Pavement Design (CBR) Calculator

Work out pavement design (cbr) instantly with clear inputs, formula shown and shareable results.

About Pavement Design (CBR) Calculator

The CBR method sizes a flexible pavement from soaked subgrade strength and cumulative standard axles. Thickness grows with the logarithm of traffic and falls as CBR rises, scaled here from a 400 mm reference for 10 msa on a CBR 5 subgrade. The correlation E = 17.6 CBR^0.64 converts CBR to an elastic modulus for mechanistic checks.

Formulas

Total thickness

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

Subgrade modulus

E MPa = 17.6 x CBR^0.64

Worked example

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

Inputs used

Soaked subgrade CBRaccepts 1–40 %: 5 %
Cumulative design trafficminimum 0.1 msa: 10 msa
Bituminous layer thicknessminimum 20 mm: 80 mm

Results

Total pavement thickness: 400 mm
Granular layers thickness: 320 mm
Estimated subgrade modulus: 49.3 MPa
Design note: Moderate subgrade, standard design applies

Step by step

1Values used
2Total thickness
3Subgrade modulus
4Total pavement thickness= 400 mm
5Granular layers thickness= 320 mm
6Estimated subgrade modulus= 49.3 MPa
7Design note= Moderate subgrade, standard design applies

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 1 decimal place, 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 Pavement Design (CBR) figure worked out?

T mm = 200 x (log10 msa + 1) x (5 / CBR)^0.4. The CBR method sizes a flexible pavement from soaked subgrade strength and cumulative standard axles.

Why is CBR tested soaked?

Because the design condition is the worst credible moisture state. A subgrade tested at optimum moisture can show three times the soaked CBR and would produce a dangerously thin pavement.

What values do I need to enter?

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