in ISO and EN practice<\/td><td>Term used across Europe and Commonwealth countries, often interchangeable with NPS or DN<\/td><\/tr><tr><td>Units<\/td><td>Expressed in inches, such as NPS \u00bd, NPS 2, NPS 12<\/td><td>Expressed as a whole number loosely tied to millimetres, such as DN 15, DN 50, DN 300<\/td><td>Stated in inches or millimetres depending on region and project convention<\/td><\/tr><tr><td>Standard system<\/td><td>ASME B36.10M and ASME B36.19M, inch-based<\/td><td>ISO 6708 and EN standards, metric-based<\/td><td>Not a separate standard. NB is a naming convention rather than a unique dimension system<\/td><\/tr><tr><td>Relation to OD<\/td><td>For NPS \u215b to 12, OD does not equal NPS. For NPS 14 and above, OD in inches roughly equals NPS<\/td><td>DN is a label. Actual OD depends on whether ASME or EN dimensions apply<\/td><td>Often treated as equivalent to NPS in inch systems and to DN in metric systems<\/td><\/tr><tr><td>Typical usage<\/td><td>North America, oil and gas, petrochemical, ASME-based piping<\/td><td>Europe, international projects, ISO and EN-based piping and fittings<\/td><td>India, the UK, and some Commonwealth countries, often meaning NPS or DN in practice<\/td><\/tr><tr><td>Example equivalence<\/td><td>NPS \u00bd \u2248 DN 15, NPS 2 \u2248 DN 50, NPS 4 \u2248 DN 100<\/td><td>DN 15 \u2248 NPS \u00bd, DN 50 \u2248 NPS 2, DN 100 \u2248 NPS 4<\/td><td>NB \u00bd often means NPS \u00bd, NB 50 often means DN 50, depending on context<\/td><\/tr><tr><td>Important note<\/td><td>NPS is a name, not a direct measurement of ID or OD<\/td><td>DN is a name, not a direct measurement of ID or OD<\/td><td>NB is a name. Do not assume it equals the actual bore without checking the standard<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>How Pipe Size Notations Are Used in Industry<\/strong><\/h2>\n\n\n\n<figure class=\"wp-block-image size-full\"><img fetchpriority=\"high\" decoding=\"async\" width=\"1536\" height=\"1024\" src=\"http:\/\/www.thepipingmart.com\/blog\/wp-content\/uploads\/2026\/08\/how-pipe-size-notations-are-used-in-industry.png\" alt=\"How Pipe Size Notations Are Used in Industry\" class=\"wp-image-1783\"\/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Pipe size notations shape far more than paperwork. Ten areas of daily plant operation depend on reading them correctly.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Standardised Identification and Communication<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">A single pipe size notation lets suppliers and engineers in different markets specify the same pipe size using a consistent reference. For example, suppliers and engineers in cities such as Houston and Mumbai can use the same NPS or DN designation when ordering the right pipe size. Purchase orders, drawings, and datasheets all refer to NPS or DN, so nothing is misread across departments or borders.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Ensuring Compatibility Across Components<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Valves, flanges, and fittings only bolt together correctly when the notation matches the governing standard. A DN 100 flange built to an EN dimensional standard may not match an ASME NPS 4 flange in terms of dimensions, so the governing flange standard must be checked before ordering.&nbsp;<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Pressure Containment Selection via Schedules<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Nominal size and schedule work together to set the wall thickness and pressure rating. Two pipes sharing the same NPS number can carry very different pressure ratings once Schedule 40 is swapped for Schedule 160.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Hydraulic and Flow Rate Calculations<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Flow calculations need the actual internal diameter, not the nominal label. Engineers pull the true ID from schedule tables once the nominal size confirms which pipe family is being used.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Material and Weight Optimisation<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Correct notation keeps the ordering tied to the correct wall thickness. Over-specifying a schedule adds unnecessary steel weight and cost, while under-specifying risks failure under working pressure.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Global Interoperability (NPS vs. DN)<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Multinational projects routinely mix ASME and ISO documentation. Procurement teams cross-reference NPS against DN early in the design phase to avoid ordering incompatible components later in construction.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Pipe Selection and Specification<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Design engineers select pipe class using nominal size as the starting reference point, then layer schedule, material grade, and end connection type on top of that base figure.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Fabrication and Installation Reference<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Fabricators cut, bevel, and weld pipe according to nominal size callouts on isometric drawings. A wrong notation on a drawing carries straight through to a wrong cut in the shop.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Inspection and Quality Control<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Inspectors verify actual OD and wall thickness against the specified nominal size, schedule, and applicable dimensional tolerances. Any deviation beyond tolerance is flagged before the spool ships to the site.