Pavement Thickness Calculator
Traffic Volume and Subgrade Strength Pull Pavement Thickness in Opposite Directions
More traffic over a pavement's design life demands a thicker structure; a stronger subgrade beneath it allows a thinner one. This calculator captures that trade-off in a single simplified formula — thickness grows with the logarithm of design traffic (ESAL, or Equivalent Single Axle Loads) and shrinks with the square root of subgrade strength (CBR, or California Bearing Ratio) — following the general shape of full AASHTO-style pavement design without reproducing its complete methodology.
The Formula
Thickness by Traffic Level and Subgrade Strength
| ESAL | CBR 3 (weak) | CBR 6 (moderate) | CBR 10 (strong) |
|---|---|---|---|
| 10,000 | 9.24 in | 6.53 in | 5.06 in |
| 100,000 | 11.55 in | 8.16 in | 6.32 in |
| 1,000,000 | 13.86 in | 9.80 in | 7.59 in |
Across two full orders of magnitude of traffic (10,000 to 1,000,000 ESAL), required thickness only grows by roughly 50% for a given subgrade — the logarithmic relationship means traffic has to increase enormously to demand a much thicker pavement, while subgrade strength has a comparatively larger effect.
Where This Gets Used
- Early-stage pavement planning — a quick sanity check on pavement thickness before running the full AASHTO 1993 design procedure.
- Comparing subgrade improvement options — seeing how much thickness (and material cost) can be saved by improving subgrade CBR through stabilization versus building a thicker section over weak soil.
- Teaching the traffic-versus-strength trade-off — a concrete illustration of how the two dominant variables in pavement design interact.
How to Use This Calculator
- Choose Unit system — Imperial (in) or Metric (cm).
- Enter Design Traffic (ESAL) — the cumulative equivalent single axle loads expected over the design life.
- Enter Subgrade CBR Value.
- Select Calculate to see the estimated pavement thickness.
Related Calculations
Check the traffic loading behind this design with the Traffic Load Calculator, or size the culvert draining this roadway with the Culvert Sizing Calculator.