# Hex lag screws, 1/4 to 1/2 in (ASME B18.2.1): shank, head width and height, pilot holes + envelope models

> What are the head size and pilot hole for a 3/8 lag screw?

- Category: Hardware › Fasteners
- Kind: Standard part
- Basis: AFT Fasteners' ASME B18.2.1 table with Engineers Edge, Fastenermart and McMaster-Carr as cross-checks; pilot holes from Fastenermart (by wood group) and from a Portland Bolt / Haydon table; five envelope models as files
- Page: https://xemstudio.com/objects/lag-screws-hex-inch/ · JSON: https://xemstudio.com/api/geometry?object=lag-screws-hex-inch · Plain text: https://xemstudio.com/objects/lag-screws-hex-inch/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 |
|---|---|---|
| 22.0 | 19.0 | 84.9 |

**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, hex lag screws (the AFT table cites ASME B18.2.1-1996), 1/4 to 1/2: threads per inch; shank diameter E max / min; width across flats F (basic, then max / min); width across corners G max / min; head height H (basic, then max / min). AFT's flattened table was read by assigning columns from its layout, so check the standard if a figure matters. The same sizes agree with Fastenermart (F 7/16, 1/2, 9/16, 5/8, 3/4; H 11/64, 7/32, 1/4, 19/64, 11/32) and with McMaster-Carr's head heights (0.172, 0.219, 0.25, 0.344 for 1/4, 5/16, 3/8, 1/2).
- One disagreement: Engineers Edge prints a width across flats of 3/8 for the 1/4 lag screw, against 7/16 in AFT and Fastenermart (and 0.425–0.438 in AFT's max / min). Both are shown; the model uses 7/16. Thread length is a rule, not a column: AFT gives a minimum of half the screw length plus 0.50 in, capped at 5.00 in; Engineers Edge gives the same rule capped at 6.00 in. Both caps are as printed and they differ. Screws too short for the rule are threaded close to the head. AFT also prints length tolerance ±0.12 in to 6 in and ±0.25 in over 6 in, and a minimum shoulder length S (0.094, 0.125, 0.125, 0.156, 0.156 in).
- Pilot holes are not part of B18.2.1. Fastenermart: shank hole (equal to the nominal size), then threaded-portion pilot for softwood groups III and IV, medium wood group II and hardwood group I; the page calls them pilot, not lead holes, and does not explain a dagger on 7/16. Portland Bolt (credited to OBEC) and Haydon Bolts print one threaded-portion pilot per size, 15/64, 9/32, 5/16 for 3/8, 7/16, 1/2 (no 1/4 or 5/16 row); those equal Fastenermart's medium-wood column. Treat all as a guide, as Haydon does, and follow your project engineer or the screw maker.
- Files are Xem-drawn envelopes: a hexagon at the basic width across flats and the basic height (corners are F / cos 30°, which AFT's G max matches, e.g. 0.505 in for 1/4), on a plain cylinder at the nominal diameter; no thread, no chamfer, no washer face, no point. The 3 in length under the head is an example chosen by Xem for all five files, not a published length.
- McMaster-Carr sells lag screws with per-part STEP (it files them under hex head wood screws); the page is linked, not copied. Wood screws: wood-screws-flat-head-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 |
|---|---|---|---|---|
| shank | 1/4, 5/16, 3/8, 7/16, 1/2 in | shank hole = nominal size; pilot softwood III & IV 3/32, 9/64, 11/64, 13/64, 15/64; medium wood II 5/32, 3/16, 15/64, 9/32, 5/16; hardwood I 3/16, 13/64, 1/4, 19/64, 11/32 in (Fastenermart) | designed | lag-screw-* |
| head | hex across flats × height: 7/16 × 11/64 (1/4), 1/2 × 7/32, 9/16 × 1/4, 5/8 × 19/64, 3/4 × 11/32 in | hex pocket or socket space | slide | lag-screw-* |

## Sizes

| size (in) | TPI | shank E max / min (in) | across flats F, basic (AFT) | F max / min (AFT, in) | across flats F (Engineers Edge) | across corners G max / min (in) | head height H, basic (AFT) | H max / min (AFT, in) | shank hole (in) | pilot, softwood III & IV (in) | pilot, medium wood II (in) | pilot, hardwood I (in) | pilot (Portland Bolt / Haydon, in) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1/4 | 10 | .260 / .237 | 7/16 | .438 / .425 | 3/8 | .505 / .484 | 11/64 | .188 / .150 | 1/4 | 3/32 | 5/32 | 3/16 | — |
| 5/16 | 9 | .324 / .298 | 1/2 | .500 / .484 | 1/2 | .577 / .552 | 7/32 | .235 / .195 | 5/16 | 9/64 | 3/16 | 13/64 | — |
| 3/8 | 7 | .388 / .360 | 9/16 | .562 / .544 | 9/16 | .650 / .620 | 1/4 | .268 / .226 | 3/8 | 11/64 | 15/64 | 1/4 | 15/64 |
| 7/16 | 7 | .452 / .421 | 5/8 | .625 / .603 | 5/8 | .722 / .687 | 19/64 | .316 / .272 | 7/16 | 13/64 | 9/32 | 19/64 | 9/32 |
| 1/2 | 6 | .515 / .482 | 3/4 | .750 / .725 | 3/4 | .866 / .826 | 11/32 | .364 / .302 | 1/2 | 15/64 | 5/16 | 11/32 | 5/16 |

