ASTM D698 is the laboratory compaction method commonly called the Standard Proctor test. A lab compacts prepared soil at a series of moisture contents, calculates dry density and fits a moisture-density curve. The curve's peak provides maximum dry density and optimum moisture content for the tested material and method. Those values may later serve as the reference for a field compaction requirement.
What does ASTM D698 establish?
The test describes how a prepared soil responds to a defined laboratory compaction effort over a range of moisture contents. It produces a relationship between water content and dry density, not a universal density limit for every project. The project specification determines whether D698 is the required reference and how the reported maximum is applied.
The maximum dry density (MDD) and optimum moisture content (OMC) are interpreted from the fitted curve. They belong together with the test method, material identification, mold and preparation record. A field percent-compaction calculation that uses the wrong reference method can be numerically consistent yet technically wrong, so preserve method identity wherever the MDD is reused.
What should the laboratory prepare and record?
Prepare a representative sample under the current method and confirm the required procedure and mold configuration before starting. ASTM D698 uses a 5.5 lb rammer, a 12 in drop and three layers. The number of blows per layer depends on mold size: 25 for the 4-in mold and 56 for the 6-in mold. Verify the current edition for any detail not stated here.
Record each moisture point's measured water content, mold tare, mold plus wet-soil mass, mold volume and method. Retain the moisture sample identity and any oversize correction or preparation note required by the standard. A worksheet with only the calculated density is not enough to reconstruct the curve if a client questions the peak or a reviewer finds a transcription issue.
How are the Proctor points calculated?
For each point, subtract the mold tare from the combined wet mass to get net wet soil mass. Wet density is net wet mass in grams divided by 453.592 to convert to pounds, then divided by mold volume in cubic feet. Dry density is wet density divided by one plus the water-content fraction. Keep full precision through the curve fit and round only for reporting.
A point calculated by the shared Proctor function uses mold-plus-wet-soil mass of 3,900 g, mold tare of 2,000 g, a 1/30 ft³ mold and 12% moisture. It returns a net wet mass of 1,900 g, wet density of 125.7 pcf and dry density of 112.2 pcf. This is one curve point, not the MDD; the peak requires the complete set and the selected fit.
How should the curve and its peak be reviewed?
Plot dry density against moisture and fit the method's curve using the lab's approved calculation rule. The Proctor curve calculator fits a second-order polynomial for three points and a third-order polynomial for four or more. The maximum of the fitted curve is MDD and the corresponding moisture is OMC. Inspect the observations as well as the fit; mathematics cannot rescue inconsistent inputs.
If the peak falls at the edge of the tested range, add a point on the wet or dry side rather than treating an unbracketed edge as a reliable maximum. Check that the curve has a plausible shape and that no point exceeds the zero-air-voids reference for the assumed specific gravity. Document any controlled override and its reason instead of silently replacing the fitted output.
How does D698 connect to field density testing?
A field method measures soil compacted in place. Its dry density is compared with the laboratory MDD to calculate percent compaction when the project requires that comparison. The field record must identify the matching material and reference. A D698 curve from one borrow source should not be reused for a different material simply because both are described as the same soil type.
Field moisture also matters when the specification sets a working range around OMC. The project documents determine whether that check is required and the allowed limits. The laboratory reports the reference values; field staff report the in-place measurements; the project authority evaluates compliance. Keep these roles distinct on the report and in the acceptance workflow.
How is Standard Proctor different from Modified Proctor?
ASTM D698 uses Standard effort while ASTM D1557 uses Modified effort. Under the verified facts here, D1557 uses a 10 lb rammer, an 18 in drop and five layers, while D698 uses a 5.5 lb rammer, a 12 in drop and three layers. Modified effort commonly produces a higher MDD and lower OMC for the same material, but the measured curve is what matters.
Never choose the reference based on which gives a more favorable field result. Follow the project specification and record the method on both the lab report and field comparison. ASTM and AASHTO methods are separate documents even where transportation projects use corresponding procedures. See the D698 versus D1557 guide for a side-by-side explanation.
What formulas are used?
