How to read a sand sieve analysis: gradation curve, Cu and Cc

Introduction

Every bunker and every sand arena starts with a sand analysis report. Most superintendents and arena owners receive one before they buy, and most find it hard to read.

Reports come in different formats. Some list the percentage of sand passing through each sieve, others the percentage retained. Some use millimeters, others US mesh numbers or inches. Even experienced professionals need time to compare one report with another, or with a recommended range.

The quickest way to understand a sand is to see its gradation curve next to the range you are aiming for. This guide explains how to read the numbers behind that curve, using two real reports. It also shows how the free CapillaryFlow Sand Agent draws the curve for you in seconds.

This is the companion to our guide on how to choose the right bunker sand. That guide covers what to look for. This one covers how to check it.

Passing or retained, metric or imperial

A sieve analysis shakes a dried sample through a stack of sieves, from coarse at the top to fine at the bottom. The lab then weighs what is left on each sieve. There are two ways to report that same result.

• Retained is the share of the sample that stayed on each sieve, or in each size class. The numbers add up to 100%.
• Passing is the share that fell through each sieve. It starts at 100% on the coarsest sieve and drops towards 0% on the finest.

To turn retained into passing, start at 100% and subtract each retained value as you move down to finer sieves. Table 1 shows this for a real bunker sand, using the size classes in the lab report.

Table 1. Sand particle size distribution determined by sieve analysis, showing the percentage of material retained on and passing through each sieve.

The second difference is in the units used. U.S. laboratories often identify sieves by mesh number, which refers to the number of openings per linear inch, with higher mesh numbers indicating finer sieve openings. European labs usually give the opening in millimeters or micrometers. Some reports also give inches. A No. 60 sieve, a 0.25 mm sieve and a 250 µm sieve are the same sieve.

What the gradation curve tells you

The gradation curve plots the passing percentages against particle size. Particle size runs along the x-axis on a logarithmic scale while the percentage passing or retained runs along the y-axis.

Figure 1. Sample particle size distribution curves for a uniform sand and a broadly graded sand.

Three things are worth reading straight off the curve:

• Where the steep part sits. A steep section indicates that a large proportion of the sand particles falls within a relatively narrow particle-size range. If this section occurs in the medium to coarse sand range, the sand generally has larger pores and greater drainage potential. If it occurs in the fine sand range, the sand typically has smaller pores, greater capillary rise, and higher water retention.
• How steep it is. A steep curve is indicative of a uniform sand—a sand with similarly sized grains—whereas a flatter curve indicates a more broadly graded material. The smaller particles fill the voids between larger grains, resulting in a sand that packs more tightly and can provide greater stability.
• What happens at the fine end. This tail of a curve shows very fine sand and fines (silt and clay). A small change here has a large effect on drainage, because the finest particles fill the pores water needs to move through.

A recommended range is drawn as two curve envelopes. If your curve stays within the envelope, the particle sizes match that range. A distribution curve does not describe grain shape, mineral content, color or how the sand will behave once it has been played on for a few seasons. These characteristics require further laboratory analysis.

Coefficient of uniformity (Cu) and coefficient of curvature (Cc)

Two numbers sum up the shape of the curve. Both use D-values: the particle size at which a given percentage of the sample passes. D10 is the size that 10% of the sample is finer than. D60 is the size that 60% is finer than.

Cu, the coefficient of uniformity, tells you how wide the spread of grain sizes is.

Cu = D60 ÷ D10

A low Cu means the grains are similar in size. A higher Cu means a wider mix, so smaller grains fill the gaps between larger ones. In bunkers, the lab behind our bunker example notes that a higher Cu, within the recommended size ranges, generally gives firmer conditions than a more uniform sand. In arenas, a sand that is too uniform can lack grip, while too wide a mix compacts and gets hard.

Cc, the coefficient of curvature, tells you whether the middle of the curve is balanced.

Cc = D30² ÷ (D10 × D60)

Soil engineers read a Cc between 1 and 3 as an even spread of sizes. Below 1 means the finer half of the sample is weighted towards its smallest grains. Cc is less common on bunker and arena reports, but it helps when you compare two sands with a similar Cu.

