Primary reference
USCS symbols and classification boundaries
Search all 15 primary groups and the commonly required boundary/dual-symbol outcomes. Criteria are concise engineering summaries, not a reproduction of ASTM’s copyrighted classification table.
Chart actions
No USCS groups match the current filters.
| Identification | Classification controls | Details | |||||
|---|---|---|---|---|---|---|---|
| Defining criteria | Required test inputs | Boundary note | |||||
| GW | Coarse-grained | Gravel | Well-graded gravel | P200 < 5%; Cu ≥ 4; 1 ≤ Cc ≤ 3 | P200, P4, and gradation coefficients or D-values | At 5% fines, use a dual-symbol path | |
| GP | Coarse-grained | Gravel | Poorly graded gravel | P200 < 5%; fails one or both GW gradation criteria | P200, P4, and gradation coefficients or D-values | At 5% fines, use a dual-symbol path | |
| GM | Coarse-grained | Gravel | Silty gravel | P200 > 12%; fines plot below A-line or PI < 4 | P200, P4, LL, and PL | Crosshatched fines produce GC-GM | |
| GC | Coarse-grained | Gravel | Clayey gravel | P200 > 12%; fines on/above A-line with PI > 7 | P200, P4, LL, and PL | Crosshatched fines produce GC-GM | |
| SW | Coarse-grained | Sand | Well-graded sand | P200 < 5%; Cu ≥ 6; 1 ≤ Cc ≤ 3 | P200, P4, and gradation coefficients or D-values | At 5% fines, use a dual-symbol path | |
| SP | Coarse-grained | Sand | Poorly graded sand | P200 < 5%; fails one or both SW gradation criteria | P200, P4, and gradation coefficients or D-values | At 5% fines, use a dual-symbol path | |
| SM | Coarse-grained | Sand | Silty sand | P200 > 12%; fines plot below A-line or PI < 4 | P200, P4, LL, and PL | Crosshatched fines produce SC-SM | |
| SC | Coarse-grained | Sand | Clayey sand | P200 > 12%; fines on/above A-line with PI > 7 | P200, P4, LL, and PL | Crosshatched fines produce SC-SM | |
| ML | Fine-grained | Fines | Inorganic silt, low plasticity | P200 ≥ 50%; LL < 50; below A-line or PI < 4 | P200, LL, and PL | PI 4–7 on/above A-line is CL-ML | |
| CL | Fine-grained | Fines | Inorganic clay, low plasticity | P200 ≥ 50%; LL < 50; on/above A-line with PI > 7 | P200, LL, and PL | PI 4–7 on/above A-line is CL-ML | |
| OL | Organic | Fines | Organic silt or clay, low plasticity | P200 ≥ 50%; LL < 50; LLdry/LLnot dried < 0.75 | P200, LL, PL, and dried LL | Use the prescribed drying comparison | |
| MH | Fine-grained | Fines | Inorganic silt, high plasticity | P200 ≥ 50%; LL ≥ 50; plots below A-line | P200, LL, and PL | A point above the U-line calls for data review | |
| CH | Fine-grained | Fines | Inorganic clay, high plasticity | P200 ≥ 50%; LL ≥ 50; on/above A-line | P200, LL, and PL | A point above the U-line calls for data review | |
| OH | Organic | Fines | Organic silt or clay, high plasticity | P200 ≥ 50%; LL ≥ 50; LLdry/LLnot dried < 0.75 | P200, LL, PL, and dried LL | Use the prescribed drying comparison | |
| Pt | Highly organic | Organic matter | Peat | Primarily organic matter; typically fibrous, dark, and odorous | Visual-manual observation and project requirements | Do not infer Pt from a numeric threshold alone | |
| GW-GM | Coarse-grained | Gravel | Well-graded gravel with silt | 5% ≤ P200 ≤ 12%; GW gradation; silty fines | Sieve analysis, gradation, LL, and PL | First symbol = gradation; second = fines | |
| GW-GC | Coarse-grained | Gravel | Well-graded gravel with clay | 5% ≤ P200 ≤ 12%; GW gradation; clayey/crosshatched fines | Sieve analysis, gradation, LL, and PL | Crosshatched fines use the clayey second symbol | |
| GP-GM | Coarse-grained | Gravel | Poorly graded gravel with silt | 5% ≤ P200 ≤ 12%; GP gradation; silty fines | Sieve analysis, gradation, LL, and PL | First symbol = gradation; second = fines | |
| GP-GC | Coarse-grained | Gravel | Poorly graded gravel with clay | 5% ≤ P200 ≤ 12%; GP gradation; clayey/crosshatched fines | Sieve analysis, gradation, LL, and PL | Crosshatched fines use the clayey second symbol | |
| SW-SM | Coarse-grained | Sand | Well-graded sand with silt | 5% ≤ P200 ≤ 12%; SW gradation; silty fines | Sieve analysis, gradation, LL, and PL | First symbol = gradation; second = fines | |
