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USCS Soil Classification Calculator with Plasticity Chart

Classify a soil by the Unified Soil Classification System (ASTM D2487) and by AASHTO M 145 from its sieve analysis and Atterberg limits. You get the group symbol, including dual symbols and CL-ML, the full ASTM group name with its modifiers, and the AASHTO group with its group index. The point is plotted on an interactive Casagrande plasticity chart. Click anywhere on the chart to try a different LL and PI.

Gradation entered as

Passing No. 4 (4.75 mm)%

Gravel (3 in – No. 4)%

Sand (No. 4 – No. 200)%

Passing No. 10 (2.00 mm)% · AASHTO

Passing No. 40 (0.425 mm)% · AASHTO

Passing No. 200 (0.075 mm)% (fines)

D₁₀mm

D₃₀mm

D₆₀mm

Liquid limit, LL%

Plastic limit, PL%

LL after oven drying (optional)%

Organic soil or fines LL_oven/LL < 0.75

Gravel / sand / fines—%

Plasticity index, PI = LL − PL—%

C_u = D₆₀/D₁₀ · C_c = D₃₀²/(D₁₀D₆₀)—

USCS group symbol—ASTM D2487

USCS group name—

AASHTO group (GI)—M 145

Group index before rounding—

Subgrade rating—

Plasticity chart (Casagrande)

Hover to read a region; click or tap to set LL and PI.

A-lineU-lineLL = 50CL-ML zoneYour soil

Defaults: 30% gravel, 40% sand, 30% fines with LL 33 and PL 21, which gives SC, clayey sand with gravel, and AASHTO A-2-6(0). Percentages are of the material finer than 3 in (75 mm). Leave LL and PL blank for nonplastic fines. D-values come from the gradation curve; the grain-size distribution tool reads them off a photo.

$$ C_u = \frac{D_{60}}{D_{10}} \qquad C_c = \frac{D_{30}^2}{D_{10}\,D_{60}} \qquad PI = LL - PL $$

$$ \text{A-line: } PI = 0.73\,(LL - 20) \qquad \text{U-line: } PI = 0.9\,(LL - 8) $$

$$ GI = (F - 35)\left[0.2 + 0.005\,(LL - 40)\right] + 0.01\,(F - 15)(PI - 10) $$

D₁₀, D₃₀, D₆₀ grain sizes at 10, 30 and 60% passing · LL, PL liquid and plastic limits, ASTM D4318, on the minus-No. 40 fraction · F percent passing the No. 200 sieve. Well graded: gravel C_u ≥ 4, sand C_u ≥ 6, both 1 ≤ C_c ≤ 3. The group index is rounded to a whole number, and a negative value is reported as 0. For A-2-6 and A-2-7, only the second term (the partial index) is used. A-1, A-3, A-2-4 and A-2-5 always have GI = 0. A-7-5 has PI ≤ LL − 30; A-7-6 has PI > LL − 30.

USCS group symbols (ASTM D2487)

Coarse-grained soils: more than 50% retained on the No. 200 sieve
Fines	Criterion	Gravel (gravel > sand)	Sand (sand ≥ gravel)
< 5%	C_u and C_c met (gravel C_u ≥ 4, sand C_u ≥ 6; 1 ≤ C_c ≤ 3)	GW	SW
< 5%	Either criterion fails	GP	SP
5–12%	Gradation symbol + fines symbol	GW-GM, GW-GC, GP-GM, GP-GC	SW-SM, SW-SC, SP-SM, SP-SC
> 12%	Fines ML or MH	GM	SM
> 12%	Fines CL or CH	GC	SC
> 12%	Fines CL-ML	GC-GM	SC-SM

Fine-grained soils: 50% or more passing the No. 200 sieve
Symbol	Group name	Plasticity chart position
CL	Lean clay	LL < 50, PI > 7, on or above A-line
CL-ML	Silty clay	LL < 50, 4 ≤ PI ≤ 7, on or above A-line
ML	Silt	LL < 50, PI < 4 or below A-line
CH	Fat clay	LL ≥ 50, on or above A-line
MH	Elastic silt	LL ≥ 50, below A-line
OL / OH	Organic clay or organic silt	LL_{oven dried}/LL < 0.75; L or H by LL = 50
PT	Peat	Primarily organic matter, dark, organic odor (visual; not classified here)

