How this instrument works
A rivet joining two plates together needs to be sized to the material it's fastening — too thin a rivet for a thick plate leaves a weak joint prone to shearing, while an oversized rivet for a thin plate wastes material and perforates the plate more than necessary. Unwin's formula is a classic, widely cited empirical rule of thumb from structural and mechanical design practice that ties recommended rivet diameter directly to plate thickness: d (mm) = 6 × √t (mm) — a commonly rounded form; some published sources, including the reference cited below, state it more precisely as 6.04 × √t (mm), a difference of well under 1% in the result.
The square-root relationship is the interesting part — diameter doesn't scale linearly with thickness, it grows more slowly, since doubling a plate's thickness only increases the recommended rivet diameter by a factor of about 1.41 (the square root of 2), not by double. The same relationship converts cleanly to inches: 6 × √(25.4 × t) mm works out to almost exactly 1.2 × √t (in), the commonly cited inch-unit form of the same rule.
This is an empirical estimate and a useful starting reference, not a substitute for a governing structural specification. A real structural or pressure-vessel riveted joint's rivet diameter, type, spacing, and pattern should be verified against the applicable engineering code or standard — such as AISC steel construction requirements or a relevant national structural steel specification — by a qualified engineer before fabrication, rather than relying on this rule of thumb alone for a load-bearing application.
- Measure the Plate thickness, t (mm) — the thickness of the plate the rivet will pass through.
- Read Recommended rivet diameter (mm) and Recommended rivet diameter (in) beneath the input.
- Treat the result as a starting reference point, not a final specification.
- For any load-bearing or structural riveted joint, verify diameter, spacing, and rivet type against the governing structural code or engineering standard before fabrication.
Worked example — a 10 mm structural steel plate
Enter 10 into Plate thickness, t (mm) — a common mid-range structural plate thickness. Unwin's formula gives d = 6 × √10 = 6 × 3.162278 = 18.973666 mm.
Recommended rivet diameter (mm) reads 18.974, and Recommended rivet diameter (in) reads 0.7470 — dividing 18.973666 by 25.4. That's very close to the commonly cited inch-unit shortcut of 1.2 × √t(in), confirming the two unit forms of the same rule agree with each other.
Questions
What is Unwin's formula used for?
It's a classic empirical rule of thumb, widely cited in structural and mechanical design references, for choosing a recommended rivet diameter based on the thickness of the plate a rivet will join. It gives a quick starting figure — d(mm) = 6 × √t(mm), sometimes stated more precisely as 6.04 × √t(mm) depending on the source — for hand-driven or hot-driven structural rivets, rather than requiring a full joint-strength calculation for a preliminary sizing check.
Is there an inch-unit version of this formula?
Yes — d(in) = 1.2 × √t(in) is the commonly cited inch-unit shortcut, and it's mathematically consistent with the millimeter form: converting 6 × √t(mm) into inches works out to almost exactly the same 1.2 coefficient once the unit conversion is carried through, so both forms describe the same underlying relationship.
Is Unwin's formula a building code requirement?
No — it's a widely used engineering rule of thumb, not a mandated code value. A real structural or pressure application's rivet size, spacing, and pattern should be specified according to the governing structural code or standard applicable to that project (such as AISC provisions for structural steel), verified by a qualified engineer, rather than taken directly from this formula alone.
Does this formula account for the rivet's material or grade?
No — Unwin's formula relates diameter to plate thickness only; it doesn't select a rivet material, strength grade, or head style, all of which also affect a joint's actual strength and are governed by separate specifications. Use this result as a starting diameter reference, then specify material and grade according to the application's actual load and code requirements.
How is rivet sizing different from bolt sizing?
Bolts are typically sized from a standardized thread and diameter table matched to the calculated load a connection needs to carry, following a structural bolting specification, while a rivet's size has traditionally been estimated more directly from the plate thickness it passes through, as this rule of thumb does — reflecting how riveted joints were historically designed and installed before modern bolted-connection design methods became standard.