Free calculator
Proctor curve calculator: maximum dry density and OMC
Choose Standard or Modified, the mold, and enter each compaction point either as masses or as a wet density, with its moisture content. The calculator fits the curve, finds the maximum dry density and optimum moisture, and plots the zero air voids line.
By ProofGrit Editorial TeamLast reviewed:
Results update as you typeUnits: pcf, g, ft³, %Runs in your browser. Nothing is saved.
Key takeaways
- Wet density (pcf) = (net g ÷ 453.592) ÷ mold ft³; dry = wet ÷ (1 + w/100).
- Bracket the peak with dry-side and wet-side points.
- Points on or above the zero air voids line signal an error or a low Gs.
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How it's calculated
How is the result calculated?
Wet density = (net wet mass g ÷ 453.592) ÷ mold volume ft³; dry density = wet ÷ (1 + w/100); ZAV = Gs × 62.4 ÷ (1 + w/100 × Gs).
- 01Wet density (pcf) = (net wet mass in g ÷ 453.592) ÷ mold volume (ft³). For the 1/30 ft³ mold this is about net g × 0.06614
- 02Mold volumes: 4-in = 1/30 ft³, 6-in = 0.075 ft³
- 03Dry density = wet density ÷ (1 + w/100)
- 04Curve: least-squares fit, 2nd order for 3 points and 3rd order for 4 or more. The peak gives MDD; the moisture at the peak gives OMC
- 05If the peak sits at the edge of the tested moisture range, add a point on the wet or dry side
- 06Zero air voids: γd = (Gs × 62.4) ÷ (1 + (w/100) × Gs), in pcf
Rounding: densities to 0.1 pcf, moisture and compaction to 0.1 %. Full precision is kept between steps, and pass/fail uses the displayed value.
FAQ
Frequently asked questions about the proctor curve calculator
How many points do I need?
At least three for a curve. Four or more give a better fit and let the calculator use a third-order curve. Try to bracket the peak with points on both the dry and wet side.
What does the edge warning mean?
The highest point of the fitted curve falls at the driest or wettest point you tested, so the true peak may lie outside your data. Add a point on that side and refit.
Why would a point sit above the zero air voids line?
It should not. A point above the line usually means a measurement error or a specific gravity that is too low for the material. The calculator warns you when this happens.
Can I enter wet density directly?
Yes. Switch the point entry to wet density if you already calculated it, and enter the moisture content for each point.
Does the method change the math?
No. Standard and Modified use different compaction energy, which changes the curve you measure, but the density calculations are the same.
Related guides and pages
- The Proctor test: how it works and how to read the curveWhat a Proctor test measures, how each compaction point is calculated, how the curve gives MDD and OMC, and how the zero air voids line keeps you honest.
- Standard vs Modified Proctor: ASTM D698 and D1557 comparedHow Standard (D698) and Modified (D1557) Proctor differ in rammer, drop and layers, why Modified gives a higher MDD, and why the right reference matters.
- Nuclear density test: field compaction with ASTM D6938How the nuclear gauge test works, the dry density and percent compaction math, the daily standard count check, and the mistakes that cause retests.
- Soil testing softwareHow ProofGrit handles this test in the lab.
- Standards indexEvery standard explained on this site, with plain-English purpose.
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Sources and further reading
- ASTM D698, laboratory compaction using standard effortASTM International
- ASTM D1557, laboratory compaction using modified effortASTM International
- AASHTO Store (AASHTO standards, including R18, T 99 and T 180)AASHTO
Standards are summarized in our own words. Read the current edition from the publisher before you test.
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