{
 "version": "2026-10-20",
 "license": "Dimension sheets are written by Xem Studio from the cited sources and are free to use (CC BY 4.0, attribute 'Xem Studio'). Linked files belong to their publishers under the license shown; Xem Studio does not host them. 'license_verified: false' means the license shown is from memory or the publisher's page as last read and should be confirmed at the link before commercial use.",
 "slug": "bicycle-seatpost-diameters",
 "name": "Bicycle seatpost diameters: 25.4, 26.8, 27.2, 28.6, 30.9, 31.6 and 34.9 mm",
 "kind": "standard",
 "category": "Everyday mounts",
 "group": "Bicycle",
 "year": null,
 "question": "What diameter is a bike seatpost?",
 "tags": [
  "bicycle",
  "seatpost",
  "seat post",
  "bike",
  "clamp",
  "mount"
 ],
 "aliases": [
  "seatpost diameter",
  "seat post size",
  "27.2 seatpost",
  "30.9 seatpost",
  "31.6 seatpost",
  "34.9 dropper",
  "seatpost mount diameter",
  "seatpost step"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": false,
 "free_files": 7,
 "url": "https://xemstudio.com/objects/bicycle-seatpost-diameters/",
 "sheet_txt": "https://xemstudio.com/objects/bicycle-seatpost-diameters/sheet.txt",
 "sheet": {
  "basis": "Sizes from Sheldon Brown's seatpost list (27.2 the de facto size for 1 1/8 in seat tubes) and BikeRadar's dropper-post guide (30.9 and 31.6 most common, 34.9 and 27.2 also made)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "A seatpost's diameter is set by the frame's seat tube; there are dozens of sizes in 0.2 mm steps, and the right one is usually stamped on the post near the minimum-insertion mark.",
   "Most common today: 27.2 (road, gravel, older mountain bikes), 30.9 and 31.6 (most dropper posts), and 34.9 (large droppers). 25.4, 26.8 and 28.6 cover many older frames.",
   "Files: each size as a Ø × 100 mm section, axis on X, plus a DXF of the circle: the outside of the post only (walls differ and are not drawn). A saddle-bag, light or bottle mount that clamps the exposed post uses this Ø.",
   "Seat-tube clamps (the collar on the frame) are sized by the frame's outside diameter, not the post: that is a different number."
  ],
  "table": {
   "columns": [
    "post Ø (mm)",
    "where it is used",
    "from"
   ],
   "rows": [
    [
     "Ø25.4",
     "1 in posts on older and lower-cost frames",
     "Sheldon Brown"
    ],
    [
     "Ø26.8",
     "common on older frames",
     "Sheldon Brown"
    ],
    [
     "Ø27.2",
     "the de facto size for frames with a 1 1/8 in seat tube; still the usual road and gravel post",
     "Sheldon Brown, BikeRadar"
    ],
    [
     "Ø28.6",
     "1 1/8 in",
     "Sheldon Brown"
    ],
    [
     "Ø30.9",
     "one of the two most common dropper-post sizes",
     "BikeRadar"
    ],
    [
     "Ø31.6",
     "one of the two most common dropper-post sizes; common on mountain bikes",
     "BikeRadar, Sheldon Brown"
    ],
    [
     "Ø34.9",
     "large dropper posts",
     "BikeRadar"
    ]
   ]
  }
 },
 "sheet_text": "Bicycle seatpost diameters: 25.4, 26.8, 27.2, 28.6, 30.9, 31.6 and 34.9 mm — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/bicycle-seatpost-diameters/  (CC BY 4.0)\nBasis: Sizes from Sheldon Brown's seatpost list (27.2 the de facto size for 1 1/8 in seat tubes) and BikeRadar's dropper-post guide (30.9 and 31.6 most common, 34.9 and 27.2 also made)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- A seatpost's diameter is set by the frame's seat tube; there are dozens of sizes in 0.2 mm steps, and the right one is usually stamped on the post near the minimum-insertion mark.\n- Most common today: 27.2 (road, gravel, older mountain bikes), 30.9 and 31.6 (most dropper posts), and 34.9 (large droppers). 25.4, 26.8 and 28.6 cover many older frames.\n- Files: each size as a Ø × 100 mm section, axis on X, plus a DXF of the circle: the outside of the post only (walls differ and are not drawn). A saddle-bag, light or bottle mount that clamps the exposed post uses this Ø.\n- Seat-tube clamps (the collar on the frame) are sized by the frame's outside diameter, not the post: that is a different number.\n\npost Ø (mm)  where it is used  from\nØ25.4  1 in posts on older and lower-cost frames  Sheldon Brown\nØ26.8  common on older frames  Sheldon Brown\nØ27.2  the de facto size for frames with a 1 1/8 in seat tube; still the usual road and gravel post  Sheldon Brown, BikeRadar\nØ28.6  1 1/8 in  Sheldon Brown\nØ30.9  one of the two most common dropper-post sizes  BikeRadar\nØ31.6  one of the two most common dropper-post sizes; common on mountain bikes  BikeRadar, Sheldon Brown\nØ34.9  large dropper posts  BikeRadar\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Sheldon Brown (Harris Cyclery): Seatpost sizes by make and model (27.2 the de facto size for 1 1/8 in seat tubes) — web — reference [verify license at link] — https://www.sheldonbrown.com/seatpost-sizes.html\n- BikeRadar: The anatomy of a dropper post: 30.9 and 31.6 most common, 34.9 and 27.2 also made — web — reference [verify license at link] — https://www.bikeradar.com/us/mtb/gear/article/dropper-post-explained-49097/\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 7 parts): https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/index.json\nAPI: https://xemstudio.com/api/geometry?object=bicycle-seatpost-diameters\n",
