USCS Soil Classification Calculator

The USCS Soil Classification Calculator assigns a Unified Soil Classification System group symbol and name to a soil sample. You give it the gradation (the fines content, the gravel and sand split, and three grain-size percentiles) together with the Atterberg limits of the fines, and it works through the classification logic to a symbol such as SW, CL or GM.

Along the way it reports the coefficients of uniformity and curvature, the plasticity index on the A-line at your liquid limit, and whether the sample plots above or below that line, so the reasoning behind the symbol is visible rather than hidden.

Percentages are by weight, grain sizes in millimetres, and the Atterberg limits in percentage points of water content.

Use it to identify a soil for geotechnical description and correlation. It classifies a sample you have already tested; it measures nothing.

Purpose and model boundary

This model assigns a Unified Soil Classification System group symbol and name from entered gradation fractions, characteristic grain sizes, Atterberg limits and an organic-soil indicator. It exposes the decision path and plots the sample against the A-line and U-line. It is a classification aid, not a substitute for laboratory testing or professional logging.

The spreadsheet remains the calculation authority. The page submits the named inputs through the calculation service and displays the workbook's returned classification, decision trace, chart and status.

Inputs and units

The gravel, sand and fines entries describe the whole sample and should sum to 100%.

Governing relationships

For coarse soils, Cu = D60/D10 and Cc = D30²/(D10D60). The plasticity references are PIA = 0.73(LL − 20) for the A-line and PIU = 0.9(LL − 8) for the U-line.

The decision sequence first separates coarse- and fine-grained soils at 50% fines. Coarse soils are divided into gravel- or sand-dominant material. Fines below 5% use clean-soil gradation rules, fines above 12% use the plasticity character of the fines, and the 5–12% interval produces a dual symbol. Fine soils use organic status, the LL = 50 boundary, and position relative to the A-line. Borderline A-line regions can also produce a dual symbol.

Calculation sequence

1. Validate entered fractions, grain sizes, Atterberg values and organic selector.
2. Determine coarse- or fine-grained branch from percent passing No. 200.
3. For coarse soil, select gravel/sand dominance and evaluate the clean, dual-symbol or fines-controlled branch.
4. For fine soil, evaluate organic status, liquid-limit range and A-line position.
5. Return group symbol, group name and five-step decision trace.
6. Plot the sample against the A-line and U-line and evaluate workbook status.

Outputs and interpretation

The group symbol is the primary classification. The group name expands it in words. Cu, Cc, the A-line value and A-line position show the quantitative basis. The five classification-step outputs record which branches were reached; unused later steps explicitly state that they were not reached.

Validation and status logic

Assumptions and limitations

• Percentages and grain sizes come from representative laboratory testing using the applicable test methods.
• The calculator implements the declared classification logic but does not reproduce a copyrighted standard table.
• Borderline and dual symbols are intentional outputs, not numerical uncertainty estimates.
• Peat and highly organic soils, unusual particles, cementation, sensitivity, dispersivity and engineering behavior beyond the group name require separate evaluation.
• Classification does not determine design strength, compressibility, permeability or suitability by itself.

Restrictions and non-computing states

This calculator restricts percentages and selector values to their authored bounds. Grain-size coefficients are meaningful only for applicable coarse-soil branches with positive ordered D values. If status is NOT VALID, any formula residue must be ignored. A CHECK classification is returned for review rather than silently normalizing inconsistent data.

Errors and warnings

A rejected entry means the request did not satisfy the published input rules. NOT VALID means the required classification inputs are absent or unusable. CHECK means a symbol was returned but the fractions, grain-size ordering or Atterberg position needs review. A calculation-service failure is an availability error, not a soil classification.

The workbook derives its relations rather than reproducing any table, chart or figure from a specification, standard or agency publication. The A-line equation, the uniformity and curvature coefficients, and the decision thresholds are implemented from the published definitions of the classification system; no table or chart from the standard is reproduced.

• ASTM International. ASTM D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), the governing standard. Cite the edition in force for your work. https://www.astm.org/d2487-17e01.html
• Wikipedia. Unified Soil Classification System. https://en.wikipedia.org/wiki/Unified_Soil_Classification_System
• Wikipedia. Atterberg limits, for the liquid and plastic limits and the plasticity index. https://en.wikipedia.org/wiki/Atterberg_limits

Gradation percentages, grain-size percentiles and Atterberg limits must come from laboratory testing to the relevant standards. A classification is only as reliable as the tests behind it, and no trademark or organisation name appearing here implies endorsement by its owner.

Additional source notes migrated from Methodology

The workbook implements publicly documented USCS decision relationships, including the A-line, U-line, Cu and Cc. The applicable laboratory standards, project specifications and review by a qualified geotechnical professional govern formal soil classification.