Neither number replaces the curve. Two sands can share a Cu and still sit in very different size ranges, which is why the curve comes first. The CapillaryFlow Sand Agent calculates Cu and Cc for you from your gradation curve.

Hydraulic conductivity: how fast water moves through the sand

Saturated hydraulic conductivity is the rate water moves down through a saturated, compacted sample, usually reported in inches or millimeters per hour. Labs measure it in a column of sand, and it depends mostly on the finest part of the curve: a few percent more particles under 0.25 mm can cut it sharply.

The USGA suggests at least 20 inches (about 500 mm) per hour for bunker sand, and notes that sands starting at 30 to 100 inches per hour generally work well. The bunker sand in our example below measured 11.3 inches per hour, mainly because of its fine and very fine sand. The CapillaryFlow Sand Agent will estimate hydraulic conductivity directly from your gradation curve, as a first check before the lab test.

Why the USGA greens spec is not a bunker spec

The USGA does not publish an official specification for bunker sand. What it does publish is a recommended particle size range for building putting greens. This is the range the CapillaryFlow Sand Agent uses for golf, because it is the most widely accepted reference curve in the industry.

A putting green and a bunker serve different purposes. A putting green's root zone is designed to resist compaction, provide adequate drainage, and support healthy turfgrass growth. A bunker shares similar objectives regarding drainage and stability but does not need to support turfgrass roots. It also must drain fast, support a ball without plugging, stay on the faces, and play the same from bunker to bunker.

The putting greens sand ranges should be treated as a reference, not a target. For bunkers, labs typically assess sand against separate criteria, such as those by Brown and Thomas (1986) used in our bunker example and summarized in the USGA's guide to selecting bunker sand. They include no more than 3% silt and clay. The principles behind those numbers matter more than any single curve:

• Gradation controls drainage. Particles below 0.05 mm (silt and clay) and a high share of very fine sand slow infiltration and keep bunker floors wet. A very uniform medium sand can also drain slowly.
• Gradation controls stability. In a well-graded sand, smaller grains fill the gaps between larger ones, so the sand packs tightly and resists plugged lies. A very uniform sand shifts more.
• Sand changes after installation. Contamination, wash-out of bunker faces, and the accumulation of organic material at the bunker surface are part of the bunker aging process. The USGA cites field infiltration falling from 22.8 to 7.5 inches per hour within two to three years. A sand that meets a specified range at installation may not meet it after two years of use.

The same applies to arenas. Equestrian Surfaces: A Guide, from the Swedish Equestrian Federation and SLU, calls a sieve analysis a start, but not enough on its own, and recommends asking an expert or analyzing the sand in an arena you know works well. It quotes arena consultants who work to about 2 to 4% filler, the material finer than 0.063 mm, and a maximum of about 5% for fiber and wood mixes.

This is also where CapillaryFlow comes in. In bunkers and arenas built with the Capillary system, the Capillary Wash Box removes particles below 0.05 mm from the installed sand, without removing it. All it takes is a hose from the irrigation system, and the sand's gradation moves back towards its original curve.

Two real examples

Bunker sand: a golf club in North America

This sand was tested in July 2025 against the bunker criteria by Brown and Thomas (1986). Figure 2 gives passing percentages by size class, which we converted from retained values in Table 1.

Figure 2. Particle size distribution of a North American golf course bunker sand, expressed as cumulative percent passing.

Reading the report and the curve together:

• The steep part sits in the medium to fine range. 43.5% is medium sand and 37.8% is fine sand. Only 10% is coarser than 0.5 mm.
• Very fine sand is slightly over. 6.0% is very fine sand (0.05 to 0.10 mm), against a recommended maximum of 5%.
• Silt and clay are within range. Together they make up 2.7%, under the 3% maximum.
• Cu is 3.0, Cc about 0.9. The lab reports Cu. We calculated Cc from the same numbers (D10 0.10 mm, D30 0.17 mm, D60 0.31 mm). A fairly uniform sand, weighted towards its finer half.
• Drainage is lower than suggested. Measured hydraulic conductivity is 11.3 inches (287 mm) per hour. The lab notes this is below the 20 inches per hour minimum the USGA suggests for bunker sand, because of the share of particles under 0.25 mm.

