AASHTO Soil Classification Chart
Need the group fast? Start with percent passing the No. 200 sieve. AASHTO M 145 classifies mineral soils by proceeding from left to right through the group sequence; the first group whose complete criteria are satisfied controls.
No AASHTO group matches the current search and filter. Clear the filters or search another group.
Scroll within the chart if needed. The group column remains the row identifier. Sieve criteria are percent passing by mass.
| Group | Sieve criteria — % passing | LL / PI criteria | Typical material | General subgrade rating |
|---|---|---|---|---|
| A-1-a | No. 10 ≤ 50% · No. 40 ≤ 30% · No. 200 ≤ 15% | PI ≤ 6% | Stone fragments, gravel & sand | Excellent to good |
| A-1-b | No. 40 ≤ 50% · No. 200 ≤ 25% | PI ≤ 6% | Stone fragments, gravel & sand | Excellent to good |
| A-3 | No. 40 ≥ 51% · No. 200 ≤ 10% | NP | Fine sand | Excellent to good |
| A-2-4 | No. 200 ≤ 35% | LL ≤ 40% · PI ≤ 10% | Silty/clayey gravel & sand | Excellent to good |
| A-2-5 | No. 200 ≤ 35% | LL ≥ 41% · PI ≤ 10% | Silty/clayey gravel & sand | Excellent to good |
| A-2-6 | No. 200 ≤ 35% | LL ≤ 40% · PI ≥ 11% | Silty/clayey gravel & sand | Excellent to good |
| A-2-7 | No. 200 ≤ 35% | LL ≥ 41% · PI ≥ 11% | Silty/clayey gravel & sand | Excellent to good |
| A-4 | No. 200 ≥ 36% | LL ≤ 40% · PI ≤ 10% | Silty soils | Fair to poor |
| A-5 | No. 200 ≥ 36% | LL ≥ 41% · PI ≤ 10% | Silty soils | Fair to poor |
| A-6 | No. 200 ≥ 36% | LL ≤ 40% · PI ≥ 11% | Clayey soils | Fair to poor |
| A-7-5 | No. 200 ≥ 36% | LL ≥ 41% · PI ≥ 11% · PI ≤ LL − 30 | Clayey soils | Fair to poor |
| A-7-6 | No. 200 ≥ 36% | Clayey soils | Fair to poor |
This chart is an original transformed engineering summary for quick reference, not a reproduction of AASHTO’s copyrighted table, notes, figures, or full specification. Use the controlling current M 145 publication and project requirements for formal work.
AASHTO Soil Classification Chart | Turn2Engineering | M 145 quick-reference summary.
Chart actions
AASHTO Soil Classifier & Group Index Calculator
Enter reported laboratory results. The tool follows the left-to-right AASHTO group sequence, calculates GI when applicable, and withholds a result when the values are incomplete or internally inconsistent.
The reference chart remains usable without JavaScript. The automatic classifier, example loading, row highlighting, result copying, and LL/PI point require JavaScript.
Enter No. 200 plus the values required by the applicable path.
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Why this group?
How to Read the AASHTO Chart
≤35% passing stays on the granular side. >35% moves to A-4 through A-7.
For granular soils, apply the No. 10, No. 40, No. 200 and plasticity rules before considering A-2.
LL 40/41 and PI 10/11 separate the A-2 subgroups and the A-4 through A-7 groups.
Within A-7, compare PI with LL − 30 to choose A-7-5 or A-7-6.
AASHTO Group Index (GI)
Group Index supplements the group symbol with a numerical index based on percent passing the No. 200 sieve, liquid limit, and plasticity index. It is an index for relative subgrade quality, not a replacement for strength or performance testing.
F is percent passing the No. 200 sieve expressed as a whole number. A negative final GI is reported as zero and the result is rounded to the nearest whole number.
Only the PI portion is used for A-2-6 and A-2-7. A-1-a, A-1-b, A-3, A-2-4, and A-2-5 have GI = 0.
AASHTO Liquid Limit & Plasticity Index Boundaries
This schematic makes the LL = 40, PI = 10, and A-7 subgroup boundary easy to see. When the classifier has usable LL/PI data, your sample point is plotted on the diagram.
