Holding Down Bolt Size & Specification Guide
Holding down bolts (also called foundation bolts or anchor bolts) are the fixings cast into a concrete foundation to secure steel columns, stanchions and base plates. Get the size, grade and length right and the bolts transfer load cleanly from the steelwork into the foundation. This guide covers the sizes, grades, dimensions and specifications you need when specifying or ordering holding down bolts in the UK.
Everything below follows BS 7419:2012 “Specification for holding down bolts”, the British Standard for foundation bolts connecting steel structures to their foundations.
What is a holding down bolt?
A holding down bolt is a long bolt — usually a square-head bolt used with a square plate washer — set into wet concrete so its threaded end projects above the foundation. The steel base plate drops over the projecting threads and is levelled and secured with a nut and washer. The common UK pattern is the “square/square” (SQ SQ) bolt: a square head anchored in the concrete and a square plate washer under the nut on top.
Three names are used interchangeably:
- Holding down bolts — the standard UK term for cast-in foundation bolts.
- Foundation bolts — the same product; emphasises where they go.
- Anchor bolts — a broader term that also includes post-installed mechanical and chemical anchors. Cast-in holding down bolts are one type of anchor bolt.
The standard: BS 7419
BS 7419:2012 specifies the dimensions, tolerances, materials and mechanical properties for holding down bolts:
- Grades: 4.6 and 8.8 (see below).
- Product grade: C — the tolerance class for cast-in foundation bolts.
- Diameters: M12 to M48, with M16, M20, M24, M30 and M36 most commonly used.
- Form: square head and square plate washer (SQ SQ), cast into concrete.
Bolt grades: 4.6 vs 8.8
The grade is stamped on the bolt as two numbers (e.g. 8.8). The first relates to tensile strength; the second to the ratio of yield to tensile strength. Grade 8.8 is the high-tensile option most commonly specified for structural holding down bolts.
| Property | Grade 4.6 | Grade 8.8 |
|---|---|---|
| Minimum tensile strength | 400 MPa (N/mm²) | 800 MPa (N/mm²) |
| Minimum yield / proof stress | 240 MPa | 640 MPa |
| Material & process | Carbon steel, cold-formed (not heat-treated) | Carbon/alloy steel, heat-treated (quenched & tempered) |
| Typical use | Lighter / general connections | Structural steelwork & higher-load foundations |
Standard sizes and dimensions (BS 7419)
Key basic dimensions from BS 7419 for the most preferred sizes. All dimensions in millimetres.
| Thread size (d) | M16 | M20 | M24 | M30 | M36 |
|---|---|---|---|---|---|
| Across flats, square head (s) | 24 | 30 | 36 | 46 | 55 |
| Head height (k, nominal) | 10 | 12.5 | 15 | 17.7 | 22.5 |
| Threaded length (b) | 116–122 | 120–127.5 | 124–133 | 130–140.5 | 136–148 |
| Shank diameter (ds) | 16 | 20 | 24 | 30 | 36 |
Bolt length, tolerance and projection
Overall length is set by the foundation depth and the projection needed above the concrete — it is determined by the structural engineer, not a generic rule. Common holding down bolt lengths run from about 300 mm to 1000 mm, with longer bolts to order. The embedded length must develop the load into the concrete, and enough thread must project above the finished concrete for the plate washer, nut and any levelling.
BS 7419 sets the permitted tolerance on nominal length by band:
| Nominal length (mm) | Tolerance (mm) |
|---|---|
| over 250 up to 315 | ± 5.2 |
| over 315 up to 400 | ± 5.7 |
| over 400 up to 500 | ± 6.3 |
| over 500 up to 630 | ± 7.0 |
| over 630 up to 800 | ± 8.0 |
| over 800 up to 1000 | ± 9.0 |
| over 1000 up to 1250 | ± 10.5 |
The full holding down bolt assembly
A holding down bolt rarely goes in on its own. A complete cast-in assembly usually includes:
- The holding down bolt — square-head bolt, grade 4.6 or 8.8. Browse our holding down bolt sets (M16, M20 & M24).
- Nut(s) and washer — one nut to secure, sometimes a second as a locking or levelling nut. See our holding down bolt & nut range.
- Square plate washer — spreads the load under the cast-in head and/or under the nut on the base plate. See square plate washers.
- Waxed cardboard cone — forms a void around the projecting threads so the bolt can be adjusted for line and level after the concrete sets. See waxed cardboard cones.
- Bolt box — the assembled cluster of bolts held in the correct pattern on a plywood template, so the group casts into the foundation in exactly the right position.
Finishes
- Self-colour (plain / black) — standard for bolts cast into concrete, where the concrete provides protection.
- Hot-dip galvanised — added corrosion protection for exposed conditions, external structures or aggressive environments.
How holding down bolts are set
Most holding down bolts are cast in: the bolt cluster is assembled into a bolt box on a plywood template, positioned to the setting-out drawing, and concrete is poured around it. The waxed cones leave a void so each bolt can be nudged into final position before the base plate goes on. For more detail, see our guides on how anchor bolts work and fitting anchor bolts for steel columns.
Choosing the right holding down bolt
Diameter, grade, length and bolt pattern are all part of the structural design, so the definitive source is always your foundation/setting-out drawing. As a general guide: M16–M24 grade 8.8 covers most portal-frame and general structural steel work, with M30 and M36 used for heavier structures, crane bases, signs and similar. Unsure? Send us your layout drawings and we'll advise on the right assembly and current lead times.
Frequently asked questions
What grade are holding down bolts?
Holding down bolts to BS 7419 are supplied in grade 4.6 or grade 8.8. Grade 8.8 is high-tensile (800 MPa minimum tensile strength, 640 MPa yield) and is the usual choice for structural steelwork; grade 4.6 (400 MPa tensile) is used for lighter connections.
What sizes do holding down bolts come in?
BS 7419 covers M12 to M48, with M16, M20, M24, M30 and M36 the most common sizes. BoltBox supplies M16, M20 and M24 sets as standard, with other sizes available to order.
What length holding down bolt do I need?
Length is set by the structural engineer on the foundation drawing — it depends on the foundation depth and the projection needed above the concrete. Common lengths run from about 300 mm to 1000 mm, with longer bolts available to order.
What's the difference between a holding down bolt, a foundation bolt and an anchor bolt?
Holding down bolts and foundation bolts are the same thing — cast-in bolts that secure steelwork to a concrete foundation. “Anchor bolt” is a broader term that also includes post-installed mechanical and chemical anchors; a cast-in holding down bolt is one type of anchor bolt.
Should holding down bolts be galvanised?
Self-colour (plain) is standard where the bolt is cast into concrete. Hot-dip galvanising is specified for added corrosion protection in exposed or aggressive environments. BoltBox can supply both — advise your requirement and we'll confirm stock and lead times.
Need holding down bolts for your project?
Send your layout drawings, sizes or bolt schedule and we'll price them and advise on lead times. We make up your complete bolt boxes (clusters) in-house at our Burnley workshop, on plywood templates to the sizes on your drawings, and deliver to site UK-wide.
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