The Unified Soil Classification System, or USCS, organizes soil into engineering groups using particle-size distribution and plasticity information. ASTM D2487 provides the classification framework. A group symbol is a concise description of observed index properties, not a complete design recommendation. Good classification depends on representative sampling, reliable gradation and Atterberg data, and careful use of the full current standard.
What information does USCS use?
The classification begins with the amount of material passing the No. 200 sieve, then distinguishes coarse soils by the balance of gravel and sand and by gradation. Fine-grained soils are described using liquid limit, plasticity index and the plasticity chart. Organic behavior and highly organic material require additional consideration; a single gradation value cannot answer every classification question.
Sample preparation and test quality therefore matter as much as the symbol lookup. The sieve analysis, moisture-content and Atterberg-limit results should all trace to the same representative material. If a required value is missing, outside the measured range or affected by an unusual particle-size distribution, do not invent it. Record the limitation and consult the current ASTM D2487 chart and project procedure.
How are coarse- and fine-grained soils separated?
Under the verified D2487 boundaries used here, a soil with more than 50% retained on the No. 200 sieve is coarse-grained; one with 50% or more passing that sieve is fine-grained. The coarse fraction is then separated into gravel or sand using the No. 4 sieve. These percentage boundaries guide the first branch of the decision process.
Fines content affects the symbol. Fewer than 5% fines leads to a clean coarse-soil group, while more than 12% fines uses a fines description. The 5–12% interval uses dual symbols. Exact boundary values and the resulting description must be handled as the full standard specifies; the tool's output is a learning aid and is not a substitute for checking complete D2487 charts.
What do gradation coefficients describe?
For coarse soils where the required particle diameters are available, the uniformity coefficient is Cu = D60 ÷ D10 and the curvature coefficient is Cc = (D30)² ÷ (D10 × D60). The D-values are particle diameters at selected percentages passing from the gradation curve. They summarize curve shape; they do not mean the material has a particular field performance.
Interpolation is only defensible within measured points that bracket the target percent passing. Do not extrapolate a D-value beyond the sieve data. Use consistent units and preserve the source gradation. The calculator can interpolate log-linearly from its supported data, but the complete classification should still be checked against the current standard, especially for boundary, dual-symbol and descriptive-name decisions.
How does the plasticity chart affect fine-soil symbols?
The chart compares liquid limit and plasticity index. Its A-line separates clay-like from silt-like behavior in the classification scheme; the U-line is an upper reference for plotting. For low liquid-limit soils, CL, ML and the borderline CL-ML distinction depend on both the index values and their position relative to the line. Higher liquid limits separate CH and MH by the same chart relationship.
Organic soil requires comparing liquid-limit results before and after oven drying under the applicable method, while peat is identified visually. In coarse soils with sufficient fines, the fines classification informs the M or C letter, and CL-ML fines may require a dual notation. Use the full D2487 charts for group names and modifiers such as with sand or with gravel.
How should a lab document a USCS classification?
Keep the raw sieve masses, calculated percent passing, liquid-limit trials, plastic-limit trials and any organic check with the classification record. A reviewer should be able to see how the symbol follows from the test results, not just a final two-letter code. Include sample source, date, method and any preparation notes relevant to the results.
When a soil plots near a boundary or a chart condition is ambiguous, document the issue and seek a qualified review. Do not force a symbol because a spreadsheet only offers one option. If the available data do not support a complete classification, report the limited information clearly and obtain the missing tests rather than filling gaps with assumptions.
What can a USCS group symbol tell you—and what can it not?
A symbol helps engineers and contractors communicate a soil's broad particle-size and plasticity characteristics. It can organize logs, compare materials and support further investigation. It cannot by itself establish strength, settlement, drainage, frost behavior, compaction response or suitability for a particular use. Those questions require the project context and other tests.
Classification should be read with the full description and source data. Two soils with the same symbol may still differ in gradation, mineralogy, structure or moisture condition. Engineers may need additional tests and field observations. Treat USCS as one standardized index description, not a shortcut around the project's geotechnical evaluation.
What formulas are used?