The curve alone would have told you most of this in seconds: a sand on the fine side with a tail of very fine particles that will slow drainage. The lab's shape and infiltration tests complete the picture.

Arena sand example: washed silica sand for a project in Greece

For an equestrian arena project in Greece, the supplier's sieve analysis was entered into the first version of the CapillaryFlow Sand Agent calculator (Table 2). This report used US mesh numbers and grams retained.

Table 2. Equestrian sand particle size distribution determined by sieve analysis, showing the percentage of material retained on and passing through each sieve.

Reading it:

• Almost all of the sand sits in one envelope. 87% of the sample is between 0.15 and 0.5 mm, so the curve is very steep.
• Cu is about 2.0, Cc about 1.0 (our calculation: D10 0.18 mm, D30 0.26 mm, D60 0.37 mm). A very uniform sand, noticeably more uniform than the bunker sand above.
• No measurable fines. Nothing passed the finest sieve, which is what you expect from a washed sand. It will drain freely and is unlikely to compact or get dusty from fine particles.
• The curve landed just inside both boundaries of the equestrian range, so the sand passed this first check.

A sand that is this uniform has poor compactibility and thus, holds together poorly. Therefore, for any sand, its grip and stability depend heavily on its moisture content. For this reason, the uniformity and consistency of wetting the sand matter just as much as the sand itself.

No report yet? Check your bunker sand on site first

If your bunkers already drain badly, you can test the sand before you send anything to a lab. Our free Bunker Problem Solving Kit has four simple checks you can run with sand from the bottom of your bunkers:

• Fine particle test: shake sand and water in a jar and measure how much silt, clay and organic material settles on top. The USGA recommends no more than 3% silt and clay.
• Infiltration test: 2 inches of water should drain through 4 inches of compacted bunker sand in under a minute.
• Wash test: wash a sample and test it again, to see if your existing sand can be cleaned and reused.
• Grain shape check: look at dried sand through the phone microscope in the kit.

Check your own sand report

The CapillaryFlow Sand Agent reads your sand analysis report in seconds. Upload the report or type in the values, in metric or imperial, passing or retained. It draws your gradation curve against the right range for your sand and calculates Cu and Cc. Every analysis is saved, so you can compare suppliers.

Read more: How to choose golf bunker sand · Sand for horse arenas: how to choose · Managing sand moisture · Capillary Wash Box

Sand testing FAQ

What does a sand test include?

A standard physical test covers particle size (the sieve analysis) and usually grain shape. For bunker sand, labs often add infiltration rate and color. For root zone sand, they add porosity, water retention and hydraulic conductivity.

Where can I get my sand tested?

Use a physical soil testing lab that works with golf and sports turf. Labs typically follow ASTM F1632 for particle size and sand shape. Your supplier may also have a recent report for the sand you are buying, which is a good first check.

How do I take a sample?

Take small samples from several places, such as several bunkers or across the whole arena, and mix them into one. For a new sand, ask for a sample from the batch you will actually receive.

How often should I test bunker sand?

Before you buy, and again when bunkers start to drain slowly or play differently. Fines build up after installation, so a sand that tested well on delivery can fall outside the range within a few seasons.

Can I test sand myself?

You can run simple checks on site: a jar test for fines and an infiltration test with compacted sand. Our Bunker Problem Solving Kit includes both. They show whether there is a problem. A lab test shows why.

Does the CapillaryFlow Sand Agent replace a lab test?

No. It reads the lab or supplier report you already have, draws the curve and calculates Cu and Cc, so you can compare sands quickly. Grain shape, infiltration and chemistry still need a lab.

Is bunker sand tested the same way as arena sand?

The sieve analysis is the same, but the results are compared against different ranges. Bunker sand is judged on drainage and plugging. Arena sand is judged on support and grip, which depend on moisture as much as on particle size.

Sources

• Swedish Equestrian Federation and SLU, Equestrian Surfaces: A Guide, hosted by the FEI
• Brian Whitlark, A guide to selecting the right bunker sand for your course, USGA Green Section Record
• USGA putting green construction resources
• ASTM F1632, particle size analysis and sand shape grading of root zone mixes
• Managing Sand Moisture, CapillaryFlow
• Sustainable bunker care, CapillaryFlow