How to Choose the Right Key Clamp Size for Your Project

Picking the right key clamp size looks simple on paper: five numbers, five tube diameters, done. In practice, the right key clamp size for your project depends on more than which diameter looks right, it depends on which clamp fittings you will actually need, how much load the finished structure has to carry, and whether the tube suits the space it is going into. Fisher Alvin has manufactured key clamps and galvanised steel tubes for over a century, and getting the size right from the outset saves a reorder, a mismatched fitting, or a structure that is not strong enough for the job.

Understanding Key Clamp Sizes

Key clamps are sized by number, from size 5 through to size 9, and each number refers to a specific tube outer diameter and nominal bore rather than an arbitrary label. Size 5 is 26.9mm outside diameter, size 6 is 33.7mm, size 7 is 42.4mm, size 8 is 48.3mm, and size 9 is 60.3mm. If you are used to nominal bore rather than outer diameter, the size conversions run alongside: ¾”, 1″, 1¼”, 1½” and 2″ respectively. Once you know which size you need, the fittings and tube are ordered against that same number throughout the range.

What Size 7 (and the Other Sizes) Actually Mean

Size 7 is one of the most commonly used sizes across handrails, guardrails and general structural work, so it is worth using as the example. At 42.4mm outer diameter and 1¼” nominal bore, size 7 tube sits in the middle of the range: strong enough for most outdoor and indoor structures, without the bulk of size 8 or 9. The same logic applies whichever size you are looking at: the number tells you the diameter, the diameter determines which tube and fittings fit together, and the rest of your design follows from there.

Not Every Key Clamp Fitting Comes in Every Size

This is the part a size chart alone will not tell you. Several key clamp fittings are only offered in a subset of sizes rather than the full range 5 to 9. A tee or elbow you are planning to use might only be available in sizes 6, 7 and 8, for example, which means the fitting you need can rule a size in or out before load or looks even come into it. If you are unsure which fittings are available in your chosen size, check the range or contact us before finalising a design, since ordering tube in a size with no matching fitting is an easy mistake to make.

Matching Key Clamp Size to How the Structure Will Be Used

The right size depends on what the structure needs to support and where it is going. A size 5 or 6 suits lighter indoor applications: shop displays, furniture, or a short run of handrail on a staircase. A size 7 or 8 is more commonly specified for guardrails, safety barriers and structures exposed outdoors, where strength and durability matter more. Larger size 9 tube suits heavier structural applications and posts carrying greater loads. As a general rule, the more exposed or heavily used the structure, the larger the size you should be looking at, and length and post spacing both affect how far a given size can safely span.

Strength and Slip Load: Why Size Affects Safety

Every key clamp fitting locks onto its tube using a set screw, tightened with a hex key, and each screw achieves a slip load of up to 900kg when properly tightened. Larger sizes support proportionally greater loads, which is why a size 5 or 6 is suitable for lighter structures like ramps or display furniture, while size 7, 8 and 9 are the recommended choices for guardrails, walkways and structures that need to carry real weight. Strength is not only about looking sturdy, it is about choosing a size that is genuinely strong enough for the loads the structure will see.

Getting the Right Size Specified for Your Project

If you are still unsure which size is ideal, send us your drawings or sketches and we will help determine the right size and range of fittings for your project. Because these are galvanised castings to BS standards, dimensions can vary slightly, so we do not recommend pre-drilling any bolt holes before the fittings and tube arrive. Our team can also offer guidance on common applications, from indoor furniture to outdoor structures, to help you find the perfect match first time.

The Right Size Makes the Right Structure

Choosing a key clamp size is not just about the tube diameter you would like to use, it is about matching that diameter to the fittings you need, the load the structure will carry, and where it is going. Get the size right at the start and the rest of the build becomes far easier.

Speak to Fisher Alvin

Have questions about which key clamp size suits your project? Contact Fisher Alvin for expert guidance, or send your drawings for a same day quote.