Interactive classifier
Classify Soil Using the AASHTO System
Enter laboratory percent-passing results plus liquid limit and plasticity index. The checker follows the AASHTO left-to-right elimination sequence.
Use measured values for the total sample. The No. 10, No. 40, and No. 200 percentages must decrease or stay equal as sieve opening decreases. LL is not needed when the data resolve to A-1 or A-3; otherwise enter LL to finish the A-2 through A-7 decision. If the fines are reported nonplastic, select Nonplastic rather than entering PI = 0.
Server-rendered reference
AASHTO Soil Classification Groups and Boundaries
A transformed reference summary of the laboratory boundaries used to distinguish the mineral-soil groups A-1 through A-7.
Use the table as a lookup reference, but classify by proceeding from left to right through the group sequence. A single threshold does not establish the final group.
No AASHTO groups match these table filters. Reset filters or search another group or criterion.
Scroll horizontally to view additional columns and vertically to view additional rows. The group column and headers remain visible while scrolling.
| Sieve analysis — percent passing | Fraction passing No. 40 | Additional subgroup rule | Copy | |||||
|---|---|---|---|---|---|---|---|---|
| No. 10 | No. 40 | No. 200 | LL | PI | ||||
| A-1-a | Granular | ≤ 50% | ≤ 30% | ≤ 15% | — | ≤ 6% | First qualifying group | |
| A-1-b | Granular | — | ≤ 50% | ≤ 25% | — | ≤ 6% | After A-1-a | |
| A-3 | Granular | — | ≥ 51% | ≤ 10% | — | NP | Nonplastic fines | |
| A-2-4 | Granular | — | — | ≤ 35% | ≤ 40% | ≤ 10% | After A-1 / A-3 | |
| A-2-5 | Granular | — | — | ≤ 35% | ≥ 41% | ≤ 10% | After A-1 / A-3 | |
| A-2-6 | Granular | — | — | ≤ 35% | ≤ 40% | ≥ 11% | After A-1 / A-3 | |
| A-2-7 | Granular | — | — | ≤ 35% | ≥ 41% | ≥ 11% | After A-1 / A-3 | |
| A-4 | Silt-clay | — | — | ≥ 36% | ≤ 40% | ≤ 10% | >35% No. 200 | |
| A-5 | Silt-clay | — | — | ≥ 36% | ≥ 41% | ≤ 10% | >35% No. 200 | |
| A-6 | Silt-clay | — | — | ≥ 36% | ≤ 40% | ≥ 11% | >35% No. 200 | |
| A-7-5 | Silt-clay | — | — | ≥ 36% | ≥ 41% | ≥ 11% | PI ≤ LL − 30 | |
| A-7-6 | Silt-clay | — | — | ≥ 36% | ≥ 41% | ≥ 11% | PI > LL − 30 | |
Scope: mineral soils and soil-aggregate mixtures represented by A-1 through A-7. A-8 peat/muck identification is outside this laboratory checker. The boundary table and the companion description/GI table are original transformed summaries, not reproductions of the AASHTO standard table.
| Group | Typical constituent description | Group Index treatment |
|---|---|---|
| A-1-a | Stone fragments, gravel & sand | GI = 0 |
| A-1-b | Stone fragments, gravel & sand | GI = 0 |
| A-3 | Fine sand | GI = 0 |
| A-2-4 | Silty/clayey gravel & sand | GI = 0 |
| A-2-5 | Silty/clayey gravel & sand | GI = 0 |
| A-2-6 | Silty/clayey gravel & sand | A-2-6/7 GI equation |
| A-2-7 | Silty/clayey gravel & sand | A-2-6/7 GI equation |
| A-4 | Silty soils | General GI equation |
| A-5 | Silty soils | General GI equation |
| A-6 | Clayey soils | General GI equation |
| A-7-5 | Clayey soils | General GI equation |
| A-7-6 | Clayey soils | General GI equation |
Table actions
Bulk CSV download is intentionally unavailable because the controlling AASHTO standard is copyrighted.
