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 standard loads are 13.2 kN at 2.5 mm penetration and 19.9 kN at 5.0 mm penetration (BS 1377 / ASTM D1883). CBR is calculated at both depths; the higher value governs.

How does CBR relate to pavement thickness?

Higher CBR means the subgrade needs a thinner base course to distribute traffic loads safely. The AASHTO design method and CBR-based empirical charts (e.g., Transport Research Laboratory Road Note 31) translate CBR directly into required granular base depth.

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

Name: CBR Value Helper
Availability: InStock
Author: Nham Vu

Calculate CBR from load test data, estimate pavement base thickness, and interpret subgrade strength for road design.

Inputs

Mode

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 load test data, estimate pavement base thickness, and interpret subgrade strength for road design.

How it works

Select the calculation mode: compute CBR from test data, or estimate pavement thickness from a known CBR.
For CBR computation, enter the load applied to the piston and the standard load for 2.5 mm or 5.0 mm penetration.
The calculator divides test load by standard load and multiplies by 100 to give CBR (%).
For pavement design, enter CBR and traffic loading (ESAL) to get recommended base course thickness using the AASHTO empirical formula.
The subgrade classification panel interprets the resulting CBR value against industry ranges.
Review the output — weak subgrades (CBR < 3) typically require stabilization before paving.

Use cases

Verify lab CBR test results from Proctor compaction data.
Estimate minimum pavement base thickness for a new road project.
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: 2026-07-22 · Reviewed by Nham Vu