- 01Cu = D60 ÷ D10
- 02Cc = (D30)² ÷ (D10 × D60)
- 03A-line: PI = 0.73 × (LL − 20); U-line: PI = 0.9 × (LL − 8)
What does a worked example look like?
Inputs
Percent passing No. 4 = 85, passing No. 200 = 20, liquid limit = 30, plastic limit = 15.
Result
The shared classification function returns SC, Clayey sand. The decision path still needs comparison with the complete D2487 chart and the sample description.
Calculated by the same tested functions as the ProofGrit free calculators.
How should boundary cases be reviewed?
Recheck the original measurements, units and sample preparation, then plot the values using the complete current D2487 charts. Confirm whether the fines percentage falls in a symbol transition interval and whether the plasticity point lies on, above or below the relevant line. The output should state what the data support and identify unresolved judgments.
If the electronic tool and full standard appear to disagree, follow the standard and the lab's controlled procedure, preserve the raw values, and report the discrepancy to the tool maintainer. The chart is not a license to ignore visual description, organic checks or other conditions required by the method.
What are common USCS classification errors?
Frequent errors include using percent retained where percent passing is expected, mixing data from separate samples, estimating D-values outside the gradation range, confusing fines percentage with the fraction's plasticity, and omitting a required organic or visual check. Rounding too early can also shift a point near a classification boundary.
Keep unrounded inputs for the decision path and show units and the measurement basis. A reviewer should see the same sieve and Atterberg records used to derive the symbol. When the complete chart has an exception or modifier not covered by a simple calculator, pause and confirm the result instead of selecting the nearest available dropdown value.
Which standards support USCS classification?
ASTM D2487 is the engineering classification practice for soils using laboratory data. ASTM D6913 addresses particle-size distribution by sieving; ASTM D4318 covers liquid and plastic limits, with the linked catalog edition noted as withdrawn. Confirm the current edition and any companion requirements before testing.
The ProofGrit standards index links to the publisher pages and to related tools. The calculator intentionally provides a decision path and boundary notes, not every branch of the full standard. Use the current ASTM document and project requirements for final group naming and modifiers.
See the standards index for every standard covered on this site.
Frequently asked questions
What does USCS stand for?
Unified Soil Classification System, the engineering soil classification framework addressed by ASTM D2487.
What data are needed for a USCS classification?
Typically gradation and, for relevant soils, liquid limit and plasticity index, plus observations needed for organic or visual descriptions.
Can a calculator replace ASTM D2487?
No. It can help organize supported inputs, but full charts and descriptive modifiers must be checked in the current standard.
What are D10, D30 and D60?
They are particle diameters corresponding to 10%, 30% and 60% passing on a gradation curve.
Does a USCS symbol predict soil strength?
No. It is an index classification, not a direct measurement of engineering design properties.
How ProofGrit helps
ProofGrit keeps gradation and Atterberg inputs available for a repeatable classification workflow and links the resulting test records to the project. The free calculator shows the supported decision path but does not replace the full ASTM D2487 charts or a qualified review.
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.
- Atterberg limits: liquid limit, plastic limit and PIWhat liquid limit, plastic limit and plasticity index mean, how multipoint results are interpreted, and how the values support soil classification.
- Sieve analysis: aggregate and soil gradation guideHow sieve analysis produces percent retained and passing, how gradation curves support material description, and what the fineness modulus summarizes.
- ASTM D698: Standard Proctor test guideHow ASTM D698 establishes a Standard Proctor moisture-density reference, what to record, and how field compaction results use the reference.
- 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.
- USCS classification calculatorGroup symbol, basic group name and the decision path from gradation and Atterberg limits, with a plasticity chart.
- Atterberg limits calculatorMultipoint liquid limit from a least-squares flow curve, plastic limit from trials, plasticity index or NP, and a plasticity chart.
- Sieve analysis calculatorPercent retained, cumulative retained and percent passing for each sieve, a log-scale gradation chart and fineness modulus.
- 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 D2487, classification of soils for engineering purposes (USCS)ASTM International
- ASTM D6913/D6913M, particle-size distribution of soils by sieve analysisASTM International
- ASTM D4318, liquid limit, plastic limit and plasticity index (linked edition marked withdrawn; confirm the applicable current edition)ASTM 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.