Functional boundary diagram
Liquid Limit and Plasticity Index Boundaries
The high-LL/high-PI A-7 region is split by PI = LL − 30. A submitted LL/PI point appears after classification.
- Your LL
- —
- Your PI
- —
- LL − 30
- —
- A-7 boundary side
- —
How the AASHTO Soil Classification System Works
AASHTO M 145 is a highway-construction classification system. It uses particle-size distribution together with liquid limit and plasticity index to place mineral soils into groups A-1 through A-7. Federal Highway Administration guidance describes A-1, A-2, and A-3 as granular materials and A-4 through A-7 as silt-clay materials.
- No. 10 sieve
- 2.00 mm opening; important to the A-1-a gradation boundary.
- No. 40 sieve
- 0.425 mm opening; used for A-1/A-3 gradation and the fraction on which Atterberg characteristics are evaluated.
- No. 200 sieve
- 0.075 mm opening; 35% or less identifies the granular side of the chart, while more than 35% moves to silt-clay groups.
- LL and PI
- Atterberg-limit indices used to separate silty versus clayey behavior and low- versus high-liquid-limit groups.
Read the chart from left to right
The procedure is an elimination process. Test A-1-a first, then A-1-b, A-3, the A-2 subgroups, and finally A-4 through A-7. Do not choose a group merely because one column happens to match a single measured value.
AASHTO Group Index formula
The Group Index (GI) supplements the group symbol with a numerical index based on percent passing the No. 200 sieve, LL, and PI.
F is percent passing the No. 200 sieve. NYSDOT guidance states that a negative final GI is reported as zero and the result is rounded to the nearest whole number.
NYSDOT identifies this reduced expression specifically for A-2-6 and A-2-7. A-1-a, A-1-b, A-2-4, A-2-5, and A-3 have GI = 0 in that public guidance.
Worked AASHTO Classification Examples
Example 1 — A-1-a granular soil
Data: No. 10 = 42%, No. 40 = 25%, No. 200 = 12%, LL = 28%, PI = 4%.
The sample satisfies the first A-1-a limits: No. 10 ≤ 50%, No. 40 ≤ 30%, No. 200 ≤ 15%, and PI ≤ 6%.
Result: A-1-a (GI 0). The sanity check is that classification stops at the first qualifying group.
Example 2 — A-6 silt-clay soil
Data: No. 10 = 100%, No. 40 = 90%, No. 200 = 62%, LL = 38%, PI = 18%.
More than 35% passes No. 200, LL is at or below 40, and PI is above 10, placing the sample in A-6. Using the public NYSDOT GI equation gives 8.89, rounded to 9.
Result: A-6 (GI 9). This classification does not replace strength or pavement-design testing.
Example 3 — exact A-7 subgroup boundary
Data: No. 10 = 100%, No. 40 = 90%, No. 200 = 70%, LL = 50%, PI = 20%.
The sample reaches A-7 and PI equals LL − 30 exactly: 20 = 50 − 30. The equality belongs to A-7-5; only PI greater than LL − 30 is A-7-6.
Result: A-7-5 (GI 14) using the public GI equation and nearest-whole-number rounding.
AASHTO vs USCS Soil Classification
The systems overlap in the laboratory measurements they use, but they are not interchangeable. AASHTO classification is oriented toward highway and subgrade-material grouping, while USCS provides engineering group symbols using different particle-size and plasticity rules.
| Question | AASHTO M 145 | USCS |
|---|---|---|
| Primary transportation use | Highway/subgrade classification | General geotechnical description and classification |
| Fine-material split | 35% passing No. 200 divides granular from silt-clay side | Uses different coarse/fine-grained thresholds and group logic |
| Output | A-1 through A-7 subgroups plus GI | Symbols such as GW, SP, CL, MH |
| Direct conversion? | No exact one-to-one conversion should be assumed; classify from the applicable test data in each system. | |
For the alternate system, use the USCS Soil Classification Chart rather than trying to translate an AASHTO symbol directly.
