# Hex lag screws 5/8, 3/4, 7/8 and 1 in (ASME B18.2.1): head width and height, shank, pilot holes + envelope models

> What size is the head of a 3/4 lag screw, and what pilot hole does a 5/8 lag bolt need?

- Category: Hardware › Fasteners
- Kind: Standard part
- Basis: AFT Fasteners' ASME B18.2.1-1996 table (5/8, 3/4, 7/8) with Engineers Edge (5/8 to 1) and Fastenermart (head and pilot holes, 5/8 and 3/4) as cross-checks; four envelope models as files
- Page: https://xemstudio.com/objects/lag-screws-hex-inch-5-8-to-1/ · JSON: https://xemstudio.com/api/geometry?object=lag-screws-hex-inch-5-8-to-1 · Plain text: https://xemstudio.com/objects/lag-screws-hex-inch-5-8-to-1/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 |
|---|---|---|
| 44.0 | 38.1 | 118.7 |

**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

- Inches. Sizes 5/8, 3/4, 7/8 and 1 in extend lag-screws-hex-inch (1/4 to 1/2 in). AFT's table (it cites ASME B18.2.1-1996) stops at 7/8; the 1 in row is from Engineers Edge alone, so its basic width across flats (1-1/2) and head height (43/64) have no second source here, and its across-corners and shoulder length are left blank. AFT's flattened table was read by assigning columns from the layout (E, F, G, H, S), so check the standard if a figure matters.
- Engineers Edge agrees with AFT on E, F and H for 5/8, 3/4 and 7/8. One misprint: its 7/8 row prints a basic F of '15/16' beside a max / min of 1.312 / 1.269, which is 1-5/16 (AFT prints 1-5/16); 1-5/16 is used. Fastenermart agrees on head width and height for 5/8 and 3/4 (15/16 and 27/64; 1-1/8 and 1/2).
- Thread length is a rule, not a column: AFT gives a minimum of half the screw length plus 0.50 in, up to 5.00 in; short screws are threaded close to the head. Pilot holes are not part of B18.2.1: Fastenermart prints a shank hole equal to the nominal size and threaded-portion pilots by wood group (5/8: 5/16 softwood III and IV, 13/32 medium II, 29/64 hardwood I; 3/4: 13/32, 1/2, 9/16). The page has no pilot holes for 7/8 or 1 in, so none are shown. Treat pilots as a guide and follow the screw maker or the project engineer.
- Files: hexagon at the basic width across flats and the basic height (corners are F / cos 30°, which AFT's G max matches, e.g. 1.083 in for 5/8) on a plain cylinder at the nominal diameter; no thread, chamfer, washer face or point. The 4 in length under the head is an example chosen by Xem, not a published length. The 1 in file uses the single-source head size.
- McMaster-Carr sells lag screws with per-part STEP; its category is linked, not copied. Smaller sizes: lag-screws-hex-inch. 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 (thread drawn as plain cylinder) | Ø15.875 (5/8), 19.05 (3/4), 22.225 (7/8), 25.4 (1) mm | unthreaded clearance hole in the part being clamped: shank hole = nominal size (Fastenermart, 5/8 and 3/4) | slide | lag-screw-* |
| pilot hole in the receiving timber (5/8, 3/4 only) | 5/8: 5/16 soft, 13/32 medium, 29/64 hard; 3/4: 13/32, 1/2, 9/16 in (Fastenermart) | threaded portion of the lag screw | designed | lag-screw-* |
| hex head | across flats 15/16, 1-1/8, 1-5/16, 1-1/2 in (basic) | socket / wrench or recess that captures the head: width = F | slide | lag-screw-* |

