Free Online Grain Size Curve Plotter: D10, Cu, Cc and USCS

How to use this plotter

Enter your sieve analysis one line at a time: the aperture size in millimeters, a comma, then the cumulative percent passing that sieve. Work down from the largest sieve to the smallest, and finish with the pan value if you have one. If you want to compare materials, give each sample a name on its own line and leave a blank line between samples, and every curve will be drawn on the same axes in its own colour. A borrow pit and a specification envelope, or a wash sample before and after, sit naturally side by side this way.

Press plot and the classic semilog grading curve appears with coarse material on the left and fines on the right, exactly the way British and American standards draw it. The shaded bands behind the curve mark the gravel, sand, silt and clay size ranges so you can see at a glance which fraction dominates. The red construction lines show how D10, D30 and D60 are read: across from the 10, 30 and 60 percent marks on the vertical axis, down to the size axis.

What the numbers mean

D10 is called the effective size, and it matters far more than its modest name suggests. It is the size that only the finest tenth of the sample passes, and because the smallest particles control how water squeezes between grains, D10 drives permeability. The Hazen formula estimates hydraulic conductivity in centimetres per second as roughly D10 squared, with D10 in millimetres, which is why a sand with D10 of 0.2 mm drains freely and one with D10 of 0.02 mm barely drains at all.

The uniformity coefficient Cu is D60 divided by D10, so it measures how wide a range of sizes the soil contains. A value near 1 means every grain is nearly the same size, which is the signature of wind blown dune sand or a washed filter medium. Values above 4 for gravels or 6 for sands mean the soil is well graded, with small particles available to fill the gaps between larger ones. That matters because well graded soils compact to higher density, carry more load and settle less.

The coefficient of curvature Cc is D30 squared divided by D10 times D60, and it checks the shape of the curve between those points. A value between 1 and 3 says the distribution is smooth, with no missing size fraction. A value outside that range, especially with a high Cu, usually means the curve has a flat step in the middle: a gap graded soil, missing an intermediate size, which behaves worse than either a uniform or a well graded material because the missing fraction cannot brace the structure.

How the classification is decided

The tool follows the Unified Soil Classification System. First it reads the percentage passing 0.075 mm, the boundary between coarse and fine soils. If more than half the sample passes that sieve the soil is a silt or clay, and grading alone cannot classify it: you need Atterberg limits, and the tool says so rather than guessing. If the soil is coarse, it compares the gravel fraction above 4.75 mm with the sand fraction below it to choose between the G and S families.

Then the fines content decides the second letter. Below 5 percent fines the soil is clean, and Cu and Cc decide well graded W or poorly graded P. Above 12 percent fines the soil takes M for silty or C for clayey, which again needs Atterberg limits on the fine fraction. Between 5 and 12 percent the standard requires a dual symbol such as SW SM, and the tool reports that honestly rather than forcing a single answer.

Getting good input

Two habits protect the result. Wash the sample over the 0.075 mm sieve before dry sieving if there is any fine material, because dry sieving alone leaves fines stuck to the coarse grains and quietly understates the fines content, which is precisely the number that decides classification. And check that your percentages fall smoothly: a curve that steps backward means a sieve was skipped, weighed wrong or overloaded, and the tool will simply draw the mistake for you to spot.

Working through the demo

Press the demo button and two samples appear. The borrow pit sample spans from gravel down to a few percent fines, with Cu of 5.4 and Cc of 0.97: a broad range but a curvature just under 1, so it lands as poorly graded rather than well graded, a useful reminder that both criteria must pass. The dune sand tells a very different story: almost the entire sample sits between 0.15 and 0.425 mm, the curve is nearly vertical, and the low uniformity coefficient marks it as the classic uniform sand that compacts poorly and needs care as a fill material.

Why anyone cares beyond the symbol

Grading drives behavior in ways the two-letter symbol only hints at. Well graded soils compact to higher densities under the same effort, which is why fill specifications quietly prefer them. Uniform soils drain fast and predictably, which is why drainage blankets and filter sands are deliberately poorly graded in the USCS sense; the filter design rules that keep dams from eroding internally are written directly in terms of D15 and D85 ratios between adjacent materials, close cousins of these coefficients. And D10 itself moonlights as the effective size in Hazen's permeability approximation, so the same three numbers that classify the soil also produce the first estimate of how fast water moves through it. The one discipline worth keeping: these coefficients describe the coarse fraction only, and once fines exceed about 12 percent, plasticity takes over the classification and the grading window closes.

Related Calculators and Reference

The D10 from this analysis feeds directly into the Hazen permeability estimate, grain sizes convert between mm, phi, and Wentworth classes in the grain size converter, and when fines take over, classification moves to the Atterberg limits calculator. The USCS criteria are standardized in ASTM D2487; the Unified Soil Classification System overview lays out the full chart.