Zerdly

CBR-Based Pavement Thickness Calculator

Calculates the total flexible-pavement cover thickness required over a subgrade using the US Army Corps of Engineers (USACE) single-wheel CBR design equation. It is a quick first-pass check for haul roads, access tracks and low-volume pavements.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the wheel load P in kN and the tyre pressure p in kPa.
  2. Enter the subgrade CBR as a percentage.
  3. Read the required cover thickness in mm (and inches); the tyre contact area A is also shown.

How it works

The USACE method sizes the total thickness of material needed above the subgrade so a single wheel does not overstress it. The design equation is t = √( P/(8.1·CBR) − A/π ), with t in inches, P the wheel load in pounds, CBR in percent and A the tyre contact area in square inches.

Because Zerdly is metric-first, the calculator converts your inputs to imperial (P_lb = P × 224.809; p_psi = p × 0.145038), computes A = P_lb / p_psi, evaluates t in inches and converts to millimetres (× 25.4). If the term under the square root is not positive — a very high CBR or very light load — the required cover is effectively nil and the tool reports that.

Worked example

40 kN wheel on CBR 5. P = 40 kN → 8992 lb; p = 550 kPa → 79.77 psi; A = 112.7 in². Then t = √(8992/(8.1×5) − 112.7/π) = √(222.0 − 35.9) = √186.2 = 13.64 in = 347 mm of cover.

Common mistakes

  • Entering axle load instead of the individual wheel load P.
  • Using it for heavy, channelised traffic — this single-wheel form ignores repetitions and multi-wheel (ESAL) effects.
  • Forgetting it gives total cover thickness above the subgrade, not the thickness of any one layer.

Frequently asked questions

What is the CBR pavement thickness formula?

The USACE single-wheel equation is t = √( P/(8.1·CBR) − A/π ), where t is cover thickness (in), P is wheel load (lb), CBR is the subgrade CBR (%) and A = P/p is the tyre contact area (in²).

What units does this calculator use?

Metric inputs — wheel load in kN, tyre pressure in kPa, CBR in %. It converts internally to imperial and reports thickness in mm and inches.

Why does it sometimes say the thickness is nil?

If the CBR is high enough (or the load light enough) that P/(8.1·CBR) is smaller than A/π, the term under the root is negative — the subgrade carries the wheel with negligible cover.

Is this the right method for a modern road?

It is an empirical single-wheel check suited to unsealed and low-volume pavements. Structural design of trafficked roads should follow AUSTROADS or AASHTO mechanistic-empirical procedures.

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