Compaction Factor Calculator
A compaction factor calculator determines what percentage of maximum density has been achieved in a compacted soil layer by comparing the field-measured dry density to the laboratory Proctor maximum dry density. Adequate compaction is essential to prevent settlement, bearing capacity failure, and slope instability in embankments, road bases, utility trench backfills, and building pads. Compaction requirements are specified as a percentage of the Proctor maximum, typically 90-98% depending on application. Enter the field dry density from a nuclear gauge or density test and the laboratory Proctor maximum to check whether compaction meets the project specification.
Percent compaction formula
Percent Compaction = (Field Dry Density / Proctor Max Dry Density) x 100
Moisture Deviation = Field Moisture - Optimum Moisture Content (OMC)
A positive moisture deviation means the soil is wetter than optimum. Negative means drier than optimum. Significant deviation from OMC reduces the achievable compaction.
Frequently asked questions
What is percent compaction?
Percent compaction is the ratio of the field dry density to the maximum dry density determined in the laboratory Proctor test, expressed as a percentage. A result of 95% means the field soil density is 95% of the maximum achievable density for that soil under standard test conditions.
What is the Proctor test?
The Proctor compaction test (ASTM D698 Standard Proctor, ASTM D1557 Modified Proctor) determines the maximum dry density and optimum moisture content of a soil. These values are the benchmark against which field compaction is measured. Modified Proctor uses higher compactive effort and is used for structural fills.
What compaction percentage is required for a road base?
Road subgrade and base course typically requires 95-98% of Modified Proctor maximum dry density. Residential backfill may require 90-95%. Structural fill under foundations usually requires 95% of Modified Proctor. Requirements vary by project specification and local standards.
What is optimum moisture content?
Optimum moisture content (OMC) is the water content at which a soil achieves its maximum dry density during compaction for a given compactive effort. Compacting at or near OMC is most efficient. Soil that is too dry or too wet will not compact to maximum density regardless of roller passes.
How do I measure field dry density?
Field dry density is measured using a nuclear density gauge (ASTM D6938), the sand cone method (ASTM D1556), or the rubber balloon method (ASTM D2167). Nuclear gauges are fastest and most common on large projects. Results are compared to the lab Proctor maximum to calculate percent compaction.
Official sources
- ASTM International: ASTM D698 Standard Proctor Compaction Test.
- ASTM International: ASTM D1557 Modified Proctor Compaction Test.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.