## Sizes

| size (in) | TPI | shank E max / min (in) | across flats F, basic | F max / min (AFT) | F (Engineers Edge) | across corners G max / min (AFT) | head height H, basic | H max / min | shoulder S min (AFT) | shank hole (Fastenermart) | pilot, softwood III & IV | pilot, medium II | pilot, hardwood I |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5/8 | 5 | .642 / .605 | 15/16 | .938 / .906 | 15/16 (.938 / .906) | 1.083 / 1.033 | 27/64 | .444 / .378 | .312 | 5/8 | 5/16 | 13/32 | 29/64 |
| 3/4 | 4-1/2 | .768 / .729 | 1-1/8 | 1.125 / 1.088 | 1-1/8 (1.125 / 1.088) | 1.299 / 1.240 | 1/2 | .524 / .455 | .375 | 3/4 | 13/32 | 1/2 | 9/16 |
| 7/8 | 4 | .895 / .852 | 1-5/16 | 1.312 / 1.269 | '15/16' printed; max / min 1.312 / 1.269 | 1.516 / 1.447 | 37/64 | .604 / .531 | .375 | — | — | — | — |
| 1 | 3-1/2 | 1.022 / .976 | 1-1/2 (Engineers Edge only) | 1.500 / 1.450 | 1-1/2 (1.500 / 1.450) | — | 43/64 (Engineers Edge only) | .700 / .591 | — | — | — | — | — |

## 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, width across flats F, head height H, threads per inch, 5/8 to 1 in](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 and head dimensions (5/8 and 3/4 in rows)](https://fastenermart.com/lag-screws.html) — web — Publisher's page; dimensions are facts
- McMaster-Carr: [Hex head lag screws (McMaster files them 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 (4 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/index.json

- lag-screw-5-8-hex-4in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-5-8-hex-4in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-5-8-hex-4in-envelope.stl) — 5/8 hex lag screw envelope: hex head 0.9375 in across flats (basic) × 0.4219 in high (basic) on a plain Ø15.88 mm (5/8 in nominal) shank, example length 4 in (101.6 mm) under the head (Xem's choice); no thread, chamfer, washer face or point
- lag-screw-3-4-hex-4in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-3-4-hex-4in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-3-4-hex-4in-envelope.stl) — 3/4 hex lag screw envelope: hex head 1.125 in across flats (basic) × 0.5 in high (basic) on a plain Ø19.05 mm (3/4 in nominal) shank, example length 4 in (101.6 mm) under the head (Xem's choice); no thread, chamfer, washer face or point
- lag-screw-7-8-hex-4in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-7-8-hex-4in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-7-8-hex-4in-envelope.stl) — 7/8 hex lag screw envelope: hex head 1.3125 in across flats (basic) × 0.5781 in high (basic) on a plain Ø22.22 mm (7/8 in nominal) shank, example length 4 in (101.6 mm) under the head (Xem's choice); no thread, chamfer, washer face or point
- lag-screw-1-hex-4in-envelope: [STEP](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-1-hex-4in-envelope.step) · [STL](https://xemstudio.com/files/hardware/lag-screws-hex-inch-5-8-to-1/lag-screw-1-hex-4in-envelope.stl) — 1 hex lag screw envelope: hex head 1.5 in across flats (basic) × 0.6719 in high (basic) on a plain Ø25.40 mm (1 in nominal) shank, example length 4 in (101.6 mm) under the head (Xem's choice); no thread, chamfer, washer face or point

## Questions

**What wrench size is a 3/4 lag screw?**

1-1/8 in across flats (basic) in AFT's B18.2.1 table, Engineers Edge and Fastenermart; head height 1/2 in.

**What pilot hole for a 5/8 lag screw?**

Fastenermart: 5/16 in softwood (groups III and IV), 13/32 medium wood, 29/64 hardwood; the shank hole is 5/8 in.

**How big is the head of a 1 in lag bolt?**

Engineers Edge prints 1-1/2 in across flats and 43/64 in basic head height (max 1.500 / 1.450 and .700 / .591); no second source was found for this size.

## Fits with

- [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 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/)
- [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/)
- [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/)