{"id":51717,"date":"2018-06-12T14:30:00","date_gmt":"2018-06-12T14:30:00","guid":{"rendered":"https:\/\/discover.restek.com\/uncategorized\/how-to-determine-the-size-of-14-and-18-national-pipe-thread-tapered-npt-fittings\/"},"modified":"2026-01-28T15:54:24","modified_gmt":"2026-01-28T15:54:24","slug":"how-to-determine-the-size-of-14-and-18-national-pipe-thread-tapered-npt-fittings","status":"publish","type":"post","link":"https:\/\/discover.restek.com\/es\/blogs\/gnbl4709\/how-to-determine-the-size-of-14-and-18-national-pipe-thread-tapered-npt-fittings","title":{"rendered":"How to Determine the Size of 1\/4\u201c and 1\/8\u201c National Pipe Thread tapered (NPT) Fittings"},"content":{"rendered":"\n

Restek\u2019s Technical Service team gets quite a few questions from customers about fittings. There is a bewildering assortment of fitting types and sizes\u00a0on the Restek web site. A blog post titled, \u201cI need a fitting, but which one?<\/a>\u201d is where we often direct customers for help with these questions. One of the things discussed in that blog post is\u00a0National Pipe Thread tapered (NPT)<\/strong>\u00a0fittings as one of the main types used, but many people have difficulty identifying the correct size NPT fitting for their needs. The confusion comes from the fact that the outside diameter (OD) of an NPT fitting does not match the \u201cname\u201d of the fitting. For an example let\u2019s look at Restek catalog #\u00a023187<\/a>\u00a0shown below, which is a\u00a01\/4″ to 1\/8″ NPT Male Connector<\/strong>.<\/p>\n\n\n

\n
\"\"<\/figure>\n<\/div>\n\n\n

The 1\/4″ designation in the name refers to the compression fitting side of the fitting (on the left in the picture above) which has a nut and ferrules to accept and connect to 1\/4″ OD tubing. Makes sense, right? However, the threads on the right side of the fitting in the picture above are called 1\/8″ Male NPT, but if you measure the OD of the 1\/8\u201d NPT side you will find it is about 0.4″ in diameter\u2026certainly not very close to 1\/8″ (0.125\u201d). As a general \u201crule of thumb\u201d an NPT thread is approximately 1\/4″ (0.25\u201d) larger than its \u201cname.\u201d For a 1\/4″ NPT fitting the \u201cnominal\u201d OD is 0.533\u201d.<\/p>\n\n\n\n

NPT fittings are slightly tapered so the \u201cnominal\u201d diameter is the diameter in the middle of the threaded portion, as measured by the top (crest) of the threads. This is a bit confusing, but NPT threads are made to the ANSI B1.20.1, SAE AS71051 standard and anything complying with a standard with a name like that is bound to be bewildering. Hopefully the image below will help.<\/p>\n\n\n

\n
\"decorative\"<\/figure>\n<\/div>\n\n\n

The charts below are from the Swagelok Thread and End Connection Identification Guide with the first chart showing dimensions for male NPT threads and the second chart containing the dimensions for female NPT threads.<\/p>\n\n\n

\n
\"decorative\"<\/figure>\n<\/div>\n\n
\n
\"decorative\"<\/figure>\n<\/div>\n\n\n

Finally, you should always use PTFE tape when making a connection with an NPT fitting.<\/p>\n\n\n\n

Thanks for reading!<\/p>\n\n\n\n

<\/div>\n\n\n
\n

Products Mentioned<\/h3>\n
\n
\n
\n
\n Catalog No. 23187 <\/a> <\/div>\n
\n
Swagelok Fitting, 1\/4" to 1\/8" NPT Male Connector, Stainless Steel, 2-pk.<\/div>\n <\/div>\n
\n View Product<\/a>\n <\/div>\n <\/div>\n
\n
\n Catalog No. 22487 <\/a> <\/div>\n
\n
ResTape, Gray PTFE, 1\/2" x 260", Thread Tape for Stainless Steel<\/div>\n <\/div>\n
\n View Product<\/a>\n <\/div>\n <\/div>\n
\n
\n Catalog No. 22485 <\/a> <\/div>\n
\n
ResTape, Green PTFE, 1\/2" x 260", Thread Tape for Oxygen Service<\/div>\n <\/div>\n
\n View Product<\/a>\n <\/div>\n <\/div>\n
\n
\n Catalog No. 22486 <\/a> <\/div>\n
\n
ResTape, Yellow PTFE, 1\/2" x 260", Thread Tape for Gas Service<\/div>\n <\/div>\n
\n View Product<\/a>\n <\/div>\n <\/div>\n <\/div>\n <\/div>\n ","protected":false},"excerpt":{"rendered":"

Restek’s Technical Service team gets quite a few questions from customers about fittings. There is a bewildering assortment of fitting types and sizes as you can see here on the Restek web site. A blog post titled, “I need a…<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_kad_blocks_custom_css":"","_kad_blocks_head_custom_js":"","_kad_blocks_body_custom_js":"","_kad_blocks_footer_custom_js":"","_kadence_starter_templates_imported_post":false,"_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false,"footnotes":""},"categories":[9],"tags":[],"industries-application":[],"post-badge":[],"resource-type":[],"product-library":[2385,2372],"resource-technique":[2294,2299],"ppma_author":[605],"class_list":["post-51717","post","type-post","status-publish","format-standard","hentry","category-blogs","product-library-ferrules-fittings-connectors","product-library-gas-chromatography-products","resource-technique-gas-chromatography-gc","resource-technique-liquid-chromatography"],"acf":[],"taxonomy_info":{"category":[{"value":9,"label":"Blogs"}],"product-library":[{"value":2385,"label":"Ferrules, Fittings, Connectors"},{"value":2372,"label":"Gas Chromatography Products"}],"resource-technique":[{"value":2294,"label":"Gas Chromatography (GC)"},{"value":2299,"label":"Liquid Chromatography"}]},"featured_image_src_large":false,"author_info":{"display_name":"Brenda Cogan","author_link":"https:\/\/discover.restek.com\/es\/author\/brenda-coganrestek-com\/"},"comment_info":0,"category_info":[{"term_id":9,"name":"Blogs","slug":"blogs","term_group":0,"term_taxonomy_id":9,"taxonomy":"category","description":"","parent":0,"count":442,"filter":"raw","cat_ID":9,"category_count":442,"category_description":"","cat_name":"Blogs","category_nicename":"blogs","category_parent":0}],"tag_info":false,"authors":[{"term_id":605,"user_id":0,"is_guest":1,"slug":"robert-wiedemer","display_name":"Robert Wiedemer","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/?s=96&d=mm&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/posts\/51717","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/comments?post=51717"}],"version-history":[{"count":4,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/posts\/51717\/revisions"}],"predecessor-version":[{"id":85344,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/posts\/51717\/revisions\/85344"}],"wp:attachment":[{"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/media?parent=51717"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/categories?post=51717"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/tags?post=51717"},{"taxonomy":"industries-application","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/industries-application?post=51717"},{"taxonomy":"post-badge","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/post-badge?post=51717"},{"taxonomy":"resource-type","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/resource-type?post=51717"},{"taxonomy":"product-library","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/product-library?post=51717"},{"taxonomy":"resource-technique","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/resource-technique?post=51717"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/discover.restek.com\/es\/wp-json\/wp\/v2\/ppma_author?post=51717"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}