Common AASHTO Classification Mistakes
Using only No. 200
The 35% boundary chooses the broad granular or silt-clay side; it does not by itself identify the subgroup.
Skipping the left-to-right sequence
A soil that qualifies as A-1 or A-3 should not be reassigned to a later A-2 group simply because it also satisfies later limits.
Treating NP as PI = 0
Nonplastic is a categorical laboratory result. This page keeps NP distinct instead of silently substituting a numeric zero.
Missing the A-7 boundary
A-7-5 includes equality at PI = LL − 30; A-7-6 requires PI to be greater than LL − 30.
Engineering limitation
An AASHTO group or GI is not a CBR, resilient modulus, bearing capacity, shear strength, settlement prediction, or project approval. Detailed design requires the project-specific tests and criteria required by the governing agency and specifications.
Standard Scope, Sources, and Data Rights
Standard basis: AASHTO M 145-91 (2025). Public classification logic was cross-checked against FHWA and NYSDOT guidance on August 15, 2026.
AASHTO M 145 is the controlling commercial standard. This page presents an original transformed educational summary and calculator rather than reproducing AASHTO’s proprietary page layout, standard text, or figures.
- AASHTO — Materials Book, 45th Edition master table of contentsLists M 145-91 (2025), reconfirmed for the 2025 publication, “Classification of Soils and Soil–Aggregate Mixtures for Highway Construction Purposes.”
- FHWA Federal Lands Highway — Field Materials ManualDivision 200, §2.2.1 and Exhibits 2-3/2-4 corroborate the M 145 classification framework, test inputs, sieve sizes, and 35% No. 200 split.
- NYSDOT — Geotechnical Design Manual, Chapter 5§5.2.12.3, Tables 5-9/5-10, Equations 5-1/5-2, and Figures 5-12/5-13 corroborate classification boundaries, Group Index equations, and A-7 subgroup logic.
- ASTM International — ASTM D3282-24Public scope and significance information corroborate laboratory highway-soil classification by particle-size distribution, liquid limit, plasticity index, and group index.
Dataset and Source-Check Details
- Dataset
- AASHTO M 145 mineral-soil boundary summary, A-1-a through A-7-6
- Version label
- AASHTO M 145-91 (2025) basis
- Source checked
- August 15, 2026
- Publisher
- Turn2Engineering
- Rights decision
- Transformed educational reference; no full-standard reproduction
- Download decision
- No bulk CSV of the classification matrix
- Missing values
- — = criterion does not control; NP = nonplastic
- Excluded scope
- A-8 peat/muck field identification and project-specific design acceptance
AASHTO Soil Classification FAQ
The designations are highway-soil groups based on gradation and plasticity; A-1 through A-3 are on the granular side of the system and A-4 through A-7 are silt-clay groups.
AASHTO M 145 uses 35% passing the No. 200 sieve as the split: 35% or less is on the granular side, while more than 35% is on the silt-clay side.
The general GI equation uses percent passing No. 200, liquid limit, and plasticity index; A-2-6 and A-2-7 use the reduced expression shown above, and the public NYSDOT guidance rounds the final GI to a whole number with negative final values reported as zero.
Within the A-7 path, A-7-5 has PI less than or equal to LL − 30, while A-7-6 has PI greater than LL − 30.
No. The two systems use different classification boundaries and symbols, so a soil should be classified independently under the system required for the project.
Use the Classification as a Starting Point
The fastest reliable workflow is to enter complete sieve and Atterberg-limit results, confirm the left-to-right group decision, record the GI when applicable, and then continue to the project-specific strength, compaction, drainage, stabilization, or pavement-design checks required by the governing specification.