🛣️ Calculators

CBR — California Bearing Ratio

Calculates a soil's CBR by comparing the pressure measured in the penetration test against the standard crushed stone: 6.9 MPa at 2.54 mm and 10.3 MPa at 5.08 mm. By the standard the HIGHER of the two governs, not just the 2.54 mm one — the trap that shows up most often in subgrade reports. Enter both measured pressures.

Measured pressure at 2.54 mm penetration (MPa) Measured pressure at 5.08 mm penetration (MPa)

Result

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The two-penetration trap in every CBR test report

The press has stopped, the pressure against penetration curve is plotted and the report needs its index. That figure feeds design directly: in the Brazilian DNIT method the subgrade value sets total pavement thickness, and the value of each layer decides whether a material qualifies as capping, sub-base or base. Underestimate and both the structure and the budget swell; overestimate and the road cracks within a few years. Along the way, plenty of reports make the same slip: they read only the 2.54 mm point.

The test compares soil against a standard crushed stone. CBR = measured pressure ÷ standard pressure × 100, with a standard of 6.9 MPa, or 1,000 psi, at 2.54 mm of penetration, and 10.3 MPa, or 1,500 psi, at 5.08 mm; the larger of the two governs. With the screen values, 1.38 MPa gives 20.00% and 2.27 MPa gives 22.04%, so the answer returned reads 22.04%. As design reference points: subgrade is commonly accepted from 2%, sub-base asks for at least 20%, and a flexible pavement base asks for 60%, rising to 80% under heavy traffic. Graded crushed stone clears 100% without trouble.

Both pressures have to come from the corrected curve: the opening stretch bends upward while the piston seats and surface irregularities close, and the origin has to be shifted along the tangent to the straight portion, since the raw curve inflates the index. The specimen counts too: compaction energy, moulding moisture and the ninety-six hour soak under surcharge all move the answer, and a soaked result never matches a dry one. Where the 5.08 mm point exceeds the 2.54 mm one, the method calls for a repeat and, once confirmed, the higher figure governs. CBR stays empirical: resilient modulus near ten times CBR in MPa is an order of magnitude, not a design parameter.

Frequently asked questions

Why did the answer come from the 5.08 mm penetration?

Because it produced the larger value. With the sample numbers, the 2.54 mm reading yields 20.00% and the 5.08 mm reading yields 22.04%, and the rule says take the larger. When that happens, the test method calls for a repeat on a fresh specimen; if the repeat confirms the inversion, the 5.08 mm value stands as the official one. It usually points to a poorly levelled surface or a coarse particle under the piston, though it also turns up in soil that gains strength as the piston drives deeper.

My readings are in kgf/cm². How do I convert them?

Multiply by 0.0981 to reach MPa, or divide by 10.2 for practically the same answer. In that unit the standard pressures sit near 70 kgf/cm² at 2.54 mm and 105 kgf/cm² at 5.08 mm. If the proving ring reads pounds per square inch, divide by 145: the standards are exactly 1,000 psi and 1,500 psi, which is where the rounded 6.9 and 10.3 come from. Conversion matters here, since the page holds 6.9 and 10.3 MPa fixed as the reference and expects both readings already in megapascals.

Does a CBR of 22% work as a pavement base?

No. Under DNIT specifications, a flexible pavement base asks for at least 60%, and 80% once the traffic number N passes five million, alongside a swell limit of 0.5% and caps on plasticity. A value of 22% suits a sub-base comfortably, where the usual minimum is 20%, and it counts as strong subgrade, given how many roads live with 3% to 8%. Check the swell recorded during the soak as well, because soil with a high index and swell beyond the limit still fails.

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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.