Nuts and Bolts Sizes for Home Repair Jobs

A bolt that is only slightly too wide will not start in the nut. One with the right diameter but the wrong thread pitch can damage both parts before you have even tightened it. Getting nuts and bolts sizes right is less about owning a huge box of fixings and more about checking a few details before adding anything to the basket.

For most household repairs in the UK, metric fixings are the sensible starting point. They are widely used on modern furniture, brackets, garden equipment, shelving and general DIY projects. Older items, imported machinery and some vehicle parts may use imperial threads, so do not assume a bolt that looks close enough will fit.

Start with the diameter

The first size on a metric bolt is its thread diameter, measured across the outside of the thread. It is shown with an M, short for metric. An M6 bolt has a 6 mm thread diameter; an M8 bolt has an 8 mm thread diameter.

That diameter must match the nut, threaded insert or tapped hole exactly. An M6 nut fits an M6 bolt, but it will not fit an M8 bolt. The spanner size is different again. For example, a standard M6 hex bolt commonly takes a 10 mm spanner, while an M8 hex bolt commonly takes a 13 mm spanner. Do not use the spanner size to identify the thread size.

For a quick household reference, M4 and M5 are often found on smaller fittings, handles and electrical enclosures. M6 is a common choice for furniture, light brackets and general assembly. M8 and M10 are better suited to heavier shelving, workbench fittings and outdoor projects where the surrounding material and fixing method are also strong enough.

A thicker bolt is not automatically the better option. It needs a suitably sized hole and enough material around it. Drilling an oversized hole through a slim timber batten or metal bracket can weaken the job rather than improve it.

Measure an existing bolt properly

If you are replacing a missing fixing, measure the threaded section with a vernier calliper where possible. A ruler can help for larger bolts, but it is not reliable for telling an M5 from an M6 when the threads are worn or painted.

Measure the length from the underside of the head to the tip for standard hex-head, pan-head and button-head bolts. Countersunk bolts are the exception: their length includes the head because the head sits flush in the material. This detail matters when replacing bolts in hinges, furniture frames or brackets where there is limited clearance behind the fixing.

Length is part of choosing nuts and bolts sizes

A bolt description such as M6 x 40 means it has a 6 mm thread diameter and is 40 mm long. It does not tell you the head size, finish or thread pitch, so check the full product details if those points matter to your repair.

The right length depends on what the bolt passes through and how it is secured. When using a nut, the bolt should go through both materials, any washers and the nut, with a few threads visible beyond the nut once tightened. Too short and the nut may only grip on a small number of threads. Too long can leave a sharp projection, catch on stored items or foul a moving part.

For example, if you are bolting two 15 mm pieces of timber together with washers and an M8 nut, a 40 mm bolt may be too short once the washer and nut depth are allowed for. A 50 mm or 60 mm bolt may be more suitable, but check that the end will not protrude where it could be knocked or snagged.

Where a bolt screws into a threaded hole rather than a nut, avoid guessing. It must be long enough to engage securely, but not so long that it bottoms out inside the hole or reaches hidden wiring, a mechanism or the other side of a panel.

Thread pitch: the detail that catches people out

Thread pitch is the distance between thread peaks. Most common metric bolts use a standard coarse pitch, which is usually what you need for everyday DIY. For instance, the usual coarse pitch for M6 is 1.0 mm and for M8 it is 1.25 mm.

Fine-thread metric bolts also exist. An M8 fine-thread bolt has the same outside diameter as an ordinary M8 bolt, but the threads are closer together. It will not fit an M8 coarse nut. Forcing it can cross-thread the nut, strip the bolt or leave a fixing that feels tight but is not properly engaged.

If an existing nut runs freely on for a turn or two and then jams, stop. Check the diameter and pitch rather than reaching for more force. A clean bolt should turn into a matching nut by hand for several turns before you use a spanner.

Thread pitch becomes particularly relevant for replacement parts on bicycles, appliances, machinery, plumbing fittings and older equipment. Take the old fixing with you, or record the diameter, length and pitch before ordering.

Metric and imperial threads are not interchangeable

Metric threads are measured in millimetres. Imperial bolts may be described in inches, such as 1/4 inch or 5/16 inch, and may use thread systems including UNC, UNF, BSW or BSF. Similar diameters can be misleading. A 1/4 inch bolt is close to 6.35 mm, but it is not an M6 bolt and should not be paired with an M6 nut.

Imperial fixings are still common on older British machinery, classic vehicles, some plumbing applications and specialist equipment. If you are repairing something made decades ago, particularly an item with original hardware, inspect it before replacing every fixing with metric parts.

The safest approach is to keep metric and imperial nuts and bolts in separate labelled compartments. A mixed tub of loose fixings may look handy until a time-sensitive repair becomes a sorting job.

Choose the right head, material and nut

The thread size only tells part of the story. A hex bolt is practical where you have room for a spanner or socket. A socket-head bolt suits tighter spaces and is driven with an Allen key. A coach bolt, with its rounded head and square shoulder, is useful for timber joints because the shoulder bites into the wood and stops the bolt turning as the nut is tightened.

For indoor repairs, zinc-plated steel is a dependable general-purpose choice. For garden furniture, gates, sheds and other exposed jobs, stainless steel offers better corrosion resistance. It usually costs more, but replacing rusty fixings every season is rarely good value. Galvanised fixings are also a sensible option for many outdoor timber jobs.

Use a plain nut for ordinary assembly. A nyloc nut has a nylon insert that helps resist loosening from vibration, making it useful for moving equipment, trolleys and garden machinery. A washer spreads the load under the head or nut, helping to prevent damage to timber, plastic or painted metal. For soft materials, a larger penny washer can make a noticeable difference.

A practical way to match a replacement

Before buying, write down the diameter, length and type of the old bolt. Check whether it has a standard hex head, a countersunk head, a socket head or a coach-bolt shoulder. Then identify the material it will pass through and whether the job is indoors, outdoors or subject to vibration.

If the original bolt is missing, measure the hole diameter and the thickness of the materials being joined. Remember that a clearance hole is slightly larger than the bolt itself, so a 7 mm hole may be intended for an M6 bolt. If it is a threaded hole, test only with a known matching bolt or use a thread gauge. Never force a possible match.

It is often worth buying a few extra nuts, washers and matching bolts for routine repairs. These small essentials are easy to add alongside screws, wall plugs, tape, sealant or batteries, and they save a second order when a washer rolls under the bench or a corroded nut will not come free.

A well-chosen fixing should thread together easily, clamp the materials firmly and suit the environment it will live in. Take a minute to measure first, and the repair is far more likely to stay fixed.