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Base Course Thickness Calculator

Work out base course thickness instantly with clear inputs, formula shown and shareable results.

About Base Course Thickness Calculator

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.

Formulas

Structural number

SN = a1 D1 + a2 D2 + a3 D3 (D in inches)

Base thickness

D2 = (SN required - a1 D1) / a2

Worked example

With the default values below, Base Course Thickness Calculator returns Base course thickness required of 478. Change any input above and every figure here is recalculated in your browser.

Inputs used

Required structural numberminimum 0.5
3.5
Asphalt surface thicknessminimum 10 mm
50 mm
Surface layer coefficient a1minimum 0.05
0.44
Base layer coefficient a2minimum 0.02
0.14

Results

Base course thickness required
478 mm
Base thickness in inches
18.81 in
Structural number from surface
0.866
Structural number provided
3.500

Step by step

  1. 1Values used
  2. 2Structural number
  3. 3Base thickness
  4. 4Base course thickness required= 478 mm
  5. 5Base thickness in inches= 18.81 in
  6. 6Structural number from surface= 0.866
  7. 7Structural number provided= 3.500

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • 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.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

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.

Frequently Asked Questions

How is the Base Course Thickness figure worked out?

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.

What sets the required structural number?

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.

What values do I need to enter?

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.

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