Subgrade Modulus Calculator
Estimates the resilient modulus (Mr) of a road subgrade from its CBR using the Heukelom & Klomp / AASHTO correlation. Pavement and geotechnical engineers use Mr as the stiffness input for mechanistic-empirical flexible-pavement design.
Enter Values
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
- Enter the subgrade CBR as a percentage (from a soaked or in-situ CBR test).
- Optionally change the correlation coefficient (default 10.0 MPa per %CBR; use 10.34 for the exact 1500 psi form, or a locally calibrated value).
- Read the resilient modulus Mr in MPa (and psi).
How it works
The resilient modulus describes how stiffly the subgrade responds to repeated traffic loading. It is expensive to measure directly, so it is commonly estimated from the far cheaper CBR test.
The Heukelom & Klomp relation, adopted by AASHTO, is Mr (psi) = 1500 × CBR. Because 1500 psi ≈ 10.34 MPa, the metric shorthand is Mr (MPa) ≈ 10 × CBR. The calculator multiplies your CBR by the coefficient and converts to psi at 145.038 psi per MPa.
Worked example
CBR 5 subgrade. With CBR = 5 % and the default coefficient 10.0, Mr = 50 MPa ≈ 7252 psi. Using the exact coefficient 10.34 gives 51.7 MPa (≈ 1500 × 5 psi).
Common mistakes
- Applying the correlation to coarse, granular or stabilised subgrades — it is calibrated for fine-grained soils with CBR ≲ 10–15.
- Confusing resilient modulus Mr with the modulus of subgrade reaction k — different parameters used in flexible vs rigid design.
- Treating the estimate as a measured value; for detailed design a laboratory resilient-modulus (repeated-load triaxial) test is preferred.
Frequently asked questions
What is the formula for subgrade resilient modulus from CBR?
The Heukelom & Klomp / AASHTO correlation is Mr (psi) = 1500 × CBR, equivalently Mr (MPa) ≈ 10 × CBR (since 1500 psi ≈ 10.34 MPa).
Why does the coefficient default to 10 rather than 10.34?
10 MPa per %CBR is the common metric shorthand and easy to remember. Enter 10.34 for the exact equivalent of the 1500 psi form, or a value calibrated to your local materials.
What CBR range is this valid for?
Most reliable for fine-grained subgrades with CBR up to about 10–15. Above that the linear relation over-predicts modulus and other correlations (or direct testing) should be used.
Is this suitable for final pavement design?
No. It is a first-pass estimate. For design, measure resilient modulus directly or follow the relevant AUSTROADS / AASHTO procedure.
Related tools
- CBR to Modulus of Subgrade Reaction Converter
- CBR-Based Pavement Thickness Calculator
- Structural Number Calculator
- ESAL Traffic Loading Calculator
- AASHTO Flexible Pavement Thickness Calculator
- Rigid Pavement Thickness Calculator
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