# Carriage bolts, ASME B18.5 round head square neck, #10 to 1 in: head, square neck, body diameter + envelope models

> What are the head diameter and square neck size of a 3/8 carriage bolt, and what hole does the square need?

- Category: Hardware › Fasteners
- Kind: Standard part
- Basis: Brighton-Best's ASME B18.5 carriage bolt data sheet, cross-checked against Torqbolt's table (which adds the body diameter limits) and AFT Fasteners' thread-length rule; seven envelope models as files
- Page: https://xemstudio.com/objects/carriage-bolts-asme-b18-5/ · JSON: https://xemstudio.com/api/geometry?object=carriage-bolts-asme-b18-5 · Plain text: https://xemstudio.com/objects/carriage-bolts-asme-b18-5/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files CC BY 4.0 or their publisher's license
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

## Overall (mm)

| length | width | height |
|---|---|---|
| 21.4 | 21.4 | 81.5 |

**Exact dimensions.** Every file is drawn at true size with no tolerance added. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your process's clearance before making the part.

## Notes

- Dimensions are inches, round head square neck bolts, ASME B18.5, #10 to 1 in, each as max / min: head diameter A, head height H, square width O, square depth P (under the head) and the corner radius on the square R (max). Body diameter E max / min is from Torqbolt's table (threads are Unified UNC Class 2A per that page; for plated bolts it says the 2A maxima apply before plating). The two sheets agree on every value except two: the #10 head diameter minimum (.436 in Brighton-Best, .438 in Torqbolt) and the 7/8 head height maximum (.459 in Brighton-Best, .469 in Torqbolt); both are shown. The sheets label the columns with the standard's letters, but the letter-to-column mapping was not clear in the text read, so the columns are named by their position and size (square width ≈ the body diameter, e.g. .260 / .245 at 1/4, square depth .156 / .125). Check the standard if a figure matters.
- Thread length (Brighton-Best): for nominal lengths 6 in and shorter the minimum thread length is twice the basic diameter plus 0.25 in; for longer bolts twice the basic diameter plus 0.50 in; carriage bolts are supplied fully threaded, very long ones may be threaded only 6 in. The AFT page, as read, gives the same 0.50 in rule above 6 in but printed 0.025 in (probably a typo for 0.25) for 6 in and shorter, so the Brighton-Best figure is used. Length is measured from under the head to the end. Length tolerances were not read. The mating square hole in a steel plate or the neck pocket in timber is not tabulated by these sources; use the square width O as the size to clear.
- Not on this page: short neck, ribbed neck, fin neck and 114° countersunk square neck bolts (the page sources describe them but their tables could not be read reliably), and DIN 603 mushroom head square neck bolts (the DIN 603 table was not accessible).
- Files are Xem-drawn envelopes: a spherical cap with base diameter A max and height H max, a square neck O max wide and P max deep, and a plain cylindrical shank at the nominal diameter; the head is the maximum limit because B18.5 gives no basic value. No thread, no chamfer, no corner radius, no point. The 3 in length under the head is an example chosen by Xem for all seven files, not a published length. Sizes drawn: #10, 1/4, 5/16, 3/8, 1/2, 5/8, 3/4.
- McMaster-Carr sells carriage bolts with per-part STEP; the category is linked, not copied. Hex bolts: hex-bolts-inch; wood screws and lag screws: wood-screws-flat-head-inch, lag-screws-hex-inch.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| square neck | O max .260 in at 1/4, .388 in at 3/8, .515 in at 1/2 (width), P max .156 / .219 / .281 in (depth) | square hole in the clamped part or a square pocket in timber sized from O; the sources read give no recommended hole size | designed | carriage-bolt-* |
| shank (thread as plain cylinder) | nominal Ø #10 to 3/4 in; body E max .260 in at 1/4, .388 in at 3/8, .515 in at 1/2 (UNC Class 2A) | nut of the same UNC thread (see hex-nuts-inch) and a clearance hole in the clamped part | designed | carriage-bolt-* |

