Engineering Tools · Carriage Bolt Reference
Carriage Bolt Size & Weight Chart — M6 to M20 Head, Square Neck and Unit Weight
Family index of the carriage bolt charts: for every size from M6 to M20 the table gives the head diameter dk, head height k1, square neck width across flats s, coarse pitch, and the unit weight of a 50 mm under-head bolt in carbon steel at 7.85 g/cm³. Values come from the same engine as the per-size pages; each size opens its own page with the full length series.
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Size Index (M6–M20)
Click a size for its own page (15 under-head lengths from 20 to 150 mm with overall length and thread length, material density factors). d = nominal thread diameter; dk = head diameter; k1 = head height; s = square neck width across flats. The weight column is one 50 mm under-head bolt in carbon steel.
Basis & Disclosed Assumptions
Where the numbers come from
- Every weight is a geometry × density figure from the same engine the product catalog and the per-size pages use: a full-diameter shank, a spherical-cap head and the square neck are added as separate geometry and then multiplied by 7.85 g/cm³ for carbon steel, coating excluded.
- Head, square-neck and pitch values come from the GB/T 12-2013 series — the same table the product catalog and the per-size pages use, so each row can be cross-checked against the catalog's carriage bolt data.
- The estimate tolerance used for this family is ±10%, calibrated against the catalog with headroom for production tolerance. Coating is not part of the base figure.
Scope & limits
- Length means the under-head length L, not the overall length: overall = L + k1, so the same M12 bolt that is 50 mm under the head measures 58.45 mm overall.
- Thread length follows b = 2d + 6 capped at the shank length, so an M12×50 bolt carries 30 mm of thread and 20 mm of plain shank. Each size page states the thread length per length step.
- Head and square-neck values are nominal (maximum) standard sizes. The square neck sits under the head so that the bolt does not turn when the nut is tightened — it is not a driving feature, and the bolt is set, not screwed in.
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)
Need a length outside the table, or a batch total?
The weight calculator covers carriage bolts: size, length and material in one pass, with unit weight, batch weight and pieces per tonne — on the same engine as this table.
Open the weight calculatorCite this chart
YAXIIO. Carriage Bolt Size & Weight Chart — M6 to M20 Head, Square Neck and Unit Weight. https://www.yaxiio.com/en/tools/carriage-bolt-chart (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
FAQ
Is the weight based on the length under the head or on the overall length?
Length under the head. The weight column is the 50 mm under-head length, this family's reference value; overall length is under-head plus head height k1, so an M12×50 bolt measures 58.45 mm overall. Each size page gives both figures so you can cross-check before ordering.
What head and square-neck dimensions does this family use?
GB/T 12-2013: at M12 the head diameter dk is 25.3 mm, head height k1 is 8.45 mm, square neck width across flats s is 12.43 mm and coarse pitch is 1.75 mm. Values are nominal (maximum) standard sizes, and every size page repeats them for its own size.
Next: work out your order, or pick the size by standard
Related References
Bolt Weight Calculator
Size, length and material in one pass; the same engine and basis as this table.
Hex Nut Weight Chart
The matching nut: dimensions and unit weight for hex nuts M6–M36.
Flat Washer Weight Chart
The matching washer: DIN 125 flat washer dimensions and unit weight per size.
Carriage Bolt Product Catalog
GB/T 12-2013 round head square neck bolts — request a quote by size, length and coating.
Disclaimer: this chart provides engineering reference data for selection and quotation only; it is not a design basis or an acceptance standard.