Unified Soil Classification System Chart
Use the 15 primary USCS groups for fast lookup. Search by symbol or soil name, then include dual/boundary outcomes such as GW-GM or CL-ML only when you need them.
No USCS group matches the current search and filters. Clear the filters or include dual symbols.
Scroll within the table if needed. The first column is the USCS group symbol, followed by the group name and key classification criteria.
| Key classification criteria | Details | ||
|---|---|---|---|
| GW | Well-graded gravel | P200 < 5%; Cu ≥ 4; 1 ≤ Cc ≤ 3 | |
| GP | Poorly graded gravel | P200 < 5%; does not meet both GW gradation criteria | |
| GM | Silty gravel | P200 > 12%; fines plot below A-line or PI < 4 | |
| GC | Clayey gravel | P200 > 12%; fines on/above A-line with PI > 7 | |
| SW | Well-graded sand | P200 < 5%; Cu ≥ 6; 1 ≤ Cc ≤ 3 | |
| SP | Poorly graded sand | P200 < 5%; does not meet both SW gradation criteria | |
| SM | Silty sand | P200 > 12%; fines plot below A-line or PI < 4 | |
| SC | Clayey sand | P200 > 12%; fines on/above A-line with PI > 7 | |
| ML | Silt | P200 ≥ 50%; LL < 50; below A-line or PI < 4 | |
| CL | Lean clay | P200 ≥ 50%; LL < 50; on/above A-line with PI > 7 | |
| OL | Organic silt or organic clay | P200 ≥ 50%; LL < 50; LLdry/LLnot dried < 0.75 | |
| MH | Elastic silt | P200 ≥ 50%; LL ≥ 50; plots below A-line | |
| CH | Fat clay | P200 ≥ 50%; LL ≥ 50; on/above A-line | |
| OH | Organic silt or organic clay | P200 ≥ 50%; LL ≥ 50; LLdry/LLnot dried < 0.75 | |
| PT | Peat / highly organic soil | Primarily organic matter; typically fibrous, dark, and odorous | |
| GW-GM | Well-graded gravel with silt | 5% ≤ P200 ≤ 12%; GW gradation; silty fines | |
| GW-GC | Well-graded gravel with clay | 5% ≤ P200 ≤ 12%; GW gradation; clayey/crosshatched fines | |
| GP-GM | Poorly graded gravel with silt | 5% ≤ P200 ≤ 12%; GP gradation; silty fines | |
| GP-GC | Poorly graded gravel with clay | 5% ≤ P200 ≤ 12%; GP gradation; clayey/crosshatched fines | |
| SW-SM | Well-graded sand with silt | 5% ≤ P200 ≤ 12%; SW gradation; silty fines | |
| SW-SC | Well-graded sand with clay | 5% ≤ P200 ≤ 12%; SW gradation; clayey/crosshatched fines | |
| SP-SM | Poorly graded sand with silt | 5% ≤ P200 ≤ 12%; SP gradation; silty fines | |
| SP-SC | Poorly graded sand with clay | 5% ≤ P200 ≤ 12%; SP gradation; clayey/crosshatched fines | |
| GC-GM | Silty, clayey gravel | P200 > 12%; fines in PI 4–7 crosshatched zone | |
| SC-SM | Silty, clayey sand | P200 > 12%; fines in PI 4–7 crosshatched zone | |
| CL-ML | Silty clay | P200 ≥ 50%; LL < 50; PI 4–7 on/above A-line |
Select a group to inspect its criteria
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This is an original engineering reference summary of USCS decision rules, not a reproduction of ASTM’s copyrighted table or full standard. Final project classification should follow the controlling current standard and project requirements.
USCS Soil Classification Chart | Turn2Engineering | ASTM D2487-17(2025) laboratory classification reference.
Chart actions
USCS Symbol Key
Primary soil letters
G = gravel · S = sand · M = silt · C = clay · O = organic · PT = peat/highly organic soil.
Classification modifiers
W = well graded · P = poorly graded · L = low plasticity · H = high plasticity.
Examples: GW means well-graded gravel, CL means lean clay, and CH means fat clay.
USCS Sieve Reference
No. 4 sieve
4.75 mm opening. In the coarse fraction, material retained on No. 4 is the gravel fraction.
No. 200 sieve
0.075 mm opening. Material passing No. 200 is the fines fraction used to separate coarse- and fine-grained classification paths.
Sand fraction
Material passing No. 4 but retained on No. 200 is the sand fraction.
50% split
P200 < 50% follows the coarse-grained path; P200 ≥ 50% follows the fine-grained path.
Check a Preliminary USCS Group Symbol
Enter measured sieve and Atterberg-limit data. The checker shows the decision path and withholds a result when the entered data are incomplete or internally inconsistent.
JavaScript is optional for the chart. Use the complete server-rendered reference above. The interactive laboratory-data checker requires JavaScript.
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Decision path
- Enter laboratory data and select “Classify soil.”
Plasticity Chart
PI = LL − PL. The A-line is PI = 0.73(LL − 20); LL = 50 separates low- and high-plasticity groups. The U-line is a data-quality check, not another group boundary.
| Series | Value |
|---|---|
| Liquid limit, LL | — |
| Plasticity index, PI | — |
| A-line at submitted LL | — |
| U-line at submitted LL | — |
| Plot region | No submitted point |
How to Classify Soil Using USCS
- Split coarse from fine using No. 200. Less than 50% passing follows the coarse-grained path; 50% or more follows the fine-grained path.
