California Bearing Ratio (CBR) Calculator

A subgrade sample: 8.5 kN load at 2.5 mm penetration, 11.2 kN at 5.0 mm.

• Standard loads are ASTM D1883's reference values: 1,000 psi (3,000 lbf ≈ 13.345 kN) at 2.5 mm penetration and 1,500 psi (4,500 lbf ≈ 20.017 kN) at 5.0 mm, on the standard 3 in² piston.
• The specimen was tested at the moisture and density condition relevant to the design case (as-compacted, or after the standard 4-day soak for a soaked CBR) — the calculator does not distinguish soaked from unsoaked; that's a test-procedure choice made before this step.
• If the 5.0 mm CBR exceeds the 2.5 mm CBR, ASTM D1883 calls for the test to be repeated; if confirmed, the 5.0 mm value is used instead — this calculator reports the 2.5 mm value as governing by default and flags the condition.

What CBR value counts as a 'good' subgrade?

As a rough general guide, under 3% is a very poor subgrade, 3–7% poor to fair, 7–20% fair to good, and above 20% good to excellent — but the actual design CBR used for pavement thickness comes from your governing agency's pavement design method (e.g. AASHTO 1993 Guide), not a fixed cutoff.

Why would CBR at 5.0 mm come out higher than at 2.5 mm?

It usually signals an irregularity in the load-penetration curve — a surface bump, seating error, or a spot of dense material right at the piston. ASTM D1883 says to repeat the test if this happens; if the second test confirms it, use the 5.0 mm value as the reported CBR instead.

Soaked or unsoaked CBR — does it matter here?

The calculator's math is identical either way — soaking (or not) is a decision made in how you prepare and condition the specimen before testing, not in how the CBR percentage itself is calculated from the load readings.

My result differs slightly from another CBR calculator — why?

Some calculators use the metric-derived standard loads common in IS-code references (1,370 kgf and 2,055 kgf, ≈13.44 kN and ≈20.14 kN) instead of ASTM D1883's imperial-derived figures (13.345 kN and 20.017 kN from 1,000/1,500 psi over a 3 in² piston) used here. The two conventions differ by well under 1% — both are legitimate, just rounded from a different base unit.

• Test method, standard loads & formula

  ASTM D1883, 'Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils' — standard loads of 1,000 psi (3,000 lbf ≈ 13.345 kN) at 2.5 mm and 1,500 psi (4,500 lbf ≈ 20.017 kN) at 5.0 mm, on the standard 3 in² piston. AASHTO T193 is the equivalent AASHTO standard for the same test.