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Flexible Pavement AASHTO Structural Number (SN) Calculator

Calculate a flexible pavement's structural number (SN) from its layer thicknesses and layer coefficients, per the AASHTO 1993 design method.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Structural Number (SN)

3.54 SN

Check your inputs

Layer coefficients and drainage coefficients must be selected per AASHTO 1993 guidance (or your DOT's pavement design manual) for your actual materials — this calculator only sums the SN equation from values you provide; it does not select or validate those coefficients, nor does it determine whether the resulting SN meets your required design SN from traffic (ESAL) analysis.

Running these inputs gives 3.54 as the structural number (sn). Expect some drift against the real job; the calculation is solid but conditions on site are not. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 3.54 SN — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • AASHTO 1993 Guide for Design of Pavement Structures: SN = a1×D1 + a2×D2×m2 + a3×D3×m3, where a1/a2/a3 are layer coefficients for the surface/base/subbase layers, D1/D2/D3 are layer thicknesses (in), and m2/m3 are drainage coefficients for the base/subbase layers

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

What does the Structural Number (SN) represent?
SN is an abstract index from the AASHTO 1993 flexible pavement design method that combines each layer's thickness, structural layer coefficient, and drainage coefficient into a single number representing the pavement structure's overall load-carrying ability.
Does this calculator select the right layer coefficients or check my design for me?
No. This calculator only performs the SN summation from the coefficients and thicknesses you enter — it does not select, validate, or look up AASHTO layer or drainage coefficients for your specific materials, and it does not compare the resulting SN against the design SN required by your traffic (ESAL) analysis. Those steps must be done per AASHTO 1993 guidance or your DOT's pavement design manual.
Why do base and subbase layers include a drainage coefficient but the surface layer doesn't?
In the AASHTO 1993 equation, only the unbound or granular base and subbase layers (m2 and m3) carry a drainage coefficient reflecting how quickly water drains from them; the asphalt surface layer's coefficient (a1) does not include a drainage term.