## Where the files are

- AFT Fasteners: [Lag screws dimensions and mechanical properties (ASME B18.2.1-1996): E shank, F width across flats, G across corners, H head height, S shoulder length, thread-length rule](https://www.aftfasteners.com/lag-screws-dimensions-and-mechanical-properties/) — web — Publisher's page; dimensions are facts
- Engineers Edge: [Standard hex lag screws: shank diameter E, head width across flats F, head height H, threads per inch, thread-length rule](https://ns1.engineersedge.com/hardware/standard_hex_lag_screws_16077.htm) — web — Publisher's page; dimensions are facts
- Fastenermart: [Lag screws: pilot hole sizes by wood group (shank hole, softwood groups III & IV, medium group II, hardwood group I) and head dimensions](https://fastenermart.com/lag-screws.html) — web — Publisher's page; dimensions are facts
- Portland Bolt (table credited to OBEC Consulting Engineers): [Pilot hole sizes for lag screws: unthreaded shank portion and threaded-portion pilot, 3/8 to 1/2 in](https://portlandbolt.com/print/?table=1176) — web — Distributor table; numbers only
- Haydon Bolts: [Lag bolt pilot hole diameter: shank (unthreaded) and pilot hole, 3/8 to 1/2 in (same values as Portland Bolt's table)](https://haydonbolts.com/?p=255) — web — Maker page; numbers only
- McMaster-Carr: [Stainless steel hex head lag screws (McMaster files these as hex head wood screws) (free STEP/IGES per part, for your own design use; no redistribution)](https://www.mcmaster.com/stainless-steel-hex-head-lag-screws) — STEP, IGES, PDF — McMaster-Carr site terms: personal design use, no redistribution (verify at link)

## Free STEP/STL (5 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/lag-screws-hex-inch/index.json

- lag-screw-1-4-hex-3in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-1-4-hex-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-1-4-hex-3in-envelope.stl) — 1/4 hex lag screw envelope: hex head 7/16 in across flats (basic) × 11/64 in high (basic) on a plain Ø6.35 mm (1/4 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point
- lag-screw-5-16-hex-3in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-5-16-hex-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-5-16-hex-3in-envelope.stl) — 5/16 hex lag screw envelope: hex head 1/2 in across flats (basic) × 7/32 in high (basic) on a plain Ø7.94 mm (5/16 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point
- lag-screw-3-8-hex-3in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-3-8-hex-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-3-8-hex-3in-envelope.stl) — 3/8 hex lag screw envelope: hex head 9/16 in across flats (basic) × 1/4 in high (basic) on a plain Ø9.52 mm (3/8 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point
- lag-screw-7-16-hex-3in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-7-16-hex-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-7-16-hex-3in-envelope.stl) — 7/16 hex lag screw envelope: hex head 5/8 in across flats (basic) × 19/64 in high (basic) on a plain Ø11.11 mm (7/16 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point
- lag-screw-1-2-hex-3in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-1-2-hex-3in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch/lag-screw-1-2-hex-3in-envelope.stl) — 1/2 hex lag screw envelope: hex head 3/4 in across flats (basic) × 11/32 in high (basic) on a plain Ø12.70 mm (1/2 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point

## Questions

**What size wrench for a 3/8 lag screw?**

9/16 in across flats (basic) in AFT's B18.2.1 table, Engineers Edge and Fastenermart; head height 1/4 in.

**What pilot hole for a 1/2 lag screw?**

Fastenermart: 15/64 softwood (groups III & IV), 5/16 medium wood, 11/32 hardwood; Portland Bolt and Haydon print 5/16 for the threaded portion. The shank hole is 1/2 in.

**How much of a lag screw is threaded?**

At least half the length plus 0.50 in, capped at 5.00 in (AFT) or 6.00 in (Engineers Edge); short screws are threaded close to the head.

## Fits with

- [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 bolts, inch: 1/4-20 to 3/4-10 (ASME B18.2.1)](https://xemstudio.com/objects/hex-bolts-inch/)
- [Hex nuts, inch: #4-40 to 3/4-10 (ASME B18.2.2)](https://xemstudio.com/objects/hex-nuts-inch/)
- [Regular shoulder eye bolts, inch (drop forged), 1/4 to 1 in: eye, shoulder, overall and thread length (dimensions only)](https://xemstudio.com/objects/shoulder-eye-bolts-inch/)
- [Socket head cap screws, inch: #10-24, #10-32, 5/16-18, 3/8-16, 1/2-13 (ASME B18.3)](https://xemstudio.com/objects/socket-head-cap-screws-inch/)