CBR to Mr Converter – Resilient Modulus & CBR Conversion
Input Parameters
Calculation Results
What is California Bearing Ratio (CBR) to Resilient Modulus (Mr) Converter?
The California Bearing Ratio (CBR) is a penetration test value that indicates the strength of subgrade, subbase or base materials relative to standard crushed stone. The Resilient Modulus (Mr) is the elastic stiffness of soil under repeated traffic loading, used in modern mechanistic-empirical pavement design (MEPDG).
The California Bearing Ratio (CBR) to Resilient Modulus (Mr) Converter for Highway/Civil Engineers is a fast and accurate online tool that instantly converts CBR (%) to Mr (psi or MPa) and vice-versa using the most widely accepted empirical correlations (AASHTO 1993, NCHRP 1-37A/MEPDG Level 2, FHWA). Perfect for CBR to Mr conversion calculator, subgrade resilient modulus estimation, pavement design Mr from CBR, MEPDG Level 2 input, soaked CBR correlation, and quick highway subgrade stiffness checks.
This CBR to Mr converter provides relevant visualizations, a dedicated section for comments, analysis and recommendations, full step-by-step calculation, CSV export/download of results, and a Colorblind view for improved accessibility.
How to use CBR to Mr Converter?
Purpose: Convert laboratory or field CBR value to the resilient modulus Mr required for AASHTO 1993, MEPDG, or any mechanistic pavement design software.
Inputs you will enter:
- CBR value (%)
- Desired output units (psi or MPa)
- Soil type / classification (optional – selects best formula)
- Test condition (soaked / optimum / as-compacted)
CBR to Resilient Modulus Formula
\(M_r = 1500 \times CBR\) (AASHTO 1993 – fine-grained subgrade, psi)
\(M_r = 2555 \times CBR^{0.64}\) (NCHRP 1-37A / MEPDG Level 2, psi)
\(M_r = 17.6 \times CBR^{0.64}\) (Metric version, MPa)
\(M_r \approx 10 \times CBR\) (Simple metric approximation, MPa)
Where:
- CBR = California Bearing Ratio (%)
- Mr = Resilient Modulus (psi or MPa)
- Exponent 0.64 and constants are from regression on repeated-load triaxial tests
How to Calculate CBR to Mr (Step-by-Step)
- Enter measured CBR value (%).
- Select output units (psi or MPa).
- Choose soil type if known (auto-selects best formula).
- Select test condition (soaked is most conservative).
- Calculator applies the recommended formula and shows all alternatives.
- Inverse conversion (Mr → CBR) is also shown.
- Review range check and recommendations.
Examples
Example 1 – Typical Clayey Subgrade (Metric) Soaked CBR = 5 %, fine-grained soil Using MEPDG: \(M_r = 17.6 \times 5^{0.64} \approx 17.6 \times 2.94 \approx 52\ \text{MPa}\) (AASHTO simple: ≈ 34 MPa – more conservative)
Example 2 – Granular Subbase (US units) CBR = 25 %, granular material Using MEPDG: \(M_r = 2555 \times 25^{0.64} \approx 2555 \times 9.3 \approx 23,760\ \text{psi}\) (AASHTO linear would give only 37,500 psi – overestimates)
CBR to Mr Categories / Normal Range
| Soil Type | Typical CBR (%) | Typical Mr (psi) | Typical Mr (MPa) | Recommended Formula |
|---|---|---|---|---|
| Fine-grained clay (A-6/A-7) | 3–10 | 4,500–15,000 | 30–100 | AASHTO 1500× or MEPDG power |
| Silty sand (A-2-4/A-4) | 10–30 | 15,000–45,000 | 100–310 | MEPDG power |
| Granular base/subbase | 30–80 | 40,000–120,000 | 275–830 | MEPDG power or stress-dependent |
| Crushed stone (base) | >80 | >100,000 | >690 | Use direct lab Mr |
Limitations
- All formulas are empirical correlations — not exact mechanistic values.
- Best accuracy for soaked CBR of fine-grained subgrades (CBR < 20).
- Granular materials and high CBR values require stress-dependent Mr (not covered here).
- Regional soils may need local calibration (e.g., Indian, Australian correlations differ).
- Does not include seasonal variation, confining stress, or deviator stress effects.
Disclaimer
This calculator is provided for educational purposes, learning, and preliminary design checks only. All final pavement designs must be verified with laboratory resilient modulus testing (AASHTO T307) and approved by a qualified professional pavement/highway engineer. The developer and platform are not liable for any errors, misinterpretations, or consequences arising from the use of these results in actual road construction projects.