{"id":1984,"date":"2026-08-29T18:05:32","date_gmt":"2026-08-30T01:05:32","guid":{"rendered":"https:\/\/www.songhaiflanges.com\/?p=1984"},"modified":"2026-08-29T18:05:32","modified_gmt":"2026-08-30T01:05:32","slug":"how-to-know-correct-flange-size-ansi-bolt-pattern-guide","status":"publish","type":"post","link":"https:\/\/www.songhaiflanges.com\/es\/how-to-know-correct-flange-size-ansi-bolt-pattern-guide\/","title":{"rendered":"How to Know If Flange Size Is Correct: ANSI Flange Bolt Pattern and Selection Guide"},"content":{"rendered":"

Knowing whether a flange size is correct is more than matching the pipe name on a purchase order. A flange must match the nominal pipe size, pressure class, flange type, bore, facing, gasket, and ANSI flange bolt pattern<\/strong>. If any of these details are wrong, the flange may not bolt up to the mating part, may not seal under pressure, or may fail project inspection.<\/p>\n

This guide explains how to know if flange size is correct<\/strong>, how to check ANSI standard flanges<\/strong>, what to measure on an existing flange, and how to avoid common sizing mistakes when buying ANSI flange<\/strong> products for industrial piping.<\/p>\n

What Does \u201cCorrect Flange Size\u201d Really Mean?<\/h2>\n

A correct flange size means the flange matches the complete piping requirement, not just one dimension. In industrial projects, flange size includes nominal pipe size, pressure class, standard, flange type, face type, bolt pattern, bore, material, and sometimes pipe schedule.<\/p>\n

For example, two flanges may both be called 4 inch ANSI flanges, but they can be completely different if one is Class 150 and the other is Class 300, or if one is a weld neck flange and the other is a blind flange. That is why buyers should ask how to know correct flange size<\/strong> by checking the full specification.<\/p>\n

ANSI Standard for Flanges: Confirm the Actual Standard<\/h2>\n

The phrase ANSI standard for flanges<\/strong> is still common in the market, but many current flange dimensions are controlled by ASME standards such as ASME B16.5 and ASME B16.47. In many purchase orders, \u201cANSI flange\u201d simply means an American standard flange made to ASME dimensions.<\/p>\n

Before ordering, confirm whether the project requires ASME B16.5, ASME B16.47 Series A, ASME B16.47 Series B, ASME B16.36, or another drawing-based standard. This matters because bolt holes, outside diameter, thickness, and pressure-temperature ratings can change by standard and class.<\/p>\n

Step 1: Check the Flange Marking<\/h2>\n

The fastest way to identify a flange is to read the stamped marking. A standard flange may show size, pressure class, material grade, heat number, standard, and manufacturer. Common markings include NPS, Class 150, Class 300, ASTM A105, ASTM A182 F316L, ASME B16.5, or similar information.<\/p>\n

If the marking is clear, compare it with the project specification or mating flange. If the marking is missing, painted over, damaged, or inconsistent, measure the flange carefully and check the dimensions against the correct standard.<\/p>\n

Step 2: Confirm Nominal Pipe Size and Bore<\/h2>\n

Nominal pipe size is not always the same as the measured inside diameter. NPS is a standard naming system, while actual pipe inside diameter depends on the pipe schedule. This is especially important for weld neck flanges because the bore should match the pipe bore for proper welding and flow transition.<\/p>\n

For slip on, socket weld, threaded, and lap joint flanges, bore and fit-up details also matter. A flange that bolts correctly may still be wrong if the pipe cannot fit, the bore is too large, or the welding preparation does not match the pipe schedule.<\/p>\n

Step 3: Verify the ANSI Flange Bolt Pattern<\/h2>\n

The ANSI flange bolt pattern<\/strong> is one of the most important checks. It includes the number of bolt holes, bolt hole diameter, bolt circle diameter, and angular spacing. A flange can have the correct nominal size but still fail installation if the bolt pattern does not match the mating flange, valve, pump, vessel nozzle, or equipment connection.<\/p>\n

To check the bolt pattern, count the holes first. Then measure hole diameter and bolt circle diameter. On flanges with an even number of holes, bolt circle can often be measured across opposite holes. For more complex layouts, use a drawing, template, or coordinate measurement method instead of guessing from one center-to-center distance.<\/p>\n

Step 4: Confirm Pressure Class<\/h2>\n

ANSI and ASME flanges use pressure classes such as Class 150, 300, 400, 600, 900, 1500, and 2500. Pressure class affects flange thickness, bolt size, bolt circle, gasket seating stress, and allowable pressure-temperature rating.<\/p>\n

Do not assume Class 150 means the flange is always limited to 150 psi, and do not assume a higher class will always fit a lower class mating flange. Pressure-temperature ratings depend on material and temperature. Bolt-up compatibility depends on standard dimensions.<\/p>\n

