The calculator
Runs in your browser. Inches or millimetres at the top. Enter a thickness for each layer the rivet passes through; leave the third blank for a two-sheet joint.
How the formula works
L = grip + 1.5D. Grip is the total thickness of everything the rivet passes through — every sheet, every doubler, every shim. D is the shank diameter.
The 1.5D is not padding. It is the exact cylinder of material that gets upset by the bucking bar, and it is sized so that when it is driven flat it forms a shop head 1.5D wide and 0.5D high. Those two figures are the acceptance criteria; the 1.5D shank allowance is simply what it takes to produce them.
Then it gets rounded. Solid rivets are manufactured in 1/16" length increments, so a calculated 0.2515" becomes an ordered 5/16". Always round up — a rivet that is fractionally too long forms a slightly oversized head, while one that is too short forms no head worth the name.
Reading the dash number. The AN and MS convention catches people out because the two numbers use different fractions. In AN470AD4-5, the 4 is the diameter in thirty-seconds (4/32" = 1/8") and the 5 is the length in sixteenths (5/16"). The AD between them is the alloy — 2117.
For a countersunk AN426 rivet, grip is still the full material thickness. The countersink or dimple displaces material rather than removing it from the stack, so the length calculation does not change.
Worked example
Two sheets of 0.032" 2024-T3 joined with 1/8" AN470AD4 universal-head rivets.
- Grip = 0.032 + 0.032 = 0.064".
- D = 0.125", so the shank allowance = 1.5 × 0.125 = 0.1875".
- Required length = 0.064 + 0.1875 = 0.2515".
- Round up to the next 1/16": 0.2515 × 16 = 4.02, so 5 sixteenths = 0.3125".
- Dash number 5. Diameter 1/8" = 4/32", so the callout is AN470AD4-5.
- Driven correctly it should form a shop head about 0.188" across and 0.063" high — 1.5D by 0.5D.
AN470AD4-5. Note how close 4.02 sits to a whole number — a 4-dash rivet at 0.2500" falls only 0.0015" short of the requirement. That is less than the variation between nominal and actual sheet thickness, which is why the calculator flags this case rather than simply rounding past it.
Limits — when NOT to use this
The grip + 1.5D rule applies to solid rivets driven with a gun and bucking bar, or squeezed.
- Blind rivets do not work this way. CherryMAX, Olympic and pull-through blind rivets are specified by grip range, not by calculated length. Pick the rivet whose published grip range brackets your stack thickness and stop there.
- Approved data governs. Where an SRM, AMM or repair drawing calls out a rivet, that is the rivet.
- Measure the actual stack. Nominal sheet thickness and real sheet thickness differ, and the difference compounds across three layers. Callipers, not the drawing, on anything marginal.
- Sealant and primer add to grip. A wet-assembled joint with faying-surface sealant is measurably thicker than the sum of its sheets. On a fuel tank boundary this matters.
- Countersinking does not reduce grip. Machine countersinking removes material from the top sheet but the rivet still spans the full stack. Dimpling displaces material downward and can actually increase the effective stack slightly.
- The shop head is the real acceptance test. A correct calculation and a bad drive still gives you a rejectable rivet. 1.5D × 0.5D is checked with a rivet gauge, not assumed from arithmetic.
- Rivet length is not the way to fix an oversized hole. That is a diameter problem and a repair-design decision.
Questions people actually ask
What does the -5 mean in AN470AD4-5?
Length in sixteenths of an inch: 5/16". The 4 before it is the diameter in thirty-seconds: 4/32" = 1/8". Two different fractions in one part number, which is why this trips people up.
What if my calculated length lands exactly on a stock size?
Go up one. A rivet that exactly equals grip + 1.5D leaves nothing for real-world variation in sheet thickness, sealant or hole condition, and produces a marginal shop head.
Does a countersunk rivet need to be longer?
No. The countersink displaces or removes material from the top sheet, but the rivet still passes through the full stack thickness. Grip is unchanged, so the formula is unchanged.
How do I know the shop head is right?
Measure it. A rivet gauge checks both dimensions against 1.5D diameter and 0.5D height. On a 1/8" rivet that is 0.188" across and 0.063" high. Too flat and wide means it was overdriven; too tall and narrow means underdriven.
Can I use the same formula for 2024 icebox rivets?
Yes — the length formula is a function of geometry, not alloy. What changes with 2024-T4 (DD) rivets is that they must be driven within their time limit out of the freezer or re-heat-treated, and they take more force to drive than 2117 (AD).
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4 — “The total length of the rivet equals the grip length plus the amount of rivet shank needed to form a proper shop head… one and a half times the diameter of the rivet shank.” Figure 4-76 gives the formed-head minimum, preferred and maximum dimensions.
- FAA AC 43.13-1B CHG 1, Chapter 4, Figure 4-6(a) — formed rivet head dimensions and common rivet imperfections. The AC's figure gives 1½D as a minimum width and ½D as a minimum height — aim above them, not at them.
- FAA-H-8083-31, AMT Handbook — Airframe, Chapter 4 — AN/MS dash number conventions and shop head acceptance criteria.
- MIL-R-5674 / NASM standards — manufactured length increments for solid aluminum rivets.
Where to buy solid rivets
Once you have the callout, both distributors below stock the common AN470 and AN426 sizes.
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