Enter laboratory and gradation data
Use measured percentages and Atterberg limits. The calculator validates totals, computes PI and A-line, then returns USCS and AASHTO outcomes.
Formula used
Plasticity Index: PI = LL − PL
A-line: PIA-line = 0.73 × (LL − 20)
Well-graded gravel check: Cu ≥ 4 and 1 ≤ Cc ≤ 3
Well-graded sand check: Cu ≥ 6 and 1 ≤ Cc ≤ 3
AASHTO Group Index: GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10), with negative bracket terms taken as zero.
USCS first separates coarse-grained and fine-grained soils using fines content. Coarse soils then use the dominant fraction, gradation quality, and fines character. Fine soils rely on liquid limit and plasticity position relative to the A-line. Borderline zones return dual symbols where appropriate.
How to use this calculator
Enter the gravel, sand, and fines percentages so they total about 100%. Add the percentages passing the No. 10 and No. 40 sieves from your gradation test. Then enter liquid limit, plastic limit, and the gradation coefficients from your sieve analysis.
Choose the organic or peat flags only when field description or laboratory evidence supports them. After you submit, the report appears above the form with USCS symbol, AASHTO group, GI value, interpretation notes, and export buttons for CSV and PDF.
Example data table
| Sample | Gravel % | Sand % | Fines % | No. 10 % | No. 40 % | LL | PL | Cu | Cc | USCS | AASHTO |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Borehole B-3 | 22 | 58 | 20 | 86 | 47 | 32 | 22 | 7.8 | 1.6 | SC | A-2-4 |
| Test Pit T-1 | 48 | 47 | 5 | 54 | 12 | 18 | 15 | 5.0 | 2.1 | GW-GM | A-1-b |
| Core C-7 | 5 | 18 | 77 | 95 | 88 | 61 | 28 | 2.0 | 0.9 | CH | A-7-6 |
Frequently asked questions
1. What classification methods does this calculator use?
It returns a USCS soil symbol and an AASHTO group. That combination supports geotechnical screening and roadway-oriented subgrade review from one input set.
2. Why do gravel, sand, and fines need to total 100%?
The calculator uses those percentages to decide whether the sample is coarse-grained or fine-grained and to identify the dominant coarse fraction. A near-100% total keeps the decision consistent.
3. What happens when fines fall between 5% and 12%?
That range often creates dual USCS symbols because both coarse gradation and fines character matter. The calculator combines the clean coarse symbol with the fines-based modifier.
4. How is plasticity index determined?
Plasticity index equals liquid limit minus plastic limit. It measures the moisture range where the soil behaves plastically and helps separate silty behavior from clayey behavior.
5. Why does the A-line matter?
The A-line helps distinguish clay-like plasticity from silt-like plasticity on the plasticity chart. The result affects whether the soil trends toward M or C groups.
6. Can I use this result for final design?
Use it for screening, reporting, and preliminary decisions. Final foundation, pavement, or earthwork design should still rely on project-specific laboratory testing and engineering judgment.
7. What does the AASHTO group index indicate?
The group index is an empirical measure tied to expected subgrade quality. Lower values usually indicate better subgrade performance, while higher values suggest weaker behavior.
8. Does the organic flag change the outcome?
Yes. Organic fine-grained soils can shift to OL or OH, and peat gets PT. Those classifications reflect higher compressibility and stronger moisture sensitivity than mineral soils.