Road Pavement Thickness Calculator

Work out road pavement thickness instantly with clear inputs, formula shown and shareable results.

About Road Pavement Thickness Calculator

Flexible pavement thickness grows with the logarithm of design traffic and falls as subgrade strength rises. This chart approximation uses a 400 mm reference for 10 million standard axles on a CBR 5 subgrade and scales by (5/CBR)^0.4. The granular thickness left after the surfacing is split roughly 45 percent base and 55 percent sub-base.

Formulas

Total thickness

T mm = 200 x (log10 msa + 1) x (5 / CBR)^0.4

Layer split

Granular = T - surfacing; base = 45 percent of granular, sub-base the remainder

Worked example

With the default values below, Road Pavement Thickness Calculator returns Total pavement thickness of 400. Change any input above and every figure here is recalculated in your browser.

Inputs used

Subgrade CBRaccepts 1–40 %: 5 %
Design trafficminimum 0.1 msa: 10 msa
Bituminous surfacing thicknessminimum 10 mm: 50 mm

Results

Total pavement thickness: 400 mm
Granular layers thickness: 350 mm
Base course thickness: 158 mm
Sub-base thickness: 193 mm

Step by step

1Values used
2Total thickness
3Layer split
4Total pavement thickness= 400 mm
5Granular layers thickness= 350 mm
6Base course thickness= 158 mm
7Sub-base thickness= 193 mm

Assumptions and limits

The result is derived only from the 3 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 whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Pavement design must follow the design guide adopted by the road authority and be verified by a qualified highway engineer.

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 follow the design guide adopted by the road authority and be verified by a qualified highway engineer.

Frequently Asked Questions

How is the Road Pavement Thickness figure worked out?

T mm = 200 x (log10 msa + 1) x (5 / CBR)^0.4. Flexible pavement thickness grows with the logarithm of design traffic and falls as subgrade strength rises.

Why does a weak subgrade cost so much thickness?

The pavement has to spread the wheel load until the stress reaching the subgrade is tolerable. Halving CBR raises the required thickness by roughly a third, which is why subgrade improvement often pays for itself.

What values do I need to enter?

This calculator takes 3 inputs: Subgrade CBR, Design traffic, Bituminous surfacing thickness. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.