California Bearing Ratio (CBR) is a measure of subgrade soil strength expressed as a percentage of the load required to penetrate a standard crushed-stone sample. A CBR of 100% corresponds to crushed stone; most natural subgrades range from 2% to 15%.

What are the standard reference loads?

The reference loads used here are 13.2 kN at 2.5 mm penetration and 19.9 kN at 5.0 mm penetration. Follow the current edition of the laboratory standard and the client specification for specimen preparation, corrections, repeat testing, and reporting.

Can CBR alone determine pavement thickness?

No. Pavement thickness also depends on design traffic, materials, drainage, climate, reliability, construction constraints, and the selected design standard. Use an applicable pavement catalogue or a project-specific design by a qualified pavement engineer.

What CBR is good enough for a road?

Lightly trafficked rural roads typically require CBR ≥ 5%. Major highways often demand CBR ≥ 15% after compaction, or subgrade improvement to reach that level. Subgrades with CBR < 3% are generally stabilized with lime, cement, or geosynthetics.

Is lab CBR the same as in-situ CBR?

Not exactly. Lab CBR (soaked or unsoaked) measures remolded samples under controlled moisture; in-situ CBR measures the actual compacted layer. Design typically uses lab soaked CBR as a conservative estimate of the worst-case wet-season condition.

CBR Value Helper

Calculate CBR from 2.5 mm and optional 5.0 mm penetration loads, then review the subgrade-strength classification.

Inputs

Test load at 2.5 mm penetration (kN)

Test load at 5.0 mm penetration (kN) (optional)

Standard loads: 13.2 kN at 2.5 mm, 19.9 kN at 5.0 mm (ASTM D1883 / BS 1377).

Enter values and click Calculate to see results.

CBR Result

-- %

Subgrade Classification

0372050100%

Both Penetration Depths

Depth	Test Load (kN)	Std Load (kN)	CBR (%)

Summary

Calculate CBR from 2.5 mm and optional 5.0 mm penetration loads, then review the subgrade-strength classification.

How it works

Enter the measured load at 2.5 mm penetration.
Optionally enter the measured load at 5.0 mm penetration.
The calculator divides test load by standard load and multiplies by 100 to give CBR (%).
The subgrade classification panel interprets the resulting CBR value against industry ranges.
If the 5.0 mm result exceeds the 2.5 mm result, repeat the test as required by the applicable test procedure before selecting a design value.

Use cases

Verify lab CBR test results from Proctor compaction data.
Compare subgrade quality across multiple borehole or test pit samples.
Screen fill material before use as a road sub-base.
Support pavement rehabilitation decisions based on existing subgrade CBR.
Educational exercises on bearing ratio and flexible pavement design.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu