🛣️ Asphalt Thickness

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · A documented rule of thumb, pinned by automated tests · how PhDino checks its numbers

A simplified planning estimate for how thick a driveway or parking-lot pavement section needs to be.

A paved surface is really a layered system: an asphalt surface course on top of an aggregate base course, both resting on the natural subgrade soil. Each layer's job is to spread out the concentrated pressure of a tire contact patch until, by the time the load reaches the subgrade, it's spread thin enough that the underlying soil can carry it without rutting or failing.

The subgrade's own strength is commonly measured by its California Bearing Ratio (CBR), a lab test comparing the soil's resistance to a standard penetration test against a reference crushed-stone material. A weaker subgrade (lower CBR) needs a thicker pavement structure above it to spread the same traffic load down to a tolerable pressure — and that relationship is not linear: going from a very weak to a moderately weak subgrade helps a lot, while further strengthening an already-decent subgrade helps comparatively little. That diminishing-returns shape is why pavement thickness design methods relate required thickness to the logarithm of CBR rather than to CBR directly.

Key formula

Total thickness ≈ load factor × (10 − 3 × log₁₀(CBR)), in — a simplified planning relationship, not a full design method

Variables

CBR

California Bearing Ratio, a percentage measure of subgrade strength

Notes & limitations

• This is a rough planning estimate, not a substitute for a full flexible-pavement design method like the AASHTO 1993 Guide, which also accounts for traffic loading in equivalent single-axle loads over the pavement's design life, drainage, and reliability — all of which move the real answer meaningfully from a simplified CBR-only estimate.
• A residential driveway and a loading dock apron are not the same design problem even at the same subgrade CBR — traffic load category matters as much as soil strength.