Work out base course thickness instantly with clear inputs, formula shown and shareable results.
The AASHTO structural number method treats a flexible pavement as layers whose contributions add: SN = a1 D1 + a2 D2 + a3 D3, with thicknesses in inches. Given the required SN and the surface course already chosen, the base thickness is whatever is needed to make up the shortfall at the base layer coefficient, typically 0.14 for a granular base.
Structural number
SN = a1 D1 + a2 D2 + a3 D3 (D in inches)
Base thickness
D2 = (SN required - a1 D1) / a2
Preliminary estimate only. Pavement design must be carried out by a qualified highway engineer using the design method and traffic data required by the road authority.
SN = a1 D1 + a2 D2 + a3 D3 (D in inches). The AASHTO structural number method treats a flexible pavement as layers whose contributions add: SN = a1 D1 + a2 D2 + a3 D3, with thicknesses in inches.
Design traffic in equivalent single axle loads, subgrade resilient modulus, reliability and the serviceability loss allowed. It comes from the AASHTO design equation or nomograph.
This calculator takes 4 inputs: Required structural number, Asphalt surface thickness, Surface layer coefficient a1, Base layer coefficient a2. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.