Engineering Tools · Carriage Bolt Reference
M20 Carriage Bolt Size & Weight Chart
M20 round head square neck carriage bolt (GB/T 12-2013 as primary source, DIN 603 series for reference): head diameter dk=41.6 mm, head height k1=12.55 mm, square neck width s=20.52 mm, coarse pitch 2.5 mm — full length-series under-head vs overall length distinction and unit weight (g or lb) on one page, geometry fully disclosed.
M20 carriage bolt (GB/T 12-2013 round head square neck): head diameter dk=41.6 mm, square neck width s=20.52 mm, head height k1=12.55 mm, coarse pitch 2.5 mm; M20x50 has under-head length 50 mm = overall length 62.55 mm with 46 mm of thread, about 230 g/pc and 230 kg per 1,000 pcs in carbon steel.
Other Sizes
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{size} 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 (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 GB/T 12-2013 Round Head Square Neck Bolts (small dome head, max nominal values) as the primary source; DIN 603 is the equivalent German series and ISO 8677 the equivalent ISO series, 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)
{size} Length Series: Under-Head / Overall / Unit Weight
Length convention (the point this chart adds): the length L specified by GB/T 12 and DIN 603 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 ({k1} mm for {size}), 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 | 160 | 160 |
| M20×25 | 37.55 | 25 | 170 | 170 |
| M20×30 | 42.55 | 30 | 180 | 180 |
| M20×35 | 47.55 | 35 | 190 | 190 |
| M20×40 | 52.55 | 40 | 210 | 210 |
| M20×45 | 57.55 | 45 | 220 | 220 |
| M20×50 | 62.55 | 46 | 230 | 230 |
| M20×55 | 67.55 | 46 | 240 | 240 |
| M20×60 | 72.55 | 46 | 260 | 260 |
| M20×70 | 82.55 | 46 | 280 | 280 |
| M20×80 | 92.55 | 46 | 310 | 310 |
| M20×90 | 102.55 | 46 | 330 | 330 |
| M20×100 | 112.55 | 46 | 350 | 350 |
| M20×120 | 132.55 | 46 | 400 | 400 |
| M20×150 | 162.55 | 46 | 480 | 480 |
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 (whole g below 100 g, 10 g steps below 1,000 g, 100 g steps 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
Dimensions come from fastenerCalc.CB_DIMS (GB/T 12-2013 round head square neck, max nominal values, same source as the product catalog SKUs and the online weight calculator). Unit weights are all computed from geometry x density: 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), 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 when the shank is shorter, b = L), which matches the catalog spec_json threadLenMm row by row. No table values are reproduced from any copyrighted standard (e.g. ASME B18.5).
Boundary: unit weights are theoretical geometry estimates (excluding coatings and any rolled-thread material saving) for quotation, freight and packing budgets; actual weight varies with thread accuracy, head tolerance and coating (hot-dip galvanizing roughly +4.5%), and trade settlement is against actual measurement. 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.
Corresponding standard page
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FAQ
What are the head diameter and square neck size of a M20 carriage bolt?
On the GB/T 12-2013 basis: M20 has head diameter dk=41.6 mm, head height k1=12.55 mm, square neck width s=20.52 mm, nominal diameter d=M20 and coarse pitch 2.5 mm. The head is a dome with a square neck beneath it; the square neck seats in a square hole of the joined part to prevent rotation, which is what separates a carriage bolt from a hex bolt.
Is the stated carriage bolt length measured under the head or overall?
The length L specified by both GB/T 12 and DIN 603 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.
How much does one M20 carriage bolt weigh?
Computed from geometry (carbon steel at 7.85 g/cm3, coating excluded): M20x50 is about 230 g/pc (about 50.71 lb per 100 pcs) and M20x100 about 350 g/pc. 304 stainless runs about x1.010 and 316 about x1.017 (geometry unchanged, density ratio only). At the same diameter the weight grows with shank length; the length-series table on this page covers each size.
What is the difference between GB/T 12 and DIN 603?
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 GB/T 12-2013 basis and cites DIN 603 for series equivalence only, reproducing no table values 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.
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 (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 / 10 / 8)
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.