Calculate relative compaction from field dry density and maximum dry density in g/cm3. Instant pass/fail result based on the 95% DOT standard. Used by geotechnical technicians and earthwork QC crews.
Need a nuclear density gauge for field compaction testing? Troxler and Humboldt models at Express Tools.
Shop Express Tools →Relative compaction is the primary quality control metric for earthwork on civil construction projects. It measures how well field compaction of a soil matches the theoretical maximum achievable density for that soil type. A higher relative compaction means the soil has less void space, which translates to greater strength, less settlement, and better resistance to water infiltration.
The formula is simple: divide the field dry density by the maximum dry density from a Proctor test, then multiply by 100 to express the result as a percentage. This calculation strips moisture content out of the comparison — the Proctor test establishes a maximum density at optimum moisture, and the field test measures in-place density at whatever moisture exists at the time of testing. By comparing dry densities, you get a fair apples-to-apples comparison regardless of moisture conditions on test day.
| Application | Typical Requirement | Test Standard |
|---|---|---|
| Highway roadway subgrade | 95% Modified Proctor | ASTM D1557 |
| Base course (aggregate) | 98% Modified Proctor | ASTM D1557 |
| Structural fill beneath slab | 95% Standard Proctor | ASTM D698 |
| General embankment fill | 90–95% Standard Proctor | ASTM D698 |
| Trench backfill under pavement | 95–98% Modified Proctor | ASTM D1557 |
| Trench backfill in open area | 90% Standard Proctor | ASTM D698 |
| Landscape grading / topsoil | 85–90% Standard Proctor | ASTM D698 |
| Solar farm site prep | 90–95% Standard Proctor | ASTM D698 |
Requirements vary by jurisdiction and project specification. Always confirm with the geotechnical report and contract documents.
The Proctor compaction test establishes the relationship between moisture content and dry density for a specific soil. A geotechnical laboratory compacts the soil at multiple moisture levels using a standardized amount of compaction energy, then plots density versus moisture content. The peak of the resulting bell-shaped curve is the maximum dry density at the optimum moisture content.
Standard Proctor (ASTM D698) uses 12,375 ft-lb per cubic foot of compaction energy — calibrated to lighter compaction equipment common in the mid-20th century. Modified Proctor (ASTM D1557) uses 56,250 ft-lb per cubic foot, representing modern heavy rollers and vibratory compactors. Modified Proctor produces a higher maximum dry density and lower optimum moisture content than Standard Proctor for the same soil. The choice of test depends on the equipment to be used in the field and the project specification.
Matching the correct Proctor curve to the material actually being compacted in the field is the most critical step in compaction testing. When multiple borrow sources are used or when soil type changes within a single cut area, separate Proctor tests are required. Using an incorrect Proctor curve can cause consistently failing or passing results that do not reflect actual compaction quality.
When a compaction test fails, the response depends on the magnitude of the deficiency and current conditions on site. A result of 93–94.9% on a 95% specification is marginal and often correctable with additional roller passes if the soil moisture is within 1–2% of optimum. Results below 90% typically indicate the lift is too thick, moisture is significantly off, or the wrong roller is being used for the soil type.
Common causes of compaction test failure include: lift thickness exceeding the compactor's capability (typically 8–12 inches loose for vibratory rollers on cohesive soil); soil moisture too wet of optimum, which causes pumping and reduces achievable density; aggregate oversize material in the test area giving unrepresentative readings on nuclear gauges; and incorrect Proctor curves applied to changed material. Documenting the moisture content alongside every density reading is essential for diagnosing failures and making the right correction.
Save compaction test results to a Sitemark job
Log field density results with GPS location, lift number, and pass/fail status. Auto-generates compaction test summaries for geotechnical review and inspection submittals.
Start free trialRelative compaction is the ratio of the in-place (field) dry density of a soil to the maximum dry density established by a laboratory Proctor test, expressed as a percentage. The formula is: Relative Compaction (%) = (Field Dry Density / Maximum Dry Density) × 100. A result of 95% or higher meets most DOT and highway specifications for structural fills and roadway subgrade.
Field dry density in g/cm3 comes from in-place density testing methods such as the nuclear density gauge (ASTM D6938), the sand cone test (ASTM D1556), or the rubber balloon method (ASTM D2167). Nuclear gauges typically report density in g/cm3 or pcf — 1 g/cm3 = 62.428 pcf. The gauge measures both wet density and moisture content; the device calculates dry density as: dry density = wet density / (1 + moisture content).
The 95% relative compaction threshold is the most common minimum standard for roadway subgrade and structural fill in North American DOT specifications. It derives from decades of empirical performance data showing that soils compacted to at least 95% of their Proctor maximum dry density resist settlement and deformation under traffic loading. Some specifications require 98% for base course and granular fill directly beneath pavement, and 90% for general embankment fill away from load-bearing areas.
Relative compaction and compaction percent are the same calculation — both equal (field dry density / max dry density) × 100. The terms are used interchangeably in practice. Relative compaction is the more precise term used in academic and international contexts; compaction percent or percent compaction is the term most common on US DOT projects and construction submittals.
Field density slightly exceeding the Proctor maximum dry density (by 1–3%) can occur due to testing variability, aggregate oversize material in the field sample, or natural variability in the Proctor curve. Values significantly above the maximum dry density typically indicate a mismatch between the Proctor curve used and the actual material being tested, or a nuclear gauge calibration error. Always verify that the Proctor curve was run on the same soil material being tested in the field.
From nuclear gauge or sand cone test
From laboratory Proctor test