| SW-SC | Coarse-grained | Sand | Well-graded sand with clay | 5% ≤ P200 ≤ 12%; SW gradation; clayey/crosshatched fines | Sieve analysis, gradation, LL, and PL | Crosshatched fines use the clayey second symbol | |
| SP-SM | Coarse-grained | Sand | Poorly graded sand with silt | 5% ≤ P200 ≤ 12%; SP gradation; silty fines | Sieve analysis, gradation, LL, and PL | First symbol = gradation; second = fines | |
| SP-SC | Coarse-grained | Sand | Poorly graded sand with clay | 5% ≤ P200 ≤ 12%; SP gradation; clayey/crosshatched fines | Sieve analysis, gradation, LL, and PL | Crosshatched fines use the clayey second symbol | |
| GC-GM | Coarse-grained | Gravel | Silty, clayey gravel | P200 > 12%; fines in PI 4–7 crosshatched zone | P200, P4, LL, and PL | Crosshatched fine response | |
| SC-SM | Coarse-grained | Sand | Silty, clayey sand | P200 > 12%; fines in PI 4–7 crosshatched zone | P200, P4, LL, and PL | Crosshatched fine response | |
| CL-ML | Fine-grained | Fines | Silty clay, low plasticity | P200 ≥ 50%; LL < 50; PI 4–7 on/above A-line | P200, LL, and PL | Fine-grained crosshatched outcome | |
Selected row
Select a row to inspect its criteria
—
—
—
—
Interactive lab-data check
Check a preliminary USCS group symbol
Enter measured sieve and Atterberg-limit data. The checker exposes every decision and flags incomplete or internally inconsistent inputs; it does not replace the full current standard or engineering judgment.
—
—
—
—
Decision path
- Enter laboratory data and select “Classify soil.”
Plasticity chart
The plot uses PI = LL − PL, A-line PI = 0.73(LL − 20), and U-line PI = 0.90(LL − 8). The axes expand together when a submitted point exceeds the default range.
| Series | Value |
|---|---|
| Liquid limit, LL | — |
| Plasticity index, PI | — |
| A-line at submitted LL | — |
| U-line at submitted LL | — |
| Plot region | No submitted point |
Workflow
How to use this USCS chart
- Start with percent passing No. 200. At least 50% means fine-grained; less than 50% means coarse-grained.
- For coarse soils, split gravel and sand. Use No. 4 and No. 200 results on the coarse fraction—not on the whole sample—to identify the principal fraction.
- Apply the correct secondary tests. Clean and borderline coarse soils need gradation; soils with fines need LL and PL. Organic fine soils also need the prescribed dried-versus-undried LL comparison.
- Record the symbol and full group name. Preserve dual symbols at boundary ranges and verify the descriptive naming procedure in the current project standard.
Decision sequence
USCS classification path at a glance
1. Coarse or fine
P200 < 50% follows the gravel/sand path. P200 ≥ 50% follows the silt/clay/organic path.
2. Gravel or sand
For a coarse soil, gravel is the principal fraction when more than half of the coarse fraction is retained on No. 4. Otherwise the principal fraction is sand.
3. Fines amount
Fines < 5% are clean; 5–12% requires a dual symbol; fines > 12% produces a “with fines” symbol.
4. Plasticity or gradation
Use Cu/Cc for W versus P and the plasticity chart for M versus C. Use the drying comparison for OL/OH.
Atterberg limits
How to read the plasticity chart
Plot liquid limit on the horizontal axis and PI = LL − PL on the vertical axis. The A-line separates typical clay-like behavior above from silt-like behavior below. For low-plasticity inorganic fines, PI from 4 through 7 on or above the A-line is the CL-ML crosshatched zone. LL = 50 separates L and H.
The U-line is a data-quality check rather than another group boundary. A result above it is unusual enough to review calculations, specimen preparation, test procedure, and transcription before accepting the symbol.
Boundary logic
Gradation coefficients and dual symbols
Coefficient of uniformity
Cu = D60/D10. Well-graded gravel requires Cu ≥ 4; well-graded sand requires Cu ≥ 6.
Coefficient of curvature
Cc = D302/(D10D60). Both well-graded paths require 1 ≤ Cc ≤ 3.
Five through twelve percent fines
Combine the clean-soil gradation symbol with the fines symbol: for example GW-GM, GP-GC, SW-SM, or SP-SC.