- For coarse soil, identify gravel or sand. Gravel is the principal fraction when more than half of the coarse fraction is retained on No. 4; otherwise sand is principal.
- Use the fines percentage. Less than 5% is the clean-soil path, 5–12% requires a dual symbol, and more than 12% uses the gravel/sand-with-fines path.
- Apply gradation or plasticity. Cu/Cc separate W from P; LL and PI relative to the A-line separate M from C and L from H.
Dual Symbols and Boundary Cases
5–12% fines
Coarse soils in this range combine the clean-soil gradation symbol with the fines symbol, such as GW-GM, GP-GC, SW-SM, or SP-SC.
Crosshatched fines
With more than 12% fines, a PI of 4–7 on/above the A-line produces GC-GM or SC-SM. The fine-grained crosshatched outcome is CL-ML.
Gradation coefficients
Cu = D60/D10. Cc = D302/(D10D60). GW requires Cu ≥ 4; SW requires Cu ≥ 6; both require 1 ≤ Cc ≤ 3.
Near another group
ASTM D2487 also describes slash-separated borderline classifications when test results are close to another group. Use the complete current standard for those reporting cases rather than inferring them from this screen.
USCS Classification Examples
Use these examples to follow the decision path. With JavaScript enabled, “Load example” fills the checker and calculates the result.
GW — Well-graded gravel
P200 = 3%, P4 = 28%, Cu = 8, Cc = 1.6.
SP-SC — Poorly graded sand with clay
P200 = 8%, P4 = 94%, Cu = 4, Cc = 0.8, LL = 32, PL = 20.
CL — Lean clay
P200 = 72%, LL = 38, PL = 19. PI = 19, above the A-line.
OH — Organic fine soil
P200 = 80%, undried LL = 70, PL = 35, dried LL = 45. Drying ratio = 0.64.
ASTM D2487 vs ASTM D2488
ASTM D2487-17(2025)
Laboratory classification for naturally occurring soils using prescribed particle-size characteristics, liquid limit, and plasticity index when precise engineering classification is required.
ASTM D2488-26
Visual-manual description and identification. Reports using that procedure should make clear that the symbol and name are based on visual-manual identification rather than precise D2487 laboratory classification.
USCS vs AASHTO Soil Classification
USCS is widely used for geotechnical soil description and classification. AASHTO classification is commonly used for highway and subgrade applications. Do not directly convert a USCS symbol to an AASHTO group without applying the appropriate system’s test criteria.
Common USCS Classification Mistakes
- Comparing gravel and sand on the whole sample instead of within the coarse fraction.
- Treating exactly 5% or exactly 12% fines as an ordinary single-symbol case.
- Calling a clean soil well graded after checking Cu but not Cc.
- Using the A-line before calculating PI = LL − PL.
- Assigning PT from a numeric organic-content threshold alone rather than the observed highly organic character required by the classification procedure.
- Using a group symbol as though it directly supplies project-specific design properties.
Sources and Methodology
The criteria on this page were rechecked October 3, 2026 against ASTM’s active-edition catalog and public federal engineering references. The page summarizes factual decision rules and equations; it does not reproduce ASTM’s copyrighted table, figure, notes, or full wording.
- ASTM D2487-17(2025), Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) — active laboratory-classification standard and scope.
- ASTM D2488-26, Standard Practice for Description and Identification of Soils for Engineering Purposes (Visual-Manual Procedures) — active visual-manual identification practice.
- FHWA NHI-16-072, Soils and Foundations Reference Manual — public federal geotechnical reference.
- U.S. Bureau of Reclamation, Unified Soil Classification System Test Procedures — public federal classification and laboratory reference.
Related Geotechnical References
USCS Soil Classification FAQ
USCS stands for Unified Soil Classification System. It classifies soils for engineering purposes using particle-size characteristics, plasticity, and organic characteristics.
GW means well-graded gravel. For the clean-gravel path, fines are under 5%, Cu is at least 4, and Cc is from 1 to 3.
CL is lean clay: a fine-grained, low-plasticity clay classification. The plasticity chart and low-PI crosshatched rules control the final symbol.
50% or more passing the No. 200 sieve follows the fine-grained classification path. Less than 50% follows the coarse-grained path.
Coarse soils with 5–12% fines use a gradation-plus-fines dual symbol. The plasticity-chart crosshatched condition can also produce CL-ML, GC-GM, or SC-SM in the applicable paths.
The A-line is PI = 0.73(LL − 20). For inorganic soils, points on or above the A-line generally follow the clay path and points below it the silt path, subject to the low-PI crosshatched rules.
Both are sand with more than 12% fines. SM has silty/nonclayey fines; SC has clayey fines defined by the applicable plasticity response.
Both are clean gravels with less than 5% fines. GW meets both well-graded criteria: Cu ≥ 4 and 1 ≤ Cc ≤ 3. GP fails one or both of those gradation criteria.
Both are fine-grained groups with LL below 50 in their normal low-plasticity paths. ML follows the silt-side plasticity response; CL follows the clay-side response, with the PI 4–7 crosshatched range handled as CL-ML when applicable.
No. The group symbol is a qualitative classification, not a project-specific value for strength, settlement, permeability, bearing capacity, or other design behavior.