Step 5: Identify the Flange Type<\/h2>\n

Correct sizing also depends on flange type. Common ANSI or ASME-style flanges include weld neck, slip on, blind, socket weld, threaded, lap joint, and long weld neck flanges. Each type has different geometry and installation requirements.<\/p>\n

    \n
  • Weld neck flange:<\/strong> Check bore, hub dimensions, bevel, pipe schedule, and pressure class.<\/li>\n
  • Slip on flange:<\/strong> Check pipe outside diameter fit, face type, thickness, and bolt pattern.<\/li>\n
  • Blind flange:<\/strong> Check pressure class, thickness, bolt pattern, and face type.<\/li>\n
  • Socket weld flange:<\/strong> Check socket bore, socket depth, pipe schedule, and face type.<\/li>\n
  • Threaded flange:<\/strong> Check thread standard, nominal size, pressure class, and material.<\/li>\n
  • Lap joint flange:<\/strong> Check compatibility with the stub end, lap radius, bore, and backing flange dimensions.<\/li>\n<\/ul>\n

    ANSI Fixed Flanges vs Rotatable Flanges<\/h2>\n

    ANSI fixed flanges<\/strong> are flanges whose bolt holes are fixed after welding or installation. Weld neck flanges, slip on flanges, socket weld flanges, and threaded flanges usually fall into this group. Their bolt-hole orientation must be checked before final welding or assembly.<\/p>\n

    Rotatable flanges, such as lap joint backing flanges used with stub ends, can rotate before final bolt tightening. This makes field alignment easier, especially in retrofit projects, alloy piping, and skid-mounted equipment. If a project requires fixed flange orientation, do not substitute a rotatable design without engineering approval.<\/p>\n

    Step 6: Check the Facing Type and Gasket<\/h2>\n

    Flange size is incomplete without facing type. Raised face, flat face, ring type joint, tongue and groove, and special machined faces require different gaskets and sealing logic. A flange may have the correct bolt pattern but still be unsuitable if the face type does not match the mating component.<\/p>\n

    Raised face flanges usually use ring gaskets or spiral wound gaskets. Flat face flanges often use full-face gaskets, especially with cast iron or non-metallic equipment. RTJ flanges require ring joint gaskets and precise groove dimensions.<\/p>\n

    How to Measure an Existing Flange<\/h2>\n

    If you need to identify a flange in the field, measure and record the following:<\/p>\n

      \n
    • Outside diameter of the flange<\/li>\n
    • Inside bore or pipe outside diameter<\/li>\n
    • Flange thickness<\/li>\n
    • Raised face diameter and height, if applicable<\/li>\n
    • Number of bolt holes<\/li>\n
    • Bolt hole diameter<\/li>\n
    • Bolt circle diameter<\/li>\n
    • Hub diameter and hub length for weld neck flanges<\/li>\n
    • Face type and gasket seating surface condition<\/li>\n
    • Any stamped markings, material marks, or heat numbers<\/li>\n<\/ul>\n

      Photos with a tape measure or caliper can help a supplier identify the standard, but final purchasing should still be based on a confirmed drawing or standard table.<\/p>\n

      Common Reasons a Flange Size Is Wrong<\/h2>\n

      Flange sizing errors usually come from incomplete information. Common mistakes include ordering by pipe outside diameter only, ignoring pressure class, mixing ASME and EN standards, confusing Class 150 and PN16, selecting the wrong bore for weld neck flanges, or assuming all ANSI flange bolt patterns are the same.<\/p>\n

      Another common issue is replacing an old or custom flange with a standard flange without checking bolt circle and face dimensions. Older equipment, imported machinery, and pressure vessel nozzles may use non-standard patterns that need custom manufacturing.<\/p>\n

      How to Know Correct Flange Size Before Ordering<\/h2>\n

      Before sending an order, compare the flange against these questions:<\/p>\n

        \n
      • Does the flange standard match the piping specification?<\/li>\n
      • Does the NPS or DN match the pipe?<\/li>\n
      • Does the pressure class match the mating flange and design conditions?<\/li>\n
      • Does the bolt pattern match the valve, pump, nozzle, or opposite flange?<\/li>\n
      • Does the bore match the pipe schedule or drawing?<\/li>\n
      • Does the face type match the gasket and mating face?<\/li>\n
      • Does the material match pressure, temperature, and corrosion requirements?<\/li>\n
      • Does the flange type match installation method and inspection requirements?<\/li>\n<\/ul>\n

        RFQ Checklist for ANSI Standard Flanges<\/h2>\n

        For accurate quotation and production, provide complete flange specifications:<\/p>\n

