Calculator
Example data table
| Sample | Load unit | Load @ 2.5 mm | Load @ 5.0 mm | CBR 2.5 (%) | CBR 5.0 (%) | Governing CBR (%) |
|---|---|---|---|---|---|---|
| CBR-01 | kN | 6.00 | 9.20 | 44.98 | 45.68 | 45.68 |
| CBR-02 | kN | 8.40 | 12.10 | 62.97 | 60.08 | 62.97 |
| CBR-03 | kN | 10.70 | 15.80 | 80.21 | 78.45 | 80.21 |
Formula used
The California Bearing Ratio compares the measured load on a standard plunger to a reference load at the same penetration:
CBR (%) = ( Test Load at penetration / Standard Load at penetration ) × 100
- Standard load at 2.5 mm: 13.34 kN (≈1370 kgf)
- Standard load at 5.0 mm: 20.14 kN (≈2055 kgf)
- Governing CBR: the higher of the two calculated values
How to use this calculator
- Select the load unit used in your lab record.
- Enter the corrected load at 2.5 mm and/or 5.0 mm penetration.
- Optionally record moisture content, dry density, and any surcharge.
- Press Calculate CBR to view results above the form.
- Use Download CSV or Download PDF for reporting.
What CBR indicates for pavement support
California Bearing Ratio (CBR) expresses subgrade resistance to a standard penetration piston relative to a reference crushed rock. Higher CBR generally means a stiffer foundation, lowering required granular base and surfacing thickness. Because it is a ratio, consistent compaction energy, surcharge, and unit conversion are essential for comparable results. Standard loads are referenced at 2.5 mm and 5.0 mm penetrations.
Typical CBR ranges and practical meaning
Soft, saturated clays often test below 3%, where deformation and pumping risk rises under repeated axle loads. Silty soils commonly fall around 3–8% when compacted near optimum moisture and protected from standing water. Well-graded sands and gravels may reach 10–30% or more, enabling thinner granular layers. Confirm local minimums and any seasonal reductions before finalizing design. Use CBR for subgrade, not base layers.
Moisture, density, and soaking effects
CBR is highly sensitive to moisture content and dry density. Added water can soften fines and increase pore pressures during loading, while higher density typically increases interparticle friction and strength. Soaked CBR is frequently specified to represent worst-season performance; record swell, moisture, and surcharge so the value can be interpreted correctly in reviews. Track test temperature and seating to reduce scatter between operators.
Using CBR in thickness and material selection
Design catalogues often convert subgrade CBR into classes that control total pavement thickness and base quality. Low values usually require thicker base, improved drainage, and stricter compaction targets, especially for heavy traffic. When improvement is economical, consider lime or cement treatment, geosynthetics, select fill replacement, or blending and re-compaction. Use the governing CBR from 2.5 mm and 5.0 mm results based on your authority’s rule. Document the improvement method, curing time, and retest CBR for acceptance.
Field and lab checks to improve reliability
Correlate laboratory CBR with in-situ checks such as DCP, plate load, or resilient modulus where available, and reconcile differences caused by confinement and moisture. Sample multiple locations, report variability, and use representative percentiles rather than a single peak. Verify density and moisture during placement, and re-test after rainfall, borrow-source changes, or stabilization to keep the design input defensible.
FAQs
1. Which load values should I enter?
Enter the corrected load recorded at 2.5 mm and/or 5.0 mm penetration. If your sheet is in kgf or lbf, select that unit so the calculator converts it consistently before computing the ratio.
2. Should I use the 2.5 mm or 5.0 mm CBR?
Many standards report both and adopt the higher as governing, while some agencies prefer the 2.5 mm result unless the 5.0 mm value is higher by a specified margin. Follow your project authority.
3. What if I only have one penetration reading?
You can calculate CBR with a single reading. Provide either 2.5 mm or 5.0 mm load and the tool will compute that CBR and mark it as the available governing value.
4. Why is soaked CBR usually lower?
Soaking increases moisture, softens fine-grained soils, and reduces effective stress under the piston. That combination lowers penetration resistance and better represents the weakest seasonal condition.
5. Does surcharge change the CBR value?
Surcharge affects confinement and seating during the test, which can influence measured resistance. Use the surcharge field to document the test setup; the CBR formula still uses the measured load at penetration.
6. How do I export results for reports?
After calculating, use Download CSV for a spreadsheet-friendly row of inputs and outputs, or Download PDF for a formatted summary suitable for site records and submissions.