Group-name modifiers

How the calculator builds the ASTM D2487 group name
Soil	Condition	Modifier
Gravel or sand	Other coarse fraction ≥ 15%	“with sand” / “with gravel”
Gravel or sand, 5–12% fines	Fines symbol M, C or CL-ML	“with silt”, “with clay”, “with silty clay”
Fine-grained	15–29% retained on No. 200	“with sand” or “with gravel”, whichever is larger
Fine-grained	≥ 30% retained on No. 200	“sandy” or “gravelly” prefix; add “with gravel/sand” if the other fraction is ≥ 15%

AASHTO M 145 groups

Groups are tested left to right: the first one that fits is the answer
Group	No. 10	No. 40	No. 200	LL	PI	Rating
A-1-a	≤ 50	≤ 30	≤ 15	—	≤ 6	Excellent
A-1-b	—	≤ 50	≤ 25	—	≤ 6	Excellent
A-3	—	> 50	≤ 10	—	NP	Excellent to good
A-2-4 / A-2-5	—	—	≤ 35	≤ 40 / > 40	≤ 10	Excellent to good
A-2-6 / A-2-7	—	—	≤ 35	≤ 40 / > 40	> 10	Good to fair
A-4	—	—	> 35	≤ 40	≤ 10	Fair to poor
A-5	—	—	> 35	> 40	≤ 10	Fair to poor
A-6	—	—	> 35	≤ 40	> 10	Poor
A-7-5 / A-7-6	—	—	> 35	> 40	> 10	Poor

Values are percent passing. The ratings follow the USCS reference card, which also has the approximate USCS–AASHTO cross-walk and typical compacted properties by group. AASHTO calls a soil granular only up to 35% passing the No. 200, not 50%, so the two systems can disagree on the same sample. The default soil here is a clayey sand (SC) under USCS and a granular A-2-6 under AASHTO.

Worked examples

Example 1 — Sand with a little nonplastic silt

Given: 88% passing No. 4, 75% passing No. 10, 40% passing No. 40, 8% passing No. 200. D₁₀ = 0.07 mm, D₃₀ = 0.40 mm, D₆₀ = 1.2 mm. Fines are nonplastic (NP).

Gravel = 100 − 88 = 12%; sand = 88 − 8 = 80%; fines = 8%. More than 50% is retained on No. 200, and sand is greater than gravel, so this is a sand.

C_u = 1.2/0.07 = 17.1 ≥ 6; C_c = 0.40²/(0.07 × 1.2) = 1.90, which is between 1 and 3 → well graded (SW).

Fines = 8%, so between 5 and 12% → dual symbol. NP fines classify as silt (M) → SW-SM. Gravel is 12%, under 15%, so no “with gravel”.

AASHTO: F = 8 ≤ 35 → granular. The A-1-a test needs No. 10 ≤ 50; this soil has 75 → not A-1-a. A-1-b: No. 40 = 40 ≤ 50, F = 8 ≤ 25, PI = 0 ≤ 6 → A-1-b, GI = 0.

USCS: SW-SM, well-graded sand with silt · AASHTO: A-1-b(0), excellent subgrade

Example 2 — High-plasticity clay and the group index

Given: 100% passing No. 4, No. 10 and No. 40; 95% passing No. 200. LL = 60, PL = 20.

PI = 60 − 20 = 40. Fines = 95% ≥ 50 → fine-grained; LL = 60 ≥ 50 → high plasticity.

A-line at LL 60: 0.73(60 − 20) = 29.2. PI = 40 ≥ 29.2 → on or above the A-line → CH. Only 5% is retained on No. 200, under 15%, so the name takes no modifier.

AASHTO: F = 95 > 35, LL = 60 > 40, PI = 40 > 10 → A-7. LL − 30 = 30; PI = 40 > 30 → A-7-6.

