Article — Rivet Size Calculator
Rivet size calculator: diameter, length, and hole for any joint
A rivet is a permanent fastener formed by upsetting a metal pin to clamp two or more sheets together. The minimum shank diameter is D = 3t, where t is the thickness of the thickest single layer. Rivet length is L = T + 1.5D (Unwin formula), where T is total grip thickness. For material thicker than 8 mm, use Unwin's d = 6.05√t instead of the 3t rule.
Sizing rivets is more discipline than design: standards and tables already exist for every common joint. The calculator picks the nearest standard size at or above the calculated minimum, then returns matching length and hole diameter.
What is a rivet?
A rivet is a one-shot mechanical fastener. You drop a smooth-shank pin with one preformed head through aligned holes, then deform the opposite end into a second head with hammer, bucking bar, or a hydraulic squeeze tool. The clamping force holds the parts together for the life of the joint — rivets cannot be removed without drilling them out.
Rivets predate every other permanent metal fastener. The Eiffel Tower used 2.5 million hand-driven hot rivets in 1889 and is still standing. The Boeing 747 holds together with roughly 2 million aluminum AN-rivets. Modern construction has shifted to bolts and welds for most joints, but aerospace, pressure vessels, and sheet metal work still depend on rivets because they distribute load smoothly without the heat-affected zone problems of welding.
The Statue of Liberty has 300,000 wrought iron rivets holding its 31-ton copper skin to the steel armature inside. Each rivet was hand-forged on site between 1884 and 1886. Restoration crews in the 1980s found over 90% of the original rivets still sound after a century of salt-air exposure.
The 3t rivet size rule
The classic engineering rule is D = 3t. The minimum rivet diameter equals three times the thickness of the thickest single layer in the joint. For 2 mm aluminum sheet, the minimum rivet is 6 mm, which rounds up to a standard 6.4 mm rivet. For 1/16 inch steel, the minimum is 3/16 inch.
The rule balances two failure modes. Make the rivet too small and the shank shears off under load. Make it too large and the hole removes so much parent material that the sheet tears around the rivet. D = 3t hits a sweet spot for material up to about 8 mm thick. Above that, the Unwin formula d = 6.05√t takes over — the linear 3t rule starts oversizing rivets badly.
D = 3t thin stock under 8 mmd = 6.05√t Unwin (thick stock)L = T + 1.5D length with bucking allowancehole = D + 0.08 mm clearanceedge distance ≥ 2D prevents tear-outCalculating rivet length
Rivet length is total grip thickness plus enough extra shank to form the bucked head. Unwin's formula L = T + 1.5D works for solid rivets driven against a bucking bar. T is the sum of all layer thicknesses, not just the thickest. Two 2 mm sheets gives T = 4 mm.
The 1.5D allowance is empirical. Too little extra and the bucked head is undersize or absent — the joint fails immediately under shear. Too much and the rivet buckles sideways while being driven, leaving a curved shank that pinches the parts unevenly. Aircraft inspectors check head diameter (must be at least 1.4D) and head height (about 0.3D) on every rivet in a structural joint.
- T = sum of all layer thicknesses (mm or in)
- 1.5D = bucking allowance for forming head
- Min head = 1.4 × D diameter, 0.3 × D height
- Driving force = 6.4 kN for 4 mm aluminum rivet
- Hot rivets shrink as they cool, increasing clamp force
- Cold rivets rely on the upsetting force alone
Rivet hole diameter and clearance
The hole is always slightly larger than the rivet shank: D + 0.08 mm in metric, D + 0.003 inch in US sizing. The clearance lets the rivet slip through both holes without forcing, but stays tight enough that the rivet does not tip while being driven.
Holes more than 0.15 mm oversize give weak joints. The rivet cannot fully fill the hole when upset, so the joint relies on friction instead of the bearing surface of the shank. Aerospace inspectors gauge every drilled hole before fastening. Construction work is more forgiving, but sloppy holes are still the most common cause of premature rivet failure.
A steel rivet in aluminum sheet, or a copper rivet in steel, forms a galvanic cell in the presence of moisture. The less noble metal corrodes preferentially — aluminum dissolves around steel rivets, steel rusts around copper rivets. Always match rivet material to the parts, or use stainless steel (304/316) with an insulating sealant between the layers. The Statue of Liberty's century of corrosion damage came mostly from galvanic action between iron rivets and copper skin where the asbestos insulation had decayed.
Rivet types and uses
Solid rivets are the strongest and oldest type. They need access from both sides — one side held with a bucking bar while the other is driven by pneumatic gun or hammer. Aircraft structural joints, pressure vessels, and historic structures use solid rivets almost exclusively.
Blind rivets (pop rivets) install from one side using a mandrel that pulls a flared head into the hole and snaps off. About half the strength of solid rivets the same diameter, but vastly faster and require no access to the back side. Structural blind rivets (Huck, Avdel) use a controlled mandrel break and achieve solid-rivet-class strength while keeping single-side installation.
Choosing rivet material
Aluminum 2117 (AD designation) is the workhorse for aluminum aircraft structure. It is soft enough to drive cold and strong enough for most airframe joints. Aluminum 5056 handles harsher environments and slightly higher loads.
Mild steel rivets dominate construction and heavy machinery, hot-driven historically and cold-driven today. Stainless steel 304 and 316 cover marine, food-service, and corrosion-prone work. Copper and brass rivets serve copper sheet, leather, and decorative joints. Monel handles cryogenic and high-temperature applications where aluminum and steel both fail.
Match rivet hardness to the softer of the two parts. A hard rivet in soft sheet will crush the sheet around the hole before the rivet upsets properly. Aluminum 2117 rivets are softer than 2024 or 7075 sheet, which is by design.
Common rivet mistakes
The first mistake is using one big rivet where two smaller ones would work better. Doubling the diameter quadruples the rivet's shear capacity but also quadruples the hole area lost from the parent material. Two rivets at proper 3D spacing carry the load with less weakening of the sheet.
The second mistake is wrong rivet length. Aerospace inspectors reject rivets where the bucked head diameter is below 1.4D or height is below 0.3D. Both errors trace to length problems — too short and there is not enough metal to form the head; too long and the rivet buckles sideways during driving.
The third is hole quality. Drilled holes should be perpendicular to the surface, the right diameter, deburred on both sides, and free of swarf. A burr 0.1 mm tall under the rivet head leaves a gap that lets the joint work loose under vibration.
ISO, MIL, and AN rivet standards
ISO 1051 defines metric solid rivet shank diameters: 2, 2.4, 3, 3.2, 4, 4.8, 5, 6.4, 8, 10, 12, 14, 16, 18, 20 mm. DIN 660 (round head), DIN 661 (countersunk), and DIN 662 (lens head) cover the head styles. ISO 15983 defines blind rivet dimensions.
US aerospace work uses AN (Army-Navy) and MS (Military Standard) designations: AN-470 round head, AN-426 countersunk 100°, AN-455 brazier head. Dash numbers encode diameter in 1/32 inch and length in 1/16 inch. AN470AD4-6 is a 4/32 inch (1/8 in) diameter, 6/16 inch (3/8 in) length aluminum 2117 rivet with round head. NASA-STD-5020 governs structural fastening for spaceflight hardware.