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Operation, Maintenance, and Replacement<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Maintenance teams pull replacement parts using the nominal size stamped on the original component. Matching NPS, DN, or NB correctly during a shutdown avoids delays waiting on the wrong part.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Conclusion<\/strong><\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">NPS, DN, and NB all describe the same underlying idea: a standardised way to reference pipe size without listing exact dimensions on every document. None of them measures the actual bore or outside diameter directly. Reading them correctly and cross-checking against the governing standard matters at every stage from design through fabrication and maintenance. Projects mixing ASME and ISO sourcing need this cross-referencing built into procurement from day one, not fixed after fittings arrive on site.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>FAQs<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What is the difference between NPS, DN, and NB in pipe sizing?<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">NPS is the inch-based designation used under ASME standards; DN is the metric designation used under ISO and EN standards; and NB is a regional term, common in India and the UK, that often means either NPS or DN depending on context.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Why are different pipe size standards (NPS, DN, NB) used in industry?<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Different regions developed separate pipe sizing conventions as piping standards evolved around imperial and metric systems. North America primarily uses NPS under ASME standards, while DN is used extensively with ISO and EN standards. NB remains common in some Indian, UK, and Commonwealth documentation.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>How do you convert NPS to DN in pipe sizing?<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Conversion ranges through standard equivalence tables rather than a simple formula, since DN is not a direct millimetre conversion of the NPS inch value. NPS \u00bd converts to DN 15, NPS 2 converts to DN 50, and NPS 4 converts to DN 100.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>How do you denote pipe size?<\/strong><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Pipe size is denoted using the nominal designation followed by the schedule, such as NPS 4 Schedule 40 or DN 100 SCH 40, with the governing standard, ASME or ISO, referenced on the drawing or datasheet to remove any ambiguity.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Pipe size notation trips up more procurement teams than any other line on a purchase order. A single fitting can carry three different labels at once: NPS 4, DN 100, or NB 100, and none of these numbers match the pipe&#8217;s actual outside or inside diameter. That&#8217;s because Nominal Pipe Size (NPS), Nominal Diameter (DN), [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":1792,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[199],"tags":[],"class_list":["post-1770","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-pipe"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>What Are the Different Pipe Size Notations? NPS, DN &amp; NB Explained<\/title>\n<meta name=\"description\" content=\"Pipe size labels can be confusing. 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Learn what NPS, DN, and NB mean, how they relate to actual dimensions, and why the applicable standard matters.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.thepipingmart.com\/blog\/what-are-the-different-pipe-size-notations\/\" \/>\n<meta property=\"og:site_name\" content=\"PipingMart Blog\" \/>\n<meta property=\"article:published_time\" content=\"2026-08-26T06:44:32+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-08-26T06:51:25+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.thepipingmart.com\/blog\/wp-content\/uploads\/2026\/08\/pipe-size-notations.png\" \/>\n\t<meta property=\"og:image:width\" content=\"1499\" \/>\n\t<meta property=\"og:image:height\" content=\"1049\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"author\" content=\"Manish Doshi\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Manish Doshi\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"7 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/what-are-the-different-pipe-size-notations\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/what-are-the-different-pipe-size-notations\\\/\"},\"author\":{\"name\":\"Manish Doshi\",\"@id\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/#\\\/schema\\\/person\\\/842698a31d33e197cb71f77e4cca9071\"},\"headline\":\"What Are The Different Pipe Size Notations?\",\"datePublished\":\"2026-08-26T06:44:32+00:00\",\"dateModified\":\"2026-08-26T06:51:25+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/what-are-the-different-pipe-size-notations\\\/\"},\"wordCount\":1500,\"publisher\":{\"@id\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/#organization\"},\"image\":{\"@id\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/what-are-the-different-pipe-size-notations\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/wp-content\\\/uploads\\\/2026\\\/08\\\/pipe-size-notations.png\",\"articleSection\":[\"Pipe\"],\"inLanguage\":\"en-US\"},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/what-are-the-different-pipe-size-notations\\\/\",\"url\":\"https:\\\/\\\/www.thepipingmart.com\\\/blog\\\/what-are-the-different-pipe-size-notations\\\/\",\"name\":\"What Are the Different Pipe Size Notations? 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