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Soil Dry Density and Relative Compaction Calculator

Field density control compares a measured in-place soil state with an applicable laboratory compaction reference. GeoMiner starts with either measured moist or total density plus gravimetric water content, or an already reported field dry density. When moist density is entered, dry density equals moist density divided by one plus decimal water content. A water content of 10 percent is therefore 0.10 in the equation and produces a divisor of 1.10. This is the relationship published by the Federal Highway Administration. Water content should be measured for the relevant field sample under the governing procedure; the calculator does not derive it from appearance, soil name, weather, or an assumed optimum. Relative compaction, also called percent compaction in some specifications, equals field dry density divided by the maximum dry density from the specified laboratory compaction test, multiplied by 100 percent. The ratio requires compatible units. GeoMiner accepts kilograms per cubic metre, megagrams per cubic metre, grams per cubic centimetre, kilonewtons per cubic metre, and pounds per cubic foot. Megagrams per cubic metre and grams per cubic centimetre have the same numeric value. Pound-per-cubic-foot conversion follows the NIST factor. Converting between mass density and unit weight uses standard gravity, but the compaction ratio is unchanged when both field and laboratory values use the same compatible unit. The entered laboratory maximum must belong to the relevant material and the compaction method required for the work. FHWA identifies AASHTO T 99 and ASTM D 698 as common Standard Proctor references and AASHTO T 180 and ASTM D 1557 as common Modified Proctor references. These procedures apply different compactive effort and can produce different maximum dry density and optimum moisture content for the same soil. GeoMiner does not choose between them, recreate the moisture-density curve, fit a peak, or infer a maximum from one field observation. It begins with a valid reported maximum supplied by the user. A result calculated against the wrong material, borrow source, gradation, oversize correction, preparation method, compaction energy, or laboratory report is not a valid project comparison. No universal relative-compaction percentage certifies acceptance. FHWA notes that specifications often use a minimum such as 95 percent, but an example is not a de

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • undefined. Defines gravimetric water content, distinguishes moist and dry unit weight, publishes dry unit weight as total unit weight divided by one plus decimal water content, and defines relative compaction as field dry unit weight divided by the specified laboratory maximum.
  • undefined. Explains field compaction control, the influence of water content relative to optimum, and why the applicable laboratory compaction energy and project specification must remain explicit.
  • undefined. Defines percent compaction from field and laboratory dry unit weights and explicitly distinguishes relative compaction from relative density for granular soils.
  • undefined. Provides the pound-per-cubic-foot to kilogram-per-cubic-metre conversion factor used for U.S. customary density inputs and outputs.

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Common questions

How is soil dry density calculated from moist density?
Use dry density = moist density / (1 + w), where gravimetric water content w is a decimal in the equation. A water content of 10% therefore gives a divisor of 1.10.
How is relative compaction calculated?
Relative compaction is field dry density divided by the maximum dry density from the specified laboratory compaction test, multiplied by 100%.
Does 95% relative compaction always pass?
No. The governing project specification controls the required target, method, moisture window, material, location, correction rules, rounding, and acceptance decision.
Can relative compaction exceed 100%?
Yes. An arithmetic result above 100% can occur and should trigger review of material matching, field and laboratory variability, corrections, calibration, moisture, reference method, and transcription.
Is relative compaction the same as relative density?
No. FHWA distinguishes relative compaction from granular-soil relative density, which uses minimum and maximum index states and an in-place void ratio or dry density.
What does dry or wet of optimum mean?
It is the signed difference between entered field water content and entered laboratory optimum moisture content. It is not by itself a performance or acceptance result.

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