Compaction Percent Calculator
Relative compaction, also called degree of compaction, is the field dry density expressed as a percentage of the reference maximum dry density from a Proctor test: RC (%) = (field dry density ÷ maximum dry density) × 100.
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How to use this calculator
- Enter the laboratory maximum dry density (standard or modified Proctor) for the material.
- Enter the field dry density directly, OR leave it blank and enter the field bulk (wet) density and moisture content so the tool derives dry density = bulk ÷ (1 + w/100).
- Optionally enter a required compaction spec (e.g. 95%) to get an at-a-glance PASS/FAIL against the achieved relative compaction.
How it works
Relative compaction, also called degree of compaction, is the field dry density expressed as a percentage of the reference maximum dry density from a Proctor test: RC (%) = (field dry density ÷ maximum dry density) × 100. Because it is a ratio of two densities in the same unit, the percentage is dimensionless — t/m³, kg/m³ and lb/ft³ all give the same answer as long as both values use the same unit.
In the field a density gauge usually reports the bulk (wet) density and moisture content rather than dry density. Dry density is recovered with dry = bulk ÷ (1 + w/100), where w is the gravimetric moisture content in percent. This tool applies that conversion automatically when you leave the field dry density blank, then divides by the maximum dry density. Whether the reference is standard or modified Proctor changes the target value, so always state which test the maximum dry density comes from.
Worked example
A subgrade lift against a 95% spec. A nuclear density gauge reads a field bulk (wet) density of 2.04 t/m³ at a moisture content of 9.7%, and the standard Proctor maximum dry density for the material is 1.95 t/m³. Leave 'Field dry density' blank and enter the bulk density and moisture. Field dry density = 2.04 ÷ (1 + 9.7/100) = 1.860 t/m³. Relative compaction = 1.860 ÷ 1.95 × 100 = 95.4%. Entering a required spec of 95 returns PASS (required ≥ 95.0%, achieved 95.4%).
Common mistakes
- Mixing density units between the two inputs — the maximum and field densities must be in the same unit (e.g. both t/m³) or the percentage is meaningless.
- Feeding the bulk (wet) density in as the field dry density. Relative compaction is a dry-density ratio; use the moisture-content path to convert wet density to dry first.
- Comparing a modified-Proctor field result against a standard-Proctor maximum (or vice versa). The reference maximum dry density and the field test must be on the same compactive-effort basis.
Frequently asked questions
What is relative compaction?
It is the field dry density of a placed and compacted soil expressed as a percentage of the maximum dry density obtained from a laboratory Proctor test: RC = field dry density ÷ maximum dry density × 100. Earthworks specifications commonly require, for example, at least 95% of standard Proctor maximum dry density.
Do the density units matter?
Only that both densities share the same unit. Because relative compaction is a ratio, t/m³, kg/m³ or lb/ft³ all yield the identical percentage. Enter the maximum and field densities in the same unit and the result is correct.
How do I get dry density from a nuclear gauge reading?
Gauges typically give bulk (wet) density and moisture content. Dry density = bulk density ÷ (1 + w/100), where w is the moisture content in percent. Leave the field dry density field blank and enter the bulk density and moisture; the tool does this conversion for you.
Can relative compaction exceed 100%?
Yes. Field densities slightly above the laboratory maximum are common (e.g. from higher field compactive effort or material variability), so values just over 100% are normal and not an error.
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