USCS Classification Calculator Online Free
The Unified Soil Classification System (USCS, BS 5930) provides a two-letter symbol that summarises the mechanical and hydraulic behaviour of any soil. This calculator combines particle size distribution data (sieves 200, 4, ¾") with Atterberg limits (LL, PI) and resolves the decision path to reach GW, SM, CH, or any of the 15 groups. Useful for geotechnical reports, laboratory sheets, and reviewing fills on site.
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, further refinement uses Cu, Cc, LL, and PI. It is applied in foundation studies, compacted fill design, borrow classification, suitability review for embankments, 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 criterion (gravel): Cu ≥ 4 and 1 ≤ Cc ≤ 3 → GW; otherwise, GP.
Well-graded criterion (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 percentage of fines 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: satisfies 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 a good particle 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 behaviour with moisture would change.
Reference Standards
- BS 5930 — Code of practice for ground investigations (Unified Soil Classification System)
- BS EN ISO 14688-1 — Geotechnical investigation and testing — Identification and classification of soil — Part 1: Identification and description
- BS EN ISO 14688-2 — Geotechnical investigation and testing — Identification and classification of soil — Part 2: Principles for a classification
- BS 1377-2 — Methods of test for soils for civil engineering purposes — Part 2: Classification tests (including particle size distribution by sieving)
- BS EN ISO 17892-4 — Geotechnical investigation and testing — Laboratory testing of soil — Part 4: Determination of particle size distribution
- BS 1377-2 — Methods of test for soils for civil engineering purposes — Part 2: Classification tests (including Atterberg limits)
- BS EN ISO 17892-12 — Geotechnical investigation and testing — Laboratory testing of soil — Part 12: Determination of liquid and plastic limits
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, the fines already influence behaviour but the coarse fraction still dominates, therefore BS 5930 requires reporting both symbols.
What is the difference between USCS and AASHTO for the same soil?
USCS prioritises engineering behaviour (permeability, strength, compressibility) while AASHTO is oriented towards performance as a road subgrade. The same soil may 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 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. BS 5930 applies to the fraction passing the 75 mm (3") sieve. If there are cobbles or boulders larger than this, they must be reported separately in the soil description and excluded from the particle size distribution calculation that feeds the classification.