The calculator
Runs in your browser. Inches or millimetres at the top. The limits appear as soon as you pick a diameter; add a seam length for a real layout.
How the rule works
Like edge distance, pitch is expressed in multiples of D, the rivet shank diameter.
Minimum pitch: 3D. Rivets closer than three diameters apart stop behaving as independent fasteners. The stress fields around adjacent holes overlap, and the sheet fails by tearing along the rivet line rather than by bearing at each hole. On a 1/8" rivet that floor is 0.375".
Typical pitch: 4D to 6D. Where general sheet metal repair actually lives. 4D is the common default; 6D is about as far as you go before you should be checking the drawing rather than a rule of thumb.
Transverse pitch: 75% of the pitch, never below 2.5D. Rows sit closer together than rivets within a row. The 75% figure is the standard relationship; the 2.5D floor is the hard limit.
The number that surprises people is the count. A seam is not divided by the pitch — the two end rivets have to sit at their edge distance first, and only the span between them gets divided. On a 12" seam with 1/8" rivets that is 0.313" swallowed at each end before a single space is measured.
On the maximum. There is no hard ceiling the way there is a 4D maximum edge distance, but the FAA Airframe Handbook does give a range: “The average rivet pitch usually ranges from 4 to 6 rivet diameters, although in some instances rivet pitch could be as large as 10 rivet diameters.” So 10D is documented, not forbidden. What limits it in practice is the specific joint — pressurisation, fluid tightness, skin buckling between fasteners. Past 6D, stop using a rule of thumb and go to the drawing.
Worked example
A 12.000" seam, single row of AN470AD4 universal-head rivets — 1/8" shank.
- D = 0.125". Preferred edge distance = 2.5D = 0.313".
- Both ends take an edge distance, so the span from first rivet center to last is 12.000 − (2 × 0.313) = 11.375".
- Aim for a 4D pitch = 0.500". 11.375 ÷ 0.500 = 22.75 spaces — not a whole number, so it has to move.
- Round to 23 spaces, which means 24 rivets.
- Actual pitch = 11.375 ÷ 23 = 0.495", or 3.96D. Inside the 3D–6D band, so the layout stands.
- Divide the naive way — 12.000 ÷ 0.500 = 24 spaces — and you get 25 rivets with the end ones sitting outside the sheet.
24 rivets. First hole 0.313" from the end, then every 0.495" (12.6 mm).
Limits — when NOT to use this
General practice for solid aluminum rivets in sheet metal. Not authority for a specific repair.
- Approved data governs. An SRM, AMM, repair drawing or approved 8110-3 overrides every number here.
- Pressurised structure is a different problem. Fuselage pressure skins, fuel tank boundaries and fluid-tight seams have pitch requirements driven by sealing and fatigue, not by the general rule.
- Repeating an existing pattern beats calculating a new one. On a patch that ties into existing structure, match the surrounding rivet pattern. The calculator is for new layout, not for second-guessing the aircraft.
- Not for blind or special fasteners. CherryMAX, Hi-Lok and lockbolt spacing comes from the manufacturer's data.
- Not for composites. Fastener spacing in laminates is governed by layup and bearing/bypass behaviour.
- The calculator centers the pattern. It assumes equal edge distance at both ends. Real seams tie into ribs, doublers and existing holes that decide where rivets go regardless of what divides evenly.
- The layout is clamped to 3D–6D. Where a seam length would force a wider pitch, the calculator adds a rivet rather than return a pitch above 6D. The Handbook documents pitches as large as 10D, so a wider spacing is not automatically wrong — it is simply outside what this tool proposes on its own.
Questions people actually ask
What is the minimum rivet spacing?
3D center to center — three times the rivet shank diameter. For a 1/8" rivet that is 0.375"; for 3/32" it is 0.281".
Is pitch measured center to center or edge to edge?
Center to center, the same convention as edge distance. Measuring hole edge to hole edge understates the pitch by one full diameter.
How far apart should rows of rivets be?
About 75% of the pitch you are using along the row, and never closer than 2.5D. With a 0.500" pitch on 1/8" rivets that puts rows at roughly 0.375" — which is 3D, comfortably above the 0.313" floor.
Should rows be staggered or in line?
Staggered, unless the drawing says otherwise. Rivets in line across adjacent rows create a continuous perforated line through the sheet — effectively a tear-along-the-dotted-line. Staggering spreads the load path.
Why does the calculator give me 24 rivets when 12 divided by 0.5 is 24 spaces?
Because spaces and rivets are not the same count, and because the seam length is not the span being divided. Two end distances come off the length first, then the remainder is divided into spaces, and the rivet count is spaces + 1.
Sources
- FAA AC 43.13-1B CHG 1, Chapter 4, ¶4-57 and Figure 4-5 — the 2D minimum edge distance and 3D minimum pitch for single-row rivets, verbatim; row minimums for double and triple rows in Figure 4-5. Note that the AC does not state a maximum edge distance, a countersunk minimum, or a transverse-pitch percentage — those come from the handbook below.
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4 — “It should not be less than 2 or more than 4 rivet diameters and the recommended edge distance is about 2½ rivet diameters… the minimum edge distance for countersunk rivets is 2½ times the diameter of the rivet.” Also the source for transverse pitch at 75% of pitch with a 2½ D floor, and for rivet length = grip + 1.5D.
- FAA-H-8083-31, AMT Handbook — Airframe, Chapter 4 — rivet layout terminology, staggered vs in-line patterns.
Rivet layout card, printable
Edge distance, pitch and transverse pitch for all five common rivet sizes on one page, inches and mm. Sized for a toolbox lid.