Enter the dry density readings measured at several moisture contents in a compaction test and this page locates the peak of the compaction curve, returning the optimum moisture content and the maximum dry density. Three to five points can be entered, and more points make the fit steadier.
The method puts moisture content on the horizontal axis and dry density on the vertical axis, fits a quadratic by least squares, and reads the vertex coordinates. Least squares fitting and the vertex of a quadratic are mathematical definitions, so no reference table is involved, and the coefficient of determination is shown alongside so you can see how well the curve matches. The fitted coefficients are printed too, so the result can be checked against a value obtained another way. Figures current as of October 2026.
Typical optimum moisture contents by soil type, along with mold sizes and blow counts for compaction tests, are not confirmed by an authoritative source, so they are not offered here; every measured point is your own input and the calculation stays inside it. If the vertex falls outside the measured moisture range, the result says so because that is an extrapolation. Degree of compaction checks and sand cone field density conversions are not covered. This calculator is for reference and does not replace design. Review by a qualified engineer is required.
Frequently Asked Questions
Three is the minimum a quadratic curve needs. With exactly three the curve passes through every point and Rยฒ comes out as 1, so the fit quality cannot be judged. Four or five points are safer.
It means the points depart from a quadratic shape. Usually the moisture values are spaced too closely or one reading carries a large error, so recheck the points and add a test if needed.
The fitted quadratic coefficient came out positive, which makes the vertex a minimum. A compaction curve has to peak, so the calculation stops and asks you to check the values and units.