---
title: "Coefficient of Uniformity (Cu) | Intro to Civil Engineering"
description: "Coefficient of uniformity (Cu) is D60/D10, a soil gradation ratio that shows particle-size spread and helps predict compaction and soil behavior in civil engineering."
canonical: "https://fiveable.me/introduction-civil-engineering/key-terms/coefficient-of-uniformity-cu"
type: "key-term"
subject: "Intro to Civil Engineering"
unit: "Unit 6"
---

# Coefficient of Uniformity (Cu) | Intro to Civil Engineering

## Definition

The coefficient of uniformity (Cu) is the ratio D60/D10 that measures how wide a soil's particle-size range is. In Intro to Civil Engineering, it is used to judge soil gradation and likely engineering behavior.

## What It Is

The coefficient of uniformity (Cu) is a soil gradation number used in Intro to Civil Engineering to describe how spread out the particle sizes are in a sample. It is calculated as Cu = D60 / D10, where D60 is the particle size at which 60% of the soil is finer and D10 is the size at which 10% is finer.

A small Cu means the particles are clustered around a narrow size range, so the soil is more uniform. A large Cu means the sample includes a wider spread of sizes. That spread matters because a mix of particle sizes can pack together more efficiently than nearly equal-sized grains.

You usually get D10 and D60 from a grain-size distribution curve, which comes from sieve analysis for coarse soils or sedimentation testing for finer soils. The curve tells you how much of the sample passes each particle size, and Cu pulls one useful comparison out of that graph.

In practice, Cu is one piece of soil classification, not the whole story. A soil can have a high Cu and still not behave the way you want if the particle-size distribution is missing a middle range or if the soil has too many fines. That is why civil engineering classes often pair Cu with other measures like the coefficient of curvature and Atterberg limits.

A common classroom example is comparing a uniform sand with a well-graded sand. The uniform sand has particles that are all about the same size, so it has a low Cu. The well-graded sand has a wider spread of sizes, so the smaller grains can fill the voids between larger grains, which usually improves packing and can affect strength, density, and drainage.

Do not treat Cu as a simple quality score by itself. It describes the spread of sizes, and civil engineers interpret that spread in the context of compaction, permeability, stability, and the kind of structure or earthwork being designed.

## Why It Matters

Cu shows up whenever you need to predict how a soil will behave before you build on it. In Intro to Civil Engineering, that means linking grain-size data to real decisions about foundations, road subgrades, slopes, and earth fills.

If a soil is well graded, the smaller grains can fit into the spaces between larger grains. That usually leads to denser packing, fewer voids, and different drainage behavior than a soil with almost all the same-size particles. A uniform sand may drain quickly, but it may not compact or interlock the same way a broader gradation does.

This is why Cu is tied to soil classification. When you look at a gradation curve, you are not just naming the soil, you are trying to predict performance. A design choice like whether a base layer under a pavement will settle, compact well, or stay open enough for water to move through depends partly on particle-size distribution.

Cu also connects to later geotechnical topics. Once you start talking about effective stress, bearing capacity, or consolidation, the grading of the soil is part of the physical setup underneath those calculations. Cu does not replace those later tools, but it helps explain why two soils with different grain-size spreads can behave very differently under the same load.

## Connections

### Soil gradation

Coefficient of uniformity is one way to describe soil gradation. Gradation is the broader idea of how particle sizes are distributed across a sample, usually shown on a grain-size distribution curve. Cu gives you a quick ratio that summarizes the spread, but gradation tells you the full pattern, including whether sizes are missing in the middle or concentrated in one range.

### [Coefficient of curvature (cc)](/introduction-civil-engineering/key-terms/coefficient-of-curvature-cc)

Cu tells you how wide the particle-size range is, while coefficient of curvature looks at the shape of the gradation curve in the middle sizes. Engineers often use both together when checking whether a soil is truly well graded. A soil can have a decent Cu but still have a curve shape that suggests a gap in sizes.

### [Coefficient of permeability (k)](/introduction-civil-engineering/key-terms/coefficient-of-permeability-k)

Cu affects how tightly particles can pack and how much void space is left between them, which can change permeability. Soils with a broader range of sizes often compact differently and may have lower or more complex water flow than a soil made of nearly equal-sized grains. The relationship is not one-to-one, but grading is a big clue.

### [bearing capacity](/introduction-civil-engineering/key-terms/bearing-capacity)

Bearing capacity is about how much load soil can support without failing or settling too much. Cu matters because particle-size distribution influences density, friction, and how well grains interlock. A soil with better grading often behaves more favorably under load than a loose, uniform soil, though moisture and fines also matter.

## On the AP Exam

A quiz or problem set may give you a grain-size distribution curve and ask you to read D10 and D60, then calculate Cu with Cu = D60 / D10. You may also be asked to decide whether the soil looks uniformly graded or well graded and explain what that means for compaction, drainage, or stability. If the question includes a design scenario, use Cu as evidence, not as a standalone answer. Say what the ratio suggests about particle packing and how that might affect a pavement base, fill material, or foundation soil. A strong response connects the number back to soil behavior, not just the formula.

## coefficient of uniformity (cu) vs coefficient of curvature (cc)

These two are often paired on the same soil-gradation problem, but they measure different things. Cu compares the spread between coarse and fine ends of the curve, while cc describes the shape of the middle portion. If you only use Cu, you may miss whether the gradation curve has the right overall form.

## Key Takeaways

- The coefficient of uniformity (Cu) is calculated as D60 divided by D10 and summarizes how wide a soil's particle-size range is.
- A low Cu points to a more uniform soil, while a higher Cu usually means a broader range of grain sizes and better gradation.
- Civil engineers use Cu with a grain-size distribution curve to judge soil behavior before building roads, foundations, or earth fills.
- Cu matters because particle-size spread affects packing, void space, drainage, compaction, and sometimes strength.
- Cu is useful, but it is not the whole soil story. You often need coefficient of curvature and other soil tests to classify a soil correctly.

## FAQs

### What is coefficient of uniformity (Cu) in Intro to Civil Engineering?

Cu is a soil gradation ratio, equal to D60 divided by D10. It tells you how spread out the particle sizes are in a sample. In civil engineering, that spread helps you predict whether a soil is more uniform or more well graded.

### How do you calculate Cu from a grain-size distribution curve?

Find the particle size at 60% finer, which is D60, and the size at 10% finer, which is D10. Then divide D60 by D10. The result gives a quick measure of the soil's size range, but you still need the full curve to interpret the soil properly.

### Is a higher Cu always better?

Not always. A higher Cu usually means a broader mix of particle sizes, which can improve packing, but the soil still has to fit the design purpose. For example, a soil that drains well for one project may not be ideal where you want low permeability or specific compaction behavior.

### What is the difference between Cu and coefficient of curvature?

Cu measures the spread from fine to coarse sizes, while coefficient of curvature checks the shape of the middle part of the gradation curve. Engineers often use both together when deciding whether a soil is well graded. Cu alone does not tell you if the size distribution has gaps.

## Related Study Guides

- [6.1 Soil Classification and Properties](/introduction-civil-engineering/unit-6/soil-classification-properties/study-guide/jcVOir1wUsnPxLJj)

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