Free calculator
CBR calculator: California Bearing Ratio (ASTM D1883)
Enter the test unit loads in psi at 0.1 in and 0.2 in penetration, read from the corrected load-penetration curve. The calculator divides each by the standard unit load, shows the CBR at both depths, picks the value to report and tells you when the test should be repeated.
By ProofGrit Editorial TeamLast reviewed:
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Key takeaways
- Read both test unit loads from the corrected load-penetration curve.
- Report the 0.1 in value unless the 0.2 in value is greater.
- If the 0.2 in value is greater, retest; report the 0.2 in value only if it is still greater.
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How it's calculated
How is the result calculated?
CBR = test unit load ÷ standard unit load × 100, with 1,000 psi at 0.1 in and 1,500 psi at 0.2 in.
- 01Standard unit loads: 1,000 psi at 0.1 in penetration and 1,500 psi at 0.2 in penetration
- 02CBR at 0.1 in = test unit load at 0.1 in ÷ 1,000 × 100
- 03CBR at 0.2 in = test unit load at 0.2 in ÷ 1,500 × 100
- 04Report the 0.1 in value. If the 0.2 in value is greater, repeat the test; if it is still greater, report the 0.2 in value
- 05CBR is shown to 0.1, and the comparison uses the displayed values
Rounding: CBR to 0.1. Full precision is kept between steps, and the 0.1 in / 0.2 in comparison uses the displayed values.
FAQ
Frequently asked questions about the cbr calculator
What is the California Bearing Ratio?
A comparison of the unit load needed to push a piston into a soil specimen with the standard unit load at the same penetration, expressed as a percentage.
Why are there two penetration depths?
The test reads the load at 0.1 in and 0.2 in. The 0.1 in value is normally reported; the 0.2 in value decides whether a retest is needed.
What happens if the 0.2 in CBR is higher?
Repeat the test. If the 0.2 in value is still greater on the retest, report the 0.2 in value. Select "Yes, still greater" in the calculator after the retest.
Should I enter loads in lbf?
No. Enter unit loads in psi: the load on the piston divided by its end area, read from the corrected load-penetration curve.
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- Soil compaction testing: field and lab guideHow laboratory Proctor references, field density tests and project acceptance work together in soil compaction testing.
- The Proctor test: how it works and how to read the curveWhat a Proctor test measures, how each compaction point is calculated, how the curve gives MDD and OMC, and how the zero air voids line keeps you honest.
- ASTM D698: Standard Proctor test guideHow ASTM D698 establishes a Standard Proctor moisture-density reference, what to record, and how field compaction results use the reference.
- Soil testing softwareHow ProofGrit handles this test in the lab.
- Standards indexEvery standard explained on this site, with plain-English purpose.
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Sources and further reading
- ASTM InternationalASTM International
- ASTM D698, laboratory compaction using standard effortASTM International
- FHWA pavement programFederal Highway Administration
Standards are summarized in our own words. Read the current edition from the publisher before you test.
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