The right concrete anchor bolt size is selected by matching the fixing system to the fixture thickness, fixture-hole diameter, drill-hole diameter and available concrete depth. Do not choose from thread diameter or overall length alone.
Post-installed concrete anchors include expansion throughbolts, non-expansion masonry bolts and chemical resin systems using threaded bar. Each family has different dimensions, setting requirements and suitability limits.
Understanding concrete anchor sizing terms
Concrete anchor specifications contain several separate measurements:
- Nominal size: A designation such as M8, M10 or M12 normally identifies the nominal metric thread diameter. It helps determine compatible components but does not state the anchor’s capacity or suitability.
- Published length: The length recorded by the manufacturer for that product code. It does not by itself reveal how much fixture thickness the anchor can accommodate.
- Maximum fixture thickness: The manufacturer’s stated maximum thickness of fixture that can be secured using that specific anchor.
- Drill-hole diameter: The required diameter of the hole drilled into the concrete.
- Fixture-hole diameter: The clearance-hole diameter required in the bracket, plate or other fixture.
- Minimum drill-hole depth: The minimum depth that must be drilled for the selected fixing.
- Recorded edge distance: The distance from the centre of the fixing hole to the concrete edge recorded in the product data.
These terms must be interpreted using the exact product data and installation instructions. Drill-hole depth, anchor length and effective embedment must not be treated as interchangeable measurements.
Comparing TIMCO throughbolt sizes
A throughbolt is an expansion anchor designed for through-fixing into concrete. The anchor passes through the fixture before being tightened according to the manufacturer’s installation instructions.
The table below compares three sizes from the TIMCO zinc-plated throughbolt range:
| Product code | Nominal size | Drill-hole diameter | Fixture-hole diameter | Maximum fixture thickness | Minimum drill-hole depth | Recorded edge distance | Nut socket size |
|---|---|---|---|---|---|---|---|
| 0850TBB | M8 × 50 mm | 8 mm | 9 mm | 5 mm | 45 mm | 55 mm | 13 mm |
| 10100TBB | M10 × 100 mm | 10 mm | 12 mm | 35 mm | 50 mm | 70 mm | 17 mm |
| 12100TBB | M12 × 100 mm | 12 mm | 14 mm | 25 mm | 60 mm | 75 mm | 19 mm |
Comparing the TIMCO M10 × 100 mm Throughbolt with the M12 × 100 mm version shows why overall length alone cannot be used to compare anchors.
Both products have the same published length, but the M10 version has a maximum fixture thickness of 35 mm and a minimum drill-hole depth of 50 mm. The M12 version has a maximum fixture thickness of 25 mm and a minimum drill-hole depth of 60 mm.
The TIMCO M8 × 50 mm Throughbolt has a maximum fixture thickness of 5 mm. This does not establish a particular load capacity or application, but it demonstrates why the fixture thickness must be checked before ordering.
Tightening TIMCO throughbolts
TIMCO describes this throughbolt range as torque-controlled. Its current product information states that, if no torque wrench is available, the nut should be tightened three to four full turns from finger-tight.
This instruction is specific to the verified TIMCO throughbolt range. Do not apply it to other anchors or fixing systems. Always check the current instructions for the exact product code being installed.
Non-expansion masonry bolts
A self-tapping masonry bolt works differently from an expansion throughbolt. It forms a mating thread in the specified pre-drilled hole rather than relying on an expansion clip.
The TIMCO Multi-Fix Countersunk Masonry Bolt, product code MF860C, is described by TIMCO as a non-expansion through fixing.
Its published details include:
- size 8.0 × 60/M10;
- 8 mm drill-hole diameter;
- 10 mm fixture-hole diameter;
- maximum fixture thickness of 20 mm;
- countersunk head;
- TX drive;
- exterior silver organic finish.
Do not interpret the /M10 part of the displayed size as an M10 thread designation without consulting the exact manufacturer data.
The exterior silver organic finish is a product specification, not a guarantee that the fixing suits every external environment. Exposure, connected materials and project requirements must also be considered.
Chemical resin and threaded bar systems
Chemical anchoring uses a selected injection resin with a compatible threaded element. The threaded bar is only one component of the complete fixing system.
TIMCO M10 A2 Stainless Steel Threaded Bar, product code 10TBSS, can be cut to the required length and used with chemical resin.
The product is:
- M10 × 1000 mm;
- metric threaded;
- manufactured from A2-304 stainless steel.
The required drill-hole diameter, embedment, cleaning method, curing conditions and permitted substrate must come from the current technical data for the selected resin system and threaded element.
Do not assume that every resin is compatible with every threaded bar, substrate or application. Suitability of A2-304 stainless steel for the exposure, connected materials and project must also be confirmed.
A practical anchor-sizing procedure
Use the following sequence before ordering:
- Identify the fixing family. Confirm whether the application requires a throughbolt, non-expansion masonry bolt, resin system or another specified fixing.
- Measure the fixture arrangement. Compare the actual arrangement with the manufacturer’s definition of maximum fixture thickness.
- Check the fixture hole. Confirm that the bracket or plate has the required clearance-hole diameter.
- Verify maximum fixture thickness. Do not assume that overall anchor length provides enough usable fixing length.
- Confirm the drill-hole diameter. Use the exact bit diameter stated for the selected product code.
- Check minimum drill-hole depth. Confirm the available member depth, required drilling depth and risk of breaking through the far face.
- Check edge distance and spacing. Use the exact technical data or project fixing design rather than a generic rule.
- Follow the setting instructions. Use the stated drilling, cleaning and tightening method for the selected fixing.
Drilling and dust control
Use the drill type, bit diameter and drilling method specified for the selected fixing. Check that the equipment and drill bit are suitable and in serviceable condition.
Drilling concrete can produce respirable crystalline silica dust. Follow current Health and Safety Executive guidance by controlling dust at source using an appropriate method, such as suitable on-tool extraction. Correctly selected respiratory protective equipment may also be required by the risk assessment and applicable guidance.
RPE is the last line of protection and should not be treated as a substitute for controlling dust at source.
If reinforcement is encountered unexpectedly, stop and check the project requirements before continuing. Do not cut reinforcement without authorisation from the responsible designer.
When specialist specification is required
This article provides general product-selection guidance. It is not a structural fixing design.
Structural connections and other high-consequence installations must follow:
- the project-specific fixing design;
- the specified product approvals;
- the responsible designer’s requirements;
- the exact manufacturer installation instructions;
- the applicable current code of practice.
This boundary applies where failure could cause injury, structural damage or serious disruption, including safety barriers, fall-protection systems, structural steelwork, machinery anchorage and critical overhead services.
Final ordering checklist
Before ordering a concrete anchor, confirm:
- the exact fixing type and product code;
- nominal diameter and published length;
- maximum fixture thickness;
- drill-hole and fixture-hole diameters;
- minimum drill-hole depth;
- available concrete depth;
- required edge distances and spacing;
- tightening or setting instructions;
- substrate condition;
- required finish or material;
- whether a project-specific fixing design is required.
Concrete anchor bolt sizes cannot be compared from diameter and length alone. Matching the complete product specification to the actual fixture and concrete arrangement is the safer and more reliable way to choose.
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