Crosshatched fines
With more than 12% fines, the PI 4–7 crosshatched response gives GC-GM or SC-SM. In the 5–12% range, the clayey second symbol is used.
Scope
Laboratory classification versus field description
ASTM D2487 is a laboratory classification system based on measured particle-size and Atterberg-limit data. Field identification is a different procedure. A visual-manual field symbol is useful for logging and sampling, but it should not be silently substituted for a laboratory result in specifications or design records.
The complete group name can also require descriptive modifiers, particle-size ranges, and other observations beyond the symbol. Follow the controlling project specification and current licensed standard when preparing final logs or reports.
Worked checks
USCS classification examples
Assume each result comes from a representative laboratory specimen; sieve values are percent by dry mass and LL/PL values are water contents in percent. These examples screen the group symbol only and do not supply design behavior.
Clean, well-graded gravel
P200 = 3%, P4 = 28%, Cu = 8, and Cc = 1.6. The soil is coarse, gravel is the principal coarse fraction, and both gradation tests pass: GW.
Borderline sand with clay
P200 = 8%, P4 = 94%, Cu = 4, Cc = 0.8, LL = 32, PL = 20. The sand is poorly graded and its fines are clayey: SP-SC.
Low-plasticity clay
P200 = 72%, LL = 38, PL = 19. PI = 19, above the A-line and above 7 with LL below 50: CL.
High-plasticity organic fine soil
P200 = 80%, undried LL = 70, and dried LL = 45. The ratio is 0.64, below 0.75, and LL is at least 50: OH.
Quality control
Common USCS mistakes
- Computing gravel versus sand from the whole sample instead of from the coarse fraction.
- Treating exactly 5% or 12% fines as a single-symbol case; both endpoints belong to the dual-symbol range.
- Calling a clean soil well graded after checking Cu but not Cc.
- Using the A-line without first calculating PI = LL − PL.
- Assigning Pt from organic content or a laboratory number alone instead of the required observed highly organic character.
- Using a group symbol as if it directly supplied design properties.
Engineering caution
Limitations and required judgment
This page intentionally stops at the group-symbol level. Borderline specimens, unusual materials, organic coarse soils, samples above the U-line, and final descriptive group names should be resolved using the complete current ASTM procedure and the project’s governing requirements.
Basis and data rights
Sources and methodology
Criteria were checked August 10, 2026 against the public scope and current-edition status on ASTM’s catalog page, then cross-checked to public federal engineering guidance. The equations and boundary logic shown here are factual/derived engineering rules. ASTM’s copyrighted tables, figures, notes, and full wording are not reproduced, and bulk CSV export is intentionally unavailable.
- ASTM D2487-17(2025), Standard Practice for Classification of Soils for Engineering Purposes (USCS) — official current-edition listing and scope.
- FHWA NHI-16-072, Soils and Foundations Reference Manual — public federal classification guidance.
- U.S. Army Corps of Engineers TM 3-357, Unified Soil Classification System — public-domain historical technical basis.
- U.S. Bureau of Reclamation, Unified Soil Classification System Test Procedures — public federal laboratory criteria, dual-symbol paths, and group-name examples.
Dataset and Source-Check Details
- Primary authority
- ASTM International, D2487-17(2025)
- Public corroboration
- FHWA, USACE, and U.S. Bureau of Reclamation engineering guidance
- Source check
- August 10, 2026
- Reference scope
- 15 primary groups plus 11 conditional dual/boundary outcomes
- Rights treatment
- Factual equations and derived boundary summaries; no ASTM table, figure, full wording, or bulk CSV reproduction
- Interactive scope
- Preliminary laboratory group-symbol screen with visible decision path and validation warnings
Quick answers
USCS soil classification FAQ
USCS stands for Unified Soil Classification System. It groups soils by particle-size distribution, plasticity, and organic characteristics for engineering description.
A soil is fine-grained when 50% or more by dry mass passes the No. 200 sieve. Less than 50% follows the coarse-grained path.
Coarse soils with 5% through 12% fines require a gradation-plus-fines dual symbol. Crosshatched plasticity responses also create CL-ML, GC-GM, or SC-SM in their applicable paths.
The A-line is PI = 0.73(LL − 20). For inorganic soils, points on or above it generally follow the clay path; points below it follow the silt path, subject to the low-PI crosshatched rules.
No. A group symbol is a classification, not a project-specific value for strength, settlement, permeability, bearing capacity, or other design behavior.
Bottom line
USCS classification summary
Classify in a fixed order: No. 200 split, No. 4 split for coarse soils, fines percentage, then gradation or plasticity. Preserve boundary symbols, document the measurements behind the result, and use the current governing standard for the final group name and report.