Engineering Tools · Carriage Bolt Reference
M20 Carriage Bolt Size & Weight Chart
M20 Head and Square Neck Dimensions (unit mm)
Round head square neck form: a dome head with a square neck under it. The square neck seats in a square hole of the part being joined to stop the bolt from turning — common in timber structures, guardrails, pallets and conveying equipment.
| Nominal diameter d | 20 mm |
| Coarse pitch P (ISO 724 / GB/T 196 series) | 2.5 mm |
| Head diameter dk (dome max OD) | 41.6 mm |
| Head height k1 (dome height) | 12.55 mm |
| Square neck width s | 20.52 mm |
| Thread length b (partly threaded basis) | 2d + 6 mm |
Row-level standard labelling: dimensions follow the DIN 603 / ISO 8677 series (GB/T 12-2013 Round Head Square Neck Bolts, small dome head, max nominal values, is the primary source of the table values); DIN 603 and ISO 8677 are cited for series reference only — no table values are reproduced from them. Tolerances, property classes and surface finish are agreed separately against the buyer's drawing or contract.
References
- GB/T 12-2013 Round Head Square Neck Bolts (National Public Service Platform for Standards Information)
- GB/T 196-2003 General Purpose Metric Screw Threads — Basic Dimensions, coarse pitches (same table as fastenerCalc.pitchOf)
- Dimensions and unit weights cross-checked row by row against carriage bolt SKU spec_json in the product catalog (same source as product pages)
| Under-head L (mm) | Overall L+k1 (mm) | Unit weight (g/pc) |
|---|---|---|
| M20×50 | 62.55 | 231 |
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Other Sizes
M20 Length Series: Under-Head / Overall / Unit Weight
Length convention (the point this chart adds): the length L specified by DIN 603 / GB/T 12 is the under-head length — measured from the bearing face (below the square neck) to the end of the shank, excluding the dome head. Overall length including the head = L + k1. Reporting the same bolt by the wrong convention shifts the figure by one head height (12.55 mm for M20), so confirm the convention with the supplier before assembly.
| Under-head L (mm) | Overall L+k1 (mm) | Thread b (mm) | Unit weight (g/pc) | Bulk (kg/1,000 pcs) |
|---|---|---|---|---|
| M20×20 | 32.55 | 20 | 157 | 157 |
| M20×25 | 37.55 | 25 | 170 | 170 |
| M20×30 | 42.55 | 30 | 182 | 182 |
| M20×35 | 47.55 | 35 | 194 | 194 |
| M20×40 | 52.55 | 40 | 207 | 207 |
| M20×45 | 57.55 | 45 | 219 | 219 |
| M20×50 | 62.55 | 46 | 231 | 231 |
| M20×55 | 67.55 | 46 | 244 | 244 |
| M20×60 | 72.55 | 46 | 256 | 256 |
| M20×70 | 82.55 | 46 | 281 | 281 |
| M20×80 | 92.55 | 46 | 305 | 305 |
| M20×90 | 102.55 | 46 | 330 | 330 |
| M20×100 | 112.55 | 46 | 355 | 355 |
| M20×120 | 132.55 | 46 | 404 | 404 |
| M20×150 | 162.55 | 46 | 478 | 478 |
Unit weight = geometric volume of full-diameter shank (pi/4 x d^2 x L) + dome head (pi*h/6 x (3r^2 + h^2)) + square neck (s^2 x 0.5d), times 7.85 g/cm3 for carbon steel; rounded to avoid false precision (display-layer tiered decimals: at least 2 decimals below 1 kg, 1 decimal from 1 to 10 kg, whole kg at 10 kg and above). Length series covers the catalog standard 20-80 mm plus common longer sizes made to order.