## Sizes

| size (in) | body E max / min | head dia A max / min | head height H max / min | square width O max / min | square depth P max / min | corner radius on square max |
|---|---|---|---|---|---|---|
| #10 | .199 / .182 | .469 / .436 | .114 / .094 | .199 / .185 | .125 / .094 | .031 |
| 1/4 | .260 / .237 | .594 / .563 | .145 / .125 | .260 / .245 | .156 / .125 | .031 |
| 5/16 | .324 / .298 | .719 / .688 | .176 / .156 | .324 / .307 | .187 / .156 | .031 |
| 3/8 | .388 / .360 | .844 / .782 | .208 / .188 | .388 / .368 | .219 / .188 | .047 |
| 7/16 | .452 / .421 | .969 / .907 | .239 / .219 | .452 / .431 | .250 / .219 | .047 |
| 1/2 | .515 / .483 | 1.094 / 1.032 | .270 / .250 | .515 / .492 | .281 / .250 | .047 |
| 5/8 | .642 / .605 | 1.344 / 1.219 | .344 / .313 | .642 / .616 | .344 / .313 | .078 |
| 3/4 | .768 / .729 | 1.594 / 1.469 | .406 / .375 | .768 / .741 | .406 / .375 | .078 |
| 7/8 | .895 / .852 | 1.844 / 1.719 | .459 / .438 (Brighton-Best) or .469 / .438 (Torqbolt) | .895 / .865 | .469 / .438 | .094 |
| 1 | 1.022 / .976 | 2.094 / 1.969 | .531 / .500 | 1.022 / .990 | .531 / .500 | .094 |

## Where the files are

- Brighton-Best: [Carriage bolts product data sheet (ASME B18.5): head diameter, head height, square width, square depth, corner radius, #10 to 1 in; thread-length rule](https://www.brightonbest.com/download/pds/Carriage Bolts.pdf) — web — Maker data sheet; numbers only
- Torqbolt: [ASME B18.5 round head square neck bolts: body diameter E max / min, head, square width and depth, corner radius, #10 to 1 in; UNC 2A thread note](https://torqbolt.com/asme-b18-5-round-head-square-neck-bolts-dimensions-standards-specifications) — web — Distributor page; numbers only
- AFT Fasteners: [Carriage bolts: round head square neck, short neck, ribbed neck, fin neck, flat head (ASME B18.5); thread-length rule](https://www.aftfasteners.com/carriage-bolts-dimensions-mechanical-properties/) — web — Publisher's page; dimensions are facts

