Calculate Cu from D60 and D10, or add D30 for Cc and a limited clean-sand or clean-gravel gradation check. D values come from the same percent-finer-by-mass curve.

Use diameters at 10% and 60% finer by mass from the same gradation curve.

Related Calculators

Coefficient Of Uniformity Formula

The following formula is used to calculate the coefficient of uniformity for a given set of diameters.

CU = D₆₀ / D₁₀

Variables:

  • CU is the coefficient of uniformity
  • D60 is the diameter at 60% finer by mass on the cumulative gradation curve
  • D10 is the diameter at 10% finer by mass on the same cumulative gradation curve

Divide D60 by D10 using consistent diameter units. For the full check, Cc = D30² / (D10 × D60). The gradation criteria are Cu ≥ 4 for clean gravel or Cu ≥ 6 for clean sand, together with 1 ≤ Cc ≤ 3. This check assumes less than 5% fines by mass and an independently established sand/gravel category; it does not perform complete USCS identification. See USACE gradation definitions.

What is the Coefficient Of Uniformity?

The coefficient of uniformity (CU) is a measure used in soil mechanics and sedimentology to describe the uniformity of particle sizes in a given sample. It is calculated as the ratio of the diameter at the 60th percentile (D60) to the diameter at the 10th percentile (D10). A higher CU value indicates a wider range of particle sizes, while a lower CU value indicates a more uniform particle size distribution. Cu is one gradation measure. Cu alone cannot establish well-graded soil, construction suitability, slope stability or embankment safety; those assessments require additional soil data and engineering analysis.

How to Calculate Coefficient Of Uniformity?

The following steps outline how to calculate the Coefficient Of Uniformity.

  1. First, read D60 at 60% finer by mass on the cumulative gradation curve.
  2. Next, read D10 at 10% finer by mass on the same curve, using the same diameter units.
  3. Next, calculate the coefficient of uniformity using the formula CU = D60 / D10.
  4. For the example below, Cu = 0.6 / 0.2 = 3. This ratio alone does not classify the soil.
  5. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Diameter at 60th Percentile (D60) = 0.6 mm

Diameter at 10th Percentile (D10) = 0.2 mm