          \n
        • Standard: ASME B16.5, ASME B16.47 Series A\/B, ASME B16.36, or custom drawing<\/li>\n
        • Size: NPS or DN, plus pipe schedule or bore requirement<\/li>\n
        • Pressure class: Class 150, 300, 400, 600, 900, 1500, or 2500<\/li>\n
        • Flange type: weld neck, slip on, blind, socket weld, threaded, lap joint, or custom flange<\/li>\n
        • Facing type: raised face, flat face, RTJ, tongue and groove, or special face<\/li>\n
        • Bolt pattern: number of holes, hole diameter, bolt circle diameter, and bolt size if required<\/li>\n
        • Material: carbon steel, stainless steel, duplex, alloy steel, or special alloy<\/li>\n
        • Inspection: MTC, PMI, UT, PT, MT, RT, hardness, hydro test, or third-party inspection<\/li>\n
        • Quantity, marking, coating, packaging, delivery schedule, and destination<\/li>\n<\/ul>\n

          Why Supplier Communication Matters<\/h2>\n

          A good pipe flange manufacturer will not rely only on a short product name when the application is critical. If the inquiry says only \u201cANSI flange,\u201d the supplier should ask for standard, size, class, material, face, type, and quantity. If the flange is custom or for replacement, drawings and measurements are essential.<\/p>\n

          Clear communication prevents wrong bolt patterns, wrong bore sizes, mismatched face types, and costly site delays. It also helps the supplier recommend whether a standard flange is enough or whether a custom forged flange is required.<\/p>\n

          Conclusion<\/h2>\n

          How to know if flange size is correct<\/strong> comes down to checking the complete specification: standard, NPS, pressure class, flange type, bore, facing, material, and ANSI flange bolt pattern<\/strong>. A flange that matches only one dimension may still be wrong for the piping system.<\/p>\n

          Songhai Flanges supplies ANSI standard flanges, weld neck flanges, slip on flanges, blind flanges, socket weld flanges, threaded flanges, lap joint flanges, stainless steel flanges, large diameter flanges, and custom forged flanges. If you need help confirming the correct ANSI flange size, sharing drawings, markings, dimensions, and service conditions will help the engineering team recommend the right flange.<\/p>","protected":false},"excerpt":{"rendered":"

          Learn how to know if flange size is correct by checking ANSI flange bolt pattern, nominal pipe size, pressure class, flange type, facing, bore, and ANSI standard flange specifications.<\/p>","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_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":[230,253,214,268],"tags":[1019,1011,945,252,1009,944,1010,946,957,948,972,765,996,634,1012,106,186,975,976,977,367,458,459,1020,1017,995,609,155,463,1018,292,462,648,993,633,1013,1015,963,1016,188,956,1014,1008,1007,961,464,476,254,683,962,475,1021,352,479,461,465,955,478,232],"class_list":["post-1984","post","type-post","status-publish","format-standard","hentry","category-product-guide","category-product-knowledge","category-technical-guide","category-technical-reference","tag-ansi-asme-flange","tag-ansi-bolt-pattern","tag-ansi-fixed-flanges","tag-ansi-flange","tag-ansi-flange-bolt-pattern","tag-ansi-flange-dimensions","tag-ansi-flange-pattern","tag-ansi-pipe-flange-dimensions","tag-ansi-standard-flanges","tag-ansi-standard-for-flanges","tag-asme-b16-47-flange","tag-asme-b16-5-flange","tag-asme-class","tag-asme-flange","tag-asme-flange-bolt-pattern","tag-blind-flange","tag-bolt-circle-diameter","tag-class-150-flange","tag-class-300-flange","tag-class-600-flange","tag-correct-flange-size","tag-custom-flange-manufacturer","tag-custom-forged-flanges","tag-fixed-flange","tag-flange-bolt-circle","tag-flange-bolt-holes","tag-flange-bolt-pattern","tag-flange-dimensions","tag-flange-factory-china","tag-flange-hole-diameter","tag-flange-inspection","tag-flange-manufacturer-china","tag-flange-material-selection","tag-flange-outside-diameter","tag-flange-pressure-class","tag-flange-size-check","tag-flange-size-guide","tag-flange-size-standard","tag-flange-sizing-guide","tag-flange-specifications","tag-how-to-determine-flange-size","tag-how-to-find-flange-size","tag-how-to-know-correct-flange-size","tag-how-to-know-if-flange-size-is-correct","tag-how-to-know-your-flange-size","tag-industrial-flange-manufacturer","tag-industrial-flanges-supplier","tag-lap-joint-flange","tag-pipe-flange-connection","tag-pipe-flange-dimensions","tag-pipe-flange-manufacturer","tag-rotatable-flange","tag-slip-on-flange-2","tag-socket-weld-flange","tag-stainless-steel-flange-manufacturer","tag-stainless-steel-flange-supplier","tag-standard-flange-sizes","tag-threaded-flange","tag-weld-neck-flange"],"_links":{"self":[{"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/posts\/1984","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/comments?post=1984"}],"version-history":[{"count":1,"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/posts\/1984\/revisions"}],"predecessor-version":[{"id":1985,"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/posts\/1984\/revisions\/1985"}],"wp:attachment":[{"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/media?parent=1984"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/categories?post=1984"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.songhaiflanges.com\/es\/wp-json\/wp\/v2\/tags?post=1984"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}