Material Density Conversion
Table values are for carbon steel (7.85 g/cm3); other materials convert by density ratio: weight x factor (geometry unchanged).
| Material | Density (g/cm3) | Conversion Factor |
|---|---|---|
| Carbon Steel | 7.85 | ×1.000 |
| 304 Stainless Steel | 7.93 | ×1.010 |
| 316 Stainless Steel | 7.98 | ×1.017 |
| Brass | 8.5 | ×1.083 |
Calculation Basis and Assumptions Disclosed
Calculation basis
- Dimensions come from fastenerCalc.CB_DIMS (GB/T 12-2013 round head square neck, max nominal values) — the same source as the product catalog SKUs and the online weight calculator.
- Unit weights are computed from geometry × density: full-diameter shank π/4·d²·L + dome head πh/6·(3r²+h²) + square neck s²·0.5d, at 7.85 g/cm³ for carbon steel, base metal only, coating excluded.
- Thread length follows the GB/T 12 partly threaded rule b = 2d + 6 (full thread b = L when the shank is shorter), matching the catalog spec_json threadLenMm row by row.
Scope & limits
- Unit weights are theoretical geometry estimates (excluding coatings and any rolled-thread material saving) for quotation, freight and packing budgets; trade settlement is against actual measurement.
- Actual weight varies with thread accuracy, head tolerance and coating (hot-dip galvanizing ≈ +4.5%); head diameter dk is a max nominal value and may differ from the buyer’s drawing or sample.
- This chart is not an acceptance basis; theoretical estimate tolerance ±10%.
Cite this chart
YAXIIO. M20 Carriage Bolt Size & Weight Chart. https://www.yaxiio.com/en/tools/carriage-bolt-chart/m20 (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
FAQ
Is the stated carriage bolt length measured under the head or overall?
The length L specified by both DIN 603 and GB/T 12 is the under-head length - measured from the bearing face (below the square neck) to the end of the shank, excluding the dome head. Overall length including the head = L + k1. For M20x50: under-head length 50 mm, overall length 62.55 mm, a difference of one head height (12.55 mm). Size the stack thickness of rails or pallets on the under-head length; quoting the overall length instead leaves the bolt either too long to seat or too short to engage.
What is the difference between DIN 603 and GB/T 12?
They are the Chinese and German series for the same form (round head square neck carriage bolt): head and square neck geometry match and the length convention matches (under-head in both). The differences sit in tolerances, property classes and how surface finish is specified. This chart labels dimensions row by row on the DIN 603 / GB/T 12-2013 basis, with DIN 603 cited for series equivalence only and no table values reproduced from it. Order against a drawing or the standard the buyer names; head diameter tolerances differ between the two series and mixing them can upset countersink fits.
What thread length does a carriage bolt have, and is it fully threaded?
On the GB/T 12 partly threaded basis b = 2d + 6: for M20, b=46 mm. Taking M20x50 as an example, the threaded portion is 46 mm and the plain shank 4 mm; when the shank is shorter than b the bolt is fully threaded (b = L). Threads are roll formed to the GB/T 196/197 metric coarse profile (M20 pitch 2.5 mm). Fully threaded or extended-thread versions are made to drawing.
Next: work out your order, or pick the size by standard
Related References
Bolt Weight Calculator (carriage bolt)
Convert nominal diameter, under-head length, material and coating into unit and batch weight in real time - same engine and basis as this chart.
M20 Hex Nut Weight Chart
Matching nut: unit weight and kg per 1,000 pcs for the same size on the GB/T 6170 basis, so it can go straight into costing.
M20 Flat / Spring Washer Weight Chart
Matching washers: flat (ISO 7089 / GB/T 97.1) and spring (DIN 127 series) on one page, useful for the large washers common in timber connections.
M20 Bolt Torque Chart
Tightening: torque ranges and target preload by property class and friction state, for setting the tightening process.
Carriage Bolt Product Category (GB/T 12-2013 / 10-2013 / 8-2021)
Round head square neck, countersunk square neck and square head product catalog with specifications and drawing-based customization.
Disclaimer: This table is engineering estimate data for selection, quotation and freight estimation reference only, not a design basis or acceptance standard.