## Free STEP/STL (7 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/index.json

- carriage-bolt-no10-round-head-3in-envelope: [STEP](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-no10-round-head-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-no10-round-head-3in-envelope.stl) — #10 carriage bolt envelope (ASME B18.5 round head, square neck): spherical-cap head Ø11.91 × 2.90 mm (A max, H max), square neck 5.05 mm wide × 3.17 mm deep (O max, P max), plain shank Ø4.83 mm (nominal) × 76.2 mm (3 in, Xem's example length) under the head; no thread, chamfer, corner radius or point
- carriage-bolt-1-4-round-head-3in-envelope: [STEP](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-1-4-round-head-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-1-4-round-head-3in-envelope.stl) — 1/4 carriage bolt envelope (ASME B18.5 round head, square neck): spherical-cap head Ø15.09 × 3.68 mm (A max, H max), square neck 6.60 mm wide × 3.96 mm deep (O max, P max), plain shank Ø6.35 mm (nominal) × 76.2 mm (3 in, Xem's example length) under the head; no thread, chamfer, corner radius or point
- carriage-bolt-5-16-round-head-3in-envelope: [STEP](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-5-16-round-head-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-5-16-round-head-3in-envelope.stl) — 5/16 carriage bolt envelope (ASME B18.5 round head, square neck): spherical-cap head Ø18.26 × 4.47 mm (A max, H max), square neck 8.23 mm wide × 4.75 mm deep (O max, P max), plain shank Ø7.94 mm (nominal) × 76.2 mm (3 in, Xem's example length) under the head; no thread, chamfer, corner radius or point
- carriage-bolt-3-8-round-head-3in-envelope: [STEP](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-3-8-round-head-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-3-8-round-head-3in-envelope.stl) — 3/8 carriage bolt envelope (ASME B18.5 round head, square neck): spherical-cap head Ø21.44 × 5.28 mm (A max, H max), square neck 9.86 mm wide × 5.56 mm deep (O max, P max), plain shank Ø9.52 mm (nominal) × 76.2 mm (3 in, Xem's example length) under the head; no thread, chamfer, corner radius or point
- carriage-bolt-1-2-round-head-3in-envelope: [STEP](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-1-2-round-head-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-1-2-round-head-3in-envelope.stl) — 1/2 carriage bolt envelope (ASME B18.5 round head, square neck): spherical-cap head Ø27.79 × 6.86 mm (A max, H max), square neck 13.08 mm wide × 7.14 mm deep (O max, P max), plain shank Ø12.70 mm (nominal) × 76.2 mm (3 in, Xem's example length) under the head; no thread, chamfer, corner radius or point
- carriage-bolt-5-8-round-head-3in-envelope: [STEP](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-5-8-round-head-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-5-8-round-head-3in-envelope.stl) — 5/8 carriage bolt envelope (ASME B18.5 round head, square neck): spherical-cap head Ø34.14 × 8.74 mm (A max, H max), square neck 16.31 mm wide × 8.74 mm deep (O max, P max), plain shank Ø15.88 mm (nominal) × 76.2 mm (3 in, Xem's example length) under the head; no thread, chamfer, corner radius or point
- carriage-bolt-3-4-round-head-3in-envelope: [STEP](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-3-4-round-head-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/carriage-bolts-asme-b18-5/carriage-bolt-3-4-round-head-3in-envelope.stl) — 3/4 carriage bolt envelope (ASME B18.5 round head, square neck): spherical-cap head Ø40.49 × 10.31 mm (A max, H max), square neck 19.51 mm wide × 10.31 mm deep (O max, P max), plain shank Ø19.05 mm (nominal) × 76.2 mm (3 in, Xem's example length) under the head; no thread, chamfer, corner radius or point

## Questions

**What is the head diameter of a 3/8 carriage bolt?**

Brighton-Best's ASME B18.5 table prints .844 in max and .782 in min, head height .208 / .188 in; the square under the head is .388 / .368 in wide and .219 / .188 in deep.

**How much of a carriage bolt is threaded?**

At least twice the basic diameter plus 0.25 in for lengths up to 6 in (twice the diameter plus 0.50 in above 6 in) according to Brighton-Best; they are supplied fully threaded.

**How big a square hole does a carriage bolt need?**

The square neck is O max wide (e.g. .388 in at 3/8); the sources read do not give a recommended hole size, so size it from O plus your own allowance.

## Fits with

- [Hex bolts, inch: 1/4-20 to 3/4-10 (ASME B18.2.1)](https://xemstudio.com/objects/hex-bolts-inch/)
- [Hex lag screws, 1/4 to 1/2 in (ASME B18.2.1): shank, head width and height, pilot holes + envelope models](https://xemstudio.com/objects/lag-screws-hex-inch/)
- [Flat-head wood screws, inch #4–#14 (ASME B18.6.1): body and head diameter, pilot and clearance holes from three sources + envelope models](https://xemstudio.com/objects/wood-screws-flat-head-inch/)
- [Hex nuts, inch: #4-40 to 3/4-10 (ASME B18.2.2)](https://xemstudio.com/objects/hex-nuts-inch/)
- [Flat washers, inch: SAE and USS, 1/4 to 3/4 in (ASME B18.21.1 Type A)](https://xemstudio.com/objects/flat-washers-inch/)
- [Square nuts, DIN 557: M5–M20 width across flats, thickness, corner distance + 8 envelope files](https://xemstudio.com/objects/square-nuts-din-557/)
- [T-bolts / hammer-head bolts: DIN 186 (square neck, M6–M48), DIN 787 (T-slot, M5–M48) and profile-slot hammer bolts: head length, width, height + DIN 186 envelopes](https://xemstudio.com/objects/t-bolts-din-186-din-787-hammer-head/)