Free Online USCS Classification Calculator
The Unified Soil Classification System (USCS, ASTM D2487) provides a two-letter symbol that summarizes the mechanical and hydraulic behavior of any soil. This calculator combines grain-size data (No. 200, No. 4, ¾" sieves) with Atterberg limits (LL, PI) and resolves the decision path to reach GW, SM, CH, or any of the 15 groups. Useful for soil mechanics reports, laboratory sheets, and field fill review.
What is USCS and when to use it?
USCS separates soils into three families based on the percentage passing the No. 200 sieve (0.075 mm): coarse-grained soils (less than 50% passing), fine-grained soils (50% or more passing), and highly organic soils. Within each family, refinement is done using Cu, Cc, LL, and PI. It is applied in foundation studies, compacted fill design, borrow classification, embankment suitability review, and as input to other calculators such as Proctor, CBR, and bearing capacity. It is the most widely used classification in geotechnical reports alongside AASHTO.
Applied Formulas
Uniformity Coefficient: Cu = D60 / D10
Coefficient of Curvature: Cc = (D30)² / (D10 × D60)
Plasticity Index: PI = LL − PL
A-Line (Casagrande): PI = 0.73 × (LL − 20)
Well-Graded Criteria (gravel): Cu ≥ 4 and 1 ≤ Cc ≤ 3 → GW; otherwise, GP.
Well-Graded Criteria (sand): Cu ≥ 6 and 1 ≤ Cc ≤ 3 → SW; otherwise, SP.
Calculation Example
| Parameter | Value |
|---|---|
| Passing No. 200 Sieve | 8% |
| Passing No. 4 Sieve | 62% |
| D10 | 0.15 mm |
| D30 | 0.85 mm |
| D60 | 2.40 mm |
| LL (fine fraction) | 24 |
| PI (fine fraction) | 6 |
Since only 8% passes the No. 200 sieve, it is a coarse-grained soil. Half or more of the coarse fraction passes the No. 4 sieve (62%), therefore it is sand. The fines percentage is between 5 and 12%, so the classification results in a dual symbol. We calculate Cu = 2.40 / 0.15 = 16 and Cc = 0.85² / (0.15 × 2.40) = 2.0: meets Cu ≥ 6 and 1 ≤ Cc ≤ 3, well-graded sand. With LL = 24 and PI = 6 (above the Casagrande A-Line), the fines are clayey. The assigned group corresponds to well-graded sand with clayey fines.
Result: SW-SC — well-graded sand with clay.
Interpretation of Results
SW-SC is a soil with good grain-size distribution and a moderate clayey fine fraction. It has low permeability compared to pure SW, but maintains good maximum dry density and shear strength once compacted. Suitable for embankments and structural fills. If the PI had exceeded 20 or the fines were more than 12%, the classification would shift to pure SC and its behavior with moisture would change.
Reference Standards
- ASTM D2487 — Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
- ASTM D2488 — Standard Practice for Description and Identification of Soils (Visual-Manual Procedure)
- ASTM D422 / D6913 — Particle-Size Analysis (Sieve Analysis)
- ASTM D4318 — Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
Frequently Asked Questions
When are dual symbols like SW-SC or GM-GC used?
When the percentage passing the No. 200 sieve is between 5% and 12%. In this range, fines already influence behavior but the coarse fraction still dominates, so ASTM D2487 requires reporting both symbols.
What is the difference between USCS and AASHTO for the same soil?
USCS prioritizes engineering behavior (permeability, strength, compressibility) while AASHTO focuses on performance as a road subgrade. The same soil can be SC in USCS and A-2-6 in AASHTO: they describe different aspects and it is normal to use both systems in a report.
What if my soil has over 50% passing the No. 200 sieve but no measurable plasticity?
It is a non-plastic fine-grained soil (PI < 4 or LL not obtainable): it becomes ML if LL < 50 or MH if LL ≥ 50. Silts are not classified using the Casagrande chart, only with LL and by position relative to the A-Line.
Does USCS apply to soils with more than 50% gravel over 3 inches?
Outside the standard range. ASTM D2487 applies to the fraction passing the 75 mm (3") sieve. If there are cobbles or larger particles, they must be reported separately in the soil description and excluded from the grain-size calculation that feeds the classification.