- 01Wet density (pcf) = (net wet mass in g ÷ 453.592) ÷ mold volume (ft³)
- 02Dry density = wet density ÷ (1 + w/100), where w is moisture percent
- 03Peak of fitted dry-density curve = MDD; moisture at that peak = OMC
What does a worked example look like?
Inputs
Mold plus wet soil = 3,900 g; mold tare = 2,000 g; 4-in mold volume = 1/30 ft³; moisture = 12%.
Result
Shared calculation: net mass 1,900 g, wet density 125.7 pcf and dry density 112.2 pcf for this point.
Calculated by the same tested functions as the ProofGrit free calculators.
How should a lab respond to an unusual Proctor curve?
Start with the original masses, moisture determinations, mold volume and method selection. Recalculate individual points and check whether preparation, sample mixing or a transcription error explains the shape. Plot the raw points against the fitted curve and verify that the test range extends to both sides of the apparent optimum.
If the curve remains unusual, document the review and follow the lab's method for additional points or retesting. Preserve the original run and clearly identify any replacement result. Do not adjust points to create a preferred peak, and do not use a reference until a qualified reviewer accepts the record.
What are common D698 errors?
Common errors include recording the wrong mold volume, mixing a D1557 worksheet with a D698 sample, calculating dry density with moisture entered as a whole number rather than a fraction, and reporting a curve maximum at the edge without a warning. A stale MDD can also be paired with a new field test by mistake.
Use a controlled template that stores method, mold and source sample alongside every input. Verify units before calculation, keep the original data and require review of the plotted points. Link a field test to the exact reference report, not merely to a material name typed from memory.
Which standards relate to ASTM D698?
ASTM D698 is the Standard Proctor method; ASTM D1557 is the Modified Proctor method. AASHTO T 99 and T 180 are separate compaction methods often specified for transportation work. ASTM D6938 and D1556 are field density methods that may be compared with a laboratory reference when the project requires it.
Use the current edition named by the contract. The ProofGrit standards index links to verified publisher resources and related guides, but it is not a substitute for the complete test procedure or the project specification.
See the standards index for every standard covered on this site.
Frequently asked questions
What is ASTM D698?
A laboratory soil-compaction method using Standard effort to determine a moisture-density relationship.
What do MDD and OMC mean?
MDD is the maximum dry density from the fitted curve; OMC is the moisture content at that peak.
Can a D698 MDD be used for any soil?
No. Match the reference to the tested material, required method and project specification.
How does D698 differ from D1557?
They specify different compaction efforts. The project determines which method is required.
What if the curve peaks at an endpoint?
Add a point beyond that side of the tested range and review the curve rather than accepting an unbracketed peak.
How ProofGrit helps
ProofGrit records Proctor points with the selected method and calculates the curve through the shared tested library. Field-density records can then reference an identified Proctor result, reducing ambiguity when the team reviews percent compaction.
Related guides and tools
- Soil compaction testing: field and lab guideHow laboratory Proctor references, field density tests and project acceptance work together in soil compaction testing.
- ASTM D1557: Modified Proctor test guideHow ASTM D1557 establishes a Modified Proctor reference, how its curve is calculated and reviewed, and why the project must select the method.
- 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 gauge: field guideHow a nuclear density gauge supports field compaction testing, what direct transmission and backscatter mean, and why licensing and daily checks matter.
- 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.
- Sand cone test: ASTM D1556 procedure and calculationsHow the sand cone field density test works, the four-step calculation from sand in the hole to dry density, and the habits that keep results repeatable.
- Proctor curve calculatorWet and dry density per point, a fitted moisture-density curve with zero air voids line, MDD and OMC.
- Nuclear density calculatorDry density, percent compaction, moisture status and the daily standard count check for nuclear gauge tests.
- Standards indexEvery ASTM, AASHTO and ACI standard explained on this site.
- All soil testing guidesBrowse the full guide library by topic.
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
- FHWA pavement programFederal Highway Administration
Standards are summarized in our own words. Read the current edition from the publisher before you test.
This guide is a plain-English summary. Always follow the current standard and your project specifications.