Tip: If you only know CBR, choose the CBR method and the tool estimates subgrade MR using a practical rule-of-thumb.
Formula Used
This calculator estimates the required structural number (SN) from the AASHTO 1993 flexible pavement relationship:
How to Use This Calculator
- Enter total design traffic in ESALs (W18).
- Choose reliability and set So, Pi, and Pt for performance expectations.
- Provide subgrade strength using MR or estimate it from CBR.
- Adjust material layer coefficients (a1,a2,a3) and drainage factors (m2,m3) as needed.
- Set minimum/maximum layer thickness constraints and click calculate.
Example Data Table
Sample inputs and typical output ranges for illustration. Replace with your project-specific values.
| Case | W18 (ESALs) | Reliability | MR (psi) | Typical SN | Indicative Thickness (D1/D2/D3) |
|---|---|---|---|---|---|
| Urban collector | 500,000 | 90% | 10,000 | 3.0–4.0 | 4–6 in / 6–10 in / 6–12 in |
| Arterial | 2,000,000 | 95% | 8,000 | 4.0–5.5 | 5–8 in / 8–14 in / 8–18 in |
| Heavy freight route | 10,000,000 | 98% | 6,000 | 5.5–7.5 | 7–10 in / 12–18 in / 12–24 in |
Traffic and ESAL planning
W18 is the cumulative 18‑kip equivalent single axle loads over the design life. Build it from current counts, axle spectra, growth, lane distribution, and directional split. Because the AASHTO relationship uses log10(W18), increases in traffic produce diminishing returns, but heavy corridors still push SN upward and can shift thickness from granular layers to asphalt.
Reliability and serviceability targets
Reliability (R) reflects risk tolerance for traffic and material variability. Higher R makes ZR more conservative and increases the required SN; values commonly range from 80% for low‑volume roads to 95–99% for major facilities. Serviceability loss ΔPSI = Pi − Pt represents allowable ride and distress change before rehabilitation. Choosing a lower Pt or higher R is typical for strategic routes where downtime and user costs are high.
Subgrade strength inputs
Subgrade resilient modulus (MR) is the stiffness term that strongly influences design thickness. Use laboratory or back‑calculated MR where possible, and keep units consistent. If only CBR is available, the calculator estimates MR using a practical conversion for preliminary sizing. Apply seasonal adjustments when moisture softening is expected, and consider using a reduced “design MR” for conservative results.
Layer coefficients and drainage factors
Layer coefficients (a1, a2, a3) convert thickness into structural contribution. Asphalt typically provides the highest SN per inch, while base and subbase depend on gradation, compaction, and moisture sensitivity. Drainage factors (m2, m3) account for time near saturation; poor drainage lowers effective support. If drainage is uncertain, improve outlets or edge drains before simply adding thickness.
Interpreting the output and constraints
The calculator solves for required SN, then allocates D1/D2/D3 within your minimum and maximum limits. Compare SN provided versus SN required to confirm capacity and document any assumptions. “Below target” means the constraints prevented reaching SN. Increase maximum thickness, select stronger materials (higher coefficients), enhance drainage, or revisit W18 and reliability assumptions. Export PDF/CSV to document inputs, assumptions, and the final layer stack for review and documentation.
FAQs
1) What does SN represent in this calculator?
SN is the required structural capacity of the pavement system. It combines the contribution of each layer using coefficients and drainage factors, allowing different layer combinations to meet the same design demand.
2) Can I use CBR instead of MR?
Yes. Select the CBR option and enter your CBR value. The tool estimates MR using a practical conversion for preliminary sizing. Replace the estimate with tested MR when you have project data.
3) Why does higher reliability increase thickness?
Higher reliability reduces the acceptable risk of early failure. The model applies a more conservative normal deviate term, which increases the structural number required to carry the same traffic.
4) What should I do if the result shows “Below target”?
Increase one or more maximum thickness limits, choose higher quality materials (higher layer coefficients), improve drainage (higher m factors), or verify that W18, serviceability, and reliability inputs match the design intent.
5) Are thicknesses output in inches or millimeters?
The design thicknesses are calculated in inches for the structural number equation. The tool also converts each layer thickness to millimeters so you can use either unit in drawings and estimates.
6) Does the PDF export include my inputs?
Yes. The PDF report summarizes key inputs, required SN, layer thicknesses, and the provided SN status. Add optional notes in the form to capture assumptions or site conditions for reviewers.