- Your LL
- —
- Your PI
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- LL − 30
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- A-7 side
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Worked AASHTO Soil Classification Examples
A-1-a granular soil
No. 10 = 42%, No. 40 = 25%, No. 200 = 12%, LL = 28, PI = 4. The sample satisfies the first A-1-a criteria, so classification stops there.
Result: A-1-a (GI 0)
A-2-6 granular soil
No. 10 = 70%, No. 40 = 55%, No. 200 = 30%, LL = 38, PI = 18. The sample misses A-1/A-3 and falls into the low-LL, high-PI A-2 subgroup.
Result: A-2-6
A-6 silt-clay soil
No. 200 = 62%, LL = 38, PI = 18. More than 35% passes No. 200, LL is 40 or less, and PI is 11 or more.
Result: A-6 (GI 9)
A-7-5 exact boundary
No. 200 = 70%, LL = 50, PI = 20. Since PI = LL − 30 exactly, the equality remains on the A-7-5 side.
Result: A-7-5 (GI 14)
AASHTO vs USCS Soil Classification
The systems use some of the same laboratory measurements, but they answer different classification questions and do not have a reliable one-to-one symbol conversion.
| Question | AASHTO M 145 | USCS |
|---|---|---|
| Typical use | Highway, subgrade, embankment, subbase and base-material classification | General geotechnical engineering soil description and classification |
| Primary output | A-1 through A-7 groups/subgroups plus Group Index | Symbols such as GW, SP, CL and MH |
| Fine-material split | 35% passing No. 200 separates the broad granular and silt-clay sides | Uses a different coarse/fine-grained decision structure |
| Direct conversion? | No. Apply the governing classification procedure to the test data rather than translating a symbol directly. | |
Common AASHTO Classification Mistakes
Using only No. 200
The 35% boundary chooses the broad side of the chart; it does not by itself identify the final group.
Skipping left-to-right elimination
A soil that qualifies for an earlier group should not be reassigned simply because it also appears to fit later limits.
Calling PI = 0 “NP”
Nonplastic is a categorical laboratory result. The classifier keeps NP distinct from a numeric PI of zero, which matters for A-3.
Reversing A-7-5 / A-7-6
A-7-5 includes equality at PI = LL − 30. A-7-6 requires PI to be greater than LL − 30.
Standard Basis & Engineering References
Source check: October 6, 2026. AASHTO’s current M 145 product listing identifies M 145-91 and states that it incorporates September 2026 errata. The 46th Edition, 2026 is the current AASHTO Materials collection.
This page summarizes public classification logic and equations in an original layout. It does not reproduce the proprietary AASHTO specification, table design, notes, or figures.
- AASHTO — M 145, Standard Specification for Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes — current product listing and September 2026 errata notice.
- AASHTO — Standard Specifications for Transportation Materials, 46th Edition, 2026 — current Materials collection.
- FHWA NHI-16-072 — Geotechnical Site Characterization — public federal reference containing the AASHTO classification matrix, general subgrade ratings, fine-grained boundary diagram, and GI equation.
- NYSDOT Geotechnical Design Manual, Chapter 5 — public state DOT reference for AASHTO soil classification and Group Index treatment.
Related Geotechnical References
AASHTO Soil Classification FAQ
AASHTO M 145 classifies soils and soil-aggregate mixtures for highway-construction purposes using particle-size distribution, liquid limit, and plasticity index. The primary mineral-soil groups are A-1 through A-7, with subgroups and Group Index where applicable.
35% passing the No. 200 sieve is the broad split. Materials with 35% or less passing are on the granular side; more than 35% passing follows the silt-clay side.
Within A-7, A-7-5 applies when PI is less than or equal to LL − 30. A-7-6 applies when PI is greater than LL − 30.
The general GI equation uses percent passing No. 200, liquid limit, and plasticity index. A-2-6 and A-2-7 use only the PI portion. A negative final GI is reported as zero, and the value is reported to the nearest whole number.
A-8 is used for highly organic soils such as peat or muck and is based on visual identification rather than the numeric sieve/LL/PI matrix used for the primary mineral-soil groups.
No. The systems use different decision rules and outputs. Apply the classification system required by the project rather than assuming a direct conversion.