Soil Jar Test & Texture Calculator
Measure the separate settled sand, silt, and clay layers in your jar, then enter their thicknesses in the same unit. The calculator converts them to percentages, assigns a USDA texture class, and flags measurements that sit close to a class line.
USDA soil texture classLoam
- Sand
- 40%
- Silt
- 40%
- Clay
- 20%
- Texture triangle position
- x = 50.0, y = 17.3
- Nearest class boundary
- 6.5%
- Boundary confidence
- Clearly inside class
Layer depths are normalized to 100%, so they can be inches, millimeters, or any other common unit.
A near-boundary result can change class with a small difference in the measured layers.
How to use this calculator
- After the sample has settled, measure the thickness of the bottom sand layer.
- Measure the silt layer alone, rather than its height from the bottom of the jar.
- Measure the settled clay layer alone after the suspension has cleared as far as your method calls for.
- Enter all three thicknesses in the same unit and read the normalized percentages and texture class.
How the soil jar test calculation works
A jar test separates the mineral portion of a soil roughly by particle size. Sand settles first, silt forms the next layer, and the finest clay settles last. This calculator uses the thickness of each separate layer, so the numbers do not need to add to 100 and do not need to be measured in any particular unit.
Let D_sand, D_silt, and D_clay be the three layer thicknesses. Their total is T = D_sand + D_silt + D_clay. Each fraction is normalized with sand = 100 × D_sand ÷ T, silt = 100 × D_silt ÷ T, and clay = 100 × D_clay ÷ T.
The normalized point is assigned to one of the 12 USDA textural classes: sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, or clay. Its triangle coordinates are x = silt + clay ÷ 2 and y = (√3 ÷ 2) × clay. Those coordinates place every mixture in the same sand-silt-clay triangle used for USDA texture classes.
What the boundary result means
The nearest-boundary result is the shortest distance, in ternary percentage points, from your point to an internal edge of its USDA class polygon. A result within two percentage points is labeled near boundary. It is a measurement-sensitivity warning, not a statement that the soil has changed: repeating the test with a slightly different layer reading can place a near-boundary sample in the adjacent class.
What the calculator leaves out
A home jar test is a field estimate, not a laboratory particle-size analysis. Organic matter, poor dispersion, irregular jar diameter, aggregates, and clay that has not fully settled can make the visible layers differ from the mineral fractions used by the USDA triangle. Remove roots and debris before testing, and do not include floating material or an unsettled cloudy layer as clay.
Texture is only the relative proportion of mineral particle sizes. It does not measure fertility, pH, nutrient availability, compaction, soil structure, drainage at your site, or organic-matter content. Use a laboratory analysis when a management, engineering, or regulatory decision depends on a precise texture result.
Worked example
Suppose the separate layers measure 4 parts sand, 4 parts silt, and 2 parts clay. The total is 10 parts, so the normalized composition is 40% sand, 40% silt, and 20% clay. That point falls in the USDA loam polygon. Its triangle position is x = 40 + 20 ÷ 2 = 50.0 and y = (√3 ÷ 2) × 20 = 17.3.
The same result would come from 40 mm, 40 mm, and 20 mm layers, or from values entered as 40, 40, and 20. The calculator uses proportions, not an absolute jar size.
Common questions
How do I measure separate sand, silt, and clay layers in a jar?
Use the boundaries between the visible settled layers and record the thickness of each layer by itself. The sand is the bottom layer, silt is the layer immediately above it, and clay is the fine layer above the silt after it has settled. Do not enter cumulative heights from the jar bottom for silt or clay.
How long should a soil jar test settle?
Follow the timing of the method you are using, because sand, silt, and clay settle at very different rates. Clay can remain suspended much longer than the other fractions. If the water is still visibly cloudy, the clay measurement is likely incomplete.
Do the three measurements need to add to 100?
No. Enter three measurements made in the same unit, such as millimeters or fractions of an inch. The calculator divides each by their combined depth to create percentages that add to 100.
What does loam mean on the soil texture triangle?
Loam is a USDA texture class with a balanced enough combination of sand, silt, and clay to fall in the central loam polygon. It is not a measure of soil fertility or a guarantee of good garden performance. A loam can still be compacted, low in organic matter, acidic, saline, or poorly drained.
Can I enter laboratory sand, silt, and clay percentages?
Yes. Enter the three laboratory percentages as the three values, even if rounding makes their total slightly different from 100. The calculator normalizes the values before classifying them.
Why did a small measurement change produce a different class?
USDA texture classes meet at fixed boundaries, so a point close to an edge can move into a neighboring polygon after a small change. The near-boundary message calls out results within two percentage points of an internal class edge. Repeat the jar test or use a laboratory analysis when that distinction matters.