The nuclear gauge is the workhorse of earthwork inspection. In a few minutes per location it gives a wet density and a moisture content, and from those a dry density and a percent compaction against the Proctor. That speed is exactly why it deserves discipline: a technician can run many tests in a day, and a single bad habit, such as skipping the standard count or picking the wrong Proctor, repeats across every one of them. This guide covers the test as described in ASTM D6938 in plain terms, the math, and the daily check that keeps the gauge honest.
What is a nuclear density test?
The test checks whether compacted soil or aggregate in the field has reached the density and moisture the specification requires. Each reading is compared with a laboratory maximum dry density from a Proctor test on the same material, and often with a moisture window around the optimum moisture content.
The gauge measures quickly and without digging a large hole, which makes it practical for frequent testing on lifts as they are placed.
What equipment and licensing does a nuclear gauge need?
A nuclear moisture-density gauge with its reference standard block, a scraper plate and drive pin for preparing the test site and the probe hole, and the gauge's records. Because the gauge contains radioactive sources, operators need the training, licensing, storage, transport and dosimetry arrangements that apply to your lab. Those requirements come from regulators and your license, not from this guide.
Keep the gauge identifier with every test. When a result is questioned, the first questions are which gauge, which standard counts, and which Proctor.
What is the daily standard count check?
Before testing, take the density and moisture standard counts on the reference block, following the gauge manufacturer's procedure. Compare today's counts with the average of the previous four. Common default limits are within 1 % for the density standard count and within 2 % for the moisture standard count. A lab can set its own limits in its procedures.
A count outside the limit is a reason to stop and investigate, retake the standard, or follow your lab's procedure before using the gauge. Logging every count, including failed ones, gives you the history the next day's check depends on.
How is a nuclear density test performed?
Prepare a smooth, level test site. For direct transmission, make the probe hole with the drive pin and lower the probe to the chosen depth. For backscatter, seat the gauge on the prepared surface. Take the reading for the count time your procedure uses. Record test number, location, elevation or lift, probe depth, mode, wet density and moisture.
Pick the Proctor that matches the material in place, enter its MDD and OMC, and calculate the result. If the test fails, a retest at the same location after more compaction should be linked to the original failed test so the history is clear.
Which Proctor should a field test use, and what happens on a fail?
The gauge reading is only half of the result. The other half is the Proctor reference, and it should match the material in front of the gauge, not the material on the plan. When the fill changes partway through a lift, note it and switch references, or take a sample for a new Proctor.
When a test fails, record it as failed. If the contractor reworks the area, take the retest at the same location and link it to the original. A clear chain of failure, rework and passing retest is a sign of a working inspection process, not something to hide.
How do you calculate percent compaction?
Dry density = wet density × 100 ÷ (w + 100), where w is the moisture content in percent. Percent compaction = dry density ÷ MDD × 100. Report dry density to 0.1 pcf and compaction to 0.1 %, and compare the reported compaction with the required value, so the decision matches the number on the report.
If the specification includes a moisture window, compare the moisture with OMC plus or minus that window and record whether the test is dry of the window, within it, or wet of it.
What formulas are used?
- 01Dry density = wet density × 100 ÷ (w + 100)
- 02% compaction = dry density ÷ MDD × 100
- 03Standard counts: DS within 1 %, MS within 2 % of the average of the previous 4 (default limits)
What does a worked example look like?
Inputs
Gauge reading: wet density 125.0 pcf, moisture 10.0 %. Proctor: MDD 118.0 pcf, OMC 12.0 %. Required compaction 95 %. Density standard: previous 2,800 / 2,820 / 2,810 / 2,790, today 2,830. Moisture standard: previous 700 / 710 / 705 / 695, today 720.
Result
Dry density 113.6 pcf. Compaction 96.3 %, a pass at 95 %. Moisture is 2.0 % below OMC. Density standard: average of previous four 2,805, difference 0.9 % (pass). Moisture standard: average 702.5, difference 2.5 % (fail). The moisture standard is outside its limit, so the lab procedure should be followed before relying on the day's moisture readings.
Calculated by the same tested functions as the ProofGrit free calculators.
How do you read a nuclear density result?
Compare the percent compaction, as reported to 0.1 %, with the specified value. If the specification includes a moisture window, check the moisture against OMC as well; a test can pass compaction and still be outside the moisture window. Report both.
A failed point is a result, not an error. Record it, tell the contractor, and link any retest to the original location so the history is clear. Before blaming the fill, confirm the gauge passed its standard count check that day and that the right Proctor was used.
What are the most common nuclear density mistakes?
Skipping the standard count, or taking it and not comparing it with history.
Selecting a Proctor for the wrong material or the wrong method. A gravelly lift tested against a clay Proctor will produce a confident, wrong number.
Poor seating on a rough surface, which affects the reading.
Recording a retest as a brand-new test with no link to the original failure, which hides how many attempts a lift took.
Which standards relate to ASTM D6938?
ASTM D698 and ASTM D1557 provide the MDD reference. ASTM D1556 (sand cone) is a non-nuclear field density method some projects use to check gauge results. Licensing is handled by the U.S. Nuclear Regulatory Commission or the Agreement State where the gauge is used.
See the standards index for every standard covered on this site.
| Item | Nuclear gauge (D6938) | Sand cone (D1556) |
|---|---|---|
| What it measures | Wet density and moisture directly | Hole volume from calibrated sand |
| Moisture | From the gauge | From a sample of the removed soil |
| Speed | Minutes per point | Slower: dig, weigh, fill |
| Licensing | Radioactive materials license and training | None for radiation |
| Dry density formula | wet × 100 ÷ (w + 100) | wet ÷ (1 + w/100) |
Frequently asked questions
Why compare with the previous four counts?
A rolling average of recent counts shows gradual drift and sudden changes. Today's count is checked against that history rather than a single old value.
Can I change the 1 % and 2 % limits?
Yes. They are common defaults. Use the limits in your lab's procedure or the gauge manufacturer's guidance.
Does pass or fail use the rounded compaction?
It should. Compare the value reported to 0.1 % with the requirement, so the printed number and the decision always agree.
What if the moisture is outside the window but compaction passes?
Then the test does not meet a specification that includes a moisture window. Record both results and the status.
Is a sand cone test a substitute?
It measures in-place density by a different method. Some projects use it to check gauge results; follow the project specification.
How ProofGrit helps
The free ProofGrit nuclear density compaction calculator computes dry density, percent compaction, pass or fail and moisture status from a gauge reading, and includes the standard count check against the previous four counts with editable limits. It runs in your browser and does not store results.
Related guides and tools
- 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.
- 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.
- 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.
- Soil compaction testing: field and lab guideHow laboratory Proctor references, field density tests and project acceptance work together in soil compaction testing.
- Nuclear density calculatorDry density, percent compaction, moisture status and the daily standard count check for nuclear gauge tests.
- Proctor curve calculatorWet and dry density per point, a fitted moisture-density curve with zero air voids line, MDD and OMC.
- 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 D6938, in-place density and water content by nuclear methodsASTM International
- U.S. Nuclear Regulatory CommissionNRC
- ASTM D698, laboratory compaction using standard effortASTM International
- ASTM D1557, laboratory compaction using modified effortASTM International
- FHWA geotechnical engineering 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.