 "sources": [
  {
   "publisher": "Sheldon Brown (Harris Cyclery)",
   "title": "Seatpost sizes by make and model (27.2 the de facto size for 1 1/8 in seat tubes)",
   "url": "https://www.sheldonbrown.com/seatpost-sizes.html",
   "formats": [
    "web"
   ],
   "license": "reference",
   "license_verified": false,
   "hosted": false
  },
  {
   "publisher": "BikeRadar",
   "title": "The anatomy of a dropper post: 30.9 and 31.6 most common, 34.9 and 27.2 also made",
   "url": "https://www.bikeradar.com/us/mtb/gear/article/dropper-post-explained-49097/",
   "formats": [
    "web"
   ],
   "license": "reference",
   "license_verified": false,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "bicycle-handlebar-diameters",
  "handlebar-clamp-rings",
  "bicycle-steerer-1-1-8",
  "bicycle-bottle-cage-bosses"
 ],
 "fits": [
  {
   "role": "Parts that fit it",
   "items": [
    {
     "slug": "specialized-power-saddle",
     "why": "on the seatpost clamp"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "bicycle-handlebar-diameters",
     "why": "bar sections for mounts on the handlebar"
    }
   ]
  },
  {
   "role": "Also needed",
   "items": [
    {
     "slug": "socket-head-cap-screws-metric",
     "why": "M4 / M5 screws for a clamp on the post"
    }
   ]
  }
 ],
 "dimensions": "exact (nominal, no printing tolerance)",
 "dimension_rule": "All geometry on Xem Studio is exact (nominal, true size) with no printing tolerance. 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.",
 "mating": [
  {
   "feature": "post outside",
   "exact": "Ø25.4 / 26.8 / 27.2 / 28.6 / 30.9 / 31.6 / 34.9",
   "mates": "frame seat tube of the same Ø; mount clamps on the exposed post",
   "fit": "slide",
   "files": "seatpost-"
  }
 ],
 "fit_classes": {
  "press": "stays put: pin, tube, bearing or magnet held by interference. Typical FDM: hole +0.0 to +0.1 mm on diameter.",
  "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. Typical FDM: +0.2 mm.",
  "run": "turns or slides in use: pin in a link, rod through a guide. Typical FDM: +0.3 mm.",
  "socket": "large sliding fit: syringe barrel, sleeve, bobbin in a bore. Typical FDM: +0.3 mm.",
  "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 diameter is a bike seatpost?",
   "a": "Most often 27.2 mm on road and gravel bikes and 30.9 or 31.6 mm on mountain bikes with dropper posts; 34.9 mm on some large droppers. The size is marked on the post."
  },
  {
   "q": "What are the most common dropper post sizes?",
   "a": "30.9 and 31.6 mm; 34.9 and 27.2 mm droppers are also made."
  },
  {
   "q": "Is a 31.6 post the same as 31.8?",
   "a": "No. 31.6 is a seatpost size; 31.8 is a handlebar clamp size. Do not mix them."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "seatpost-25.4-L100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-25.4-L100.step",
   "stl": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-25.4-L100.stl",
   "note": "Seatpost section Ø25.4 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø25.4 circle"
  },
  {
   "file": "seatpost-26.8-L100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-26.8-L100.step",
   "stl": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-26.8-L100.stl",
   "note": "Seatpost section Ø26.8 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø26.8 circle"
  },
  {
   "file": "seatpost-27.2-L100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-27.2-L100.step",
   "stl": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-27.2-L100.stl",
   "note": "Seatpost section Ø27.2 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø27.2 circle"
  },
  {
   "file": "seatpost-28.6-L100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-28.6-L100.step",
   "stl": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-28.6-L100.stl",
   "note": "Seatpost section Ø28.6 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø28.6 circle"
  },
  {
   "file": "seatpost-30.9-L100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-30.9-L100.step",
   "stl": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-30.9-L100.stl",
   "note": "Seatpost section Ø30.9 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø30.9 circle"
  },
  {
   "file": "seatpost-31.6-L100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-31.6-L100.step",
   "stl": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-31.6-L100.stl",
   "note": "Seatpost section Ø31.6 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø31.6 circle"
  },
  {
   "file": "seatpost-34.9-L100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-34.9-L100.step",
   "stl": "https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-34.9-L100.stl",
   "note": "Seatpost section Ø34.9 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø34.9 circle"
  }
 ],
 "markdown": "https://xemstudio.com/objects/bicycle-seatpost-diameters/index.md"
}