GI = (95 − 35)[0.2 + 0.005(60 − 40)] + 0.01(95 − 15)(40 − 10) = 60(0.30) + 0.8(30) = 18 + 24 = 42

USCS: CH, fat clay · AASHTO: A-7-6(42), poor subgrade

Limits of this tool

It follows the laboratory procedure (ASTM D2487). The visual-manual procedure (ASTM D2488) uses the same symbols but is a field estimate. Report it as such.
Borderline results are not decided for you. A point sitting on the A-line or at LL ≈ 50, or fines very close to 5% or 12%, may warrant a borderline symbol such as CL/CH or SM/ML. Use engineering judgment.
Cobbles, boulders and peat are recorded separately. Percentages here are of the material finer than 3 in, and peat (PT) is identified visually.
Organic soils need two liquid-limit tests, one of them on oven-dried soil. Enter that value to set the organic flag automatically. For coarse soils, the calculator only flags that “with organic fines” should be added to the name.
Classification is not design. Strength, compressibility and permeability must still come from testing. Typical values by group are on the USCS reference card.

Frequently asked questions

How do you classify soil using the USCS?

Under ASTM D2487, first check the No. 200 sieve. If more than 50% is retained, the soil is coarse-grained. It is a gravel if the gravel fraction is larger than the sand fraction, and a sand otherwise. The second letter depends on the fines content:

Under 5% fines: W or P, from the gradation (C_u and C_c).
Over 12% fines: M or C, from the plasticity of the fines.
5 to 12% fines: a dual symbol that combines both.

A fine-grained soil, with 50% or more passing the No. 200, is classified by plotting LL and PI on the plasticity chart.

How do you calculate the plasticity index?

PI = LL − PL. Both limits are measured by ASTM D4318 on the fraction passing the No. 40 sieve. The soil is reported as nonplastic (NP) if the plastic limit cannot be determined, or if the plastic limit equals or exceeds the liquid limit.

What is the A-line on the plasticity chart?

The A-line is PI = 0.73(LL − 20), with a horizontal segment at PI = 4 below LL = 25.5. Soils plotting on or above the A-line are clays (CL, CH); soils below it are silts (ML, MH). The line LL = 50 separates low plasticity (L) from high plasticity (H). The U-line, PI = 0.9(LL − 8), is the approximate upper limit for natural soils. A point above it usually means a testing error.

What does a dual symbol like SP-SM mean?

A gravel or sand with 5 to 12% fines gets a dual symbol: the gradation symbol first, then the fines symbol. Examples are GW-GM, GP-GC, SW-SC and SP-SM. In this range, fines that plot as CL-ML count as clayey, which gives SP-SC and the name “with silty clay”. Above 12% fines, CL-ML fines give the dual symbols GC-GM and SC-SM.

How do you calculate the AASHTO group index?

GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10). Round the result to a whole number and report a negative value as 0. For A-2-6 and A-2-7, use only the partial index, 0.01(F − 15)(PI − 10). A-1, A-3, A-2-4 and A-2-5 soils always have GI = 0. Example 2 above works a full calculation: F = 95, LL = 60 and PI = 40 give A-7-6(42).

Why do USCS and AASHTO give different answers for the same soil?

The two systems split at different sieves and serve different purposes. USCS calls a soil coarse-grained when more than 50% is retained on the No. 200 sieve. AASHTO M 145 calls it granular only when 35% or less passes. So a sand with 30% clayey fines is SC under USCS but A-2-6 under AASHTO. With 40% fines it is still a sand under USCS, but AASHTO puts it in a silt-clay group (A-4 to A-7). USCS is the standard for geotechnical and dam work; AASHTO rates highway subgrade.

References: ASTM D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). ASTM D4318, Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. AASHTO M 145, Standard Specification for Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes. Casagrande, A. (1948), Classification and identification of soils, Transactions of the ASCE, 113, 901–930.

Related tools

Unified Soil Classification reference card — symbols, criteria and typical properties
Grain-size distribution — D₁₀, D₃₀, D₆₀ from a photo
Permeability test — constant and falling head
Soil permeability (k) by soil type
Bearing capacity — shallow foundations
Consolidation settlement
Filter criteria — Sherard / NRCS filter design
Embankment seepage — Casagrande phreatic line and seepage rate