Welcome to the first part of our Foundations in Landscaping series, Understanding Gravel Sizes, which is written to help you get a better understanding of the different gravel sizes and how they are used in garden design. Gravel, whether a decorative element or construction aggregate, is a humble yet essential part of landscaping and construction. From pebbles crunching underfoot on a garden path to the solid, compacted foundation of a driveway, the importance of these materials, whether gravelly or smooth, cannot be overstated.
In this article, we'll use the ISO 14688 to show how to assess grading from fine to coarse and then dig into which of the different grades are best suited to the most popular landscaping uses, including examples of gravel sizes with pictures. Let's rock!
Which size is best for each type of applications, such as driveway gravel sizes.
List the most common landscaping and construction applications for gravel.
Introduction to the the ISO standardisation scale.
We introduce you to our Pebble Picker, the first and only search filter for decorative aggregates.
What is Gravel?
Gravel aggregates are a mainstay in the world of building construction and landscaping. From tiny pebbles to slate chippings, it is defined as a collection of stones, which are essentially small fragments of rocks. Gravel is sourced from a range of stone types, including limestone, granite, and sandstone, meaning that a large selection of colour tones is also available. The shapes range from rounded to sub-angular, rough, and angular crushed stone. However, its size can also vary significantly.
Whether the stones are as smooth as river pebbles or rough like a boulder; they have been put through a sieve to create uniformity, and each serves a purpose. For example, the best size of gravel for driveway surfaces would be the one that doesn't get stuck in your tyres.
Gravel Sizing Guide
Whether stone chippings or gravel comes under different classifications, depending upon the particle size and classification methodology, which is either ISO or the Udden-Wentworth scale. We provide both, although the ISO classifications are adequate for determining the best sizes for each type of landscaping application, so we will use the ISO for this article.
Udden-Wentworth scale
The Udden-Wentworth scale, developed by geologists Johan August Udden and Chester K. Wentworth, is a granulometric scale that classifies sediment particles based on their size. This scale is quite technical, providing a detailed framework for categorising gravel and other sediment types. Essentially, it classifies gravel as particles ranging from 2 mm to 64 mm in diameter, offering a precise and standardised method for sediment size classification.
Granule: 2mm to 4 mm (0.079-0.157 inches)
Pebbles: 4mm to 64 mm (0.2-2.5 inches) particles.
Cobblestones: 64mm to 256 mm (2.5–10.1 inches)
Boulders: Larger than 256 mm (10.1 inches).
International Organisation for Standardisation ISO 14688:
Fine gravel: Pea gravel sizes refer to gravel with a particle size greater than 2mm but less than or equal to 6.3mm. (>2mm ≤ 6.3mm).
Medium gravel: This gravel contains particles larger than 6.3mm but smaller or equal to 20mm. (>6.3mm ≤ 20mm).
Coarse gravel: This gravel consists of particles with a size between 20mm and 63mm. (>20mm ≤ 63mm).
Next, for simplicity and your convenience, we have tabulated the common applications in landscaping and design and matched the sizes according to their suitability. The classifications range from the most ideal size gravel to unsuitable or undesirable sizes for each use.
Need to work out how much gravel your project needs? Try our free gravel calculator.
Choosing the Best Size for Applications
Considering the gravel size is important when it comes to selecting the right material for your landscaping project, as it can impact the functionality, longevity and future maintenance requirements of your outdoor feature. Due to the natural variation that occurs during mechanical processing through a screen, there is a range of diameters that will fit into each general size category, fine, medium or coarse. Please bear in mind that this is just a guide and there are always exceptions to every rule.
Pea Gravel: Small, usually round and smooth, it is a very versatile and affordable option for pathways and playgrounds. Its texture makes it comfortable underfoot, and pea gravel size typically ranges from about 2mm to 14mm.
2
Crushed Stone: Known for its durability and sharp, angular shape, crushed stone is suitable for areas that require a sturdy surface. It's a common choice for high-traffic areas due to its resilience and superior interlocking, ranging from 8-30mm diameter.
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River Rock: Materials like river gravel, have a naturally smooth and polished appearance. However, they are not ideal for driveways, where uniformly compacted gravel is often more desirable for stability. Available from 5mm to 30mm and larger.
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Slate Chippings: Unique in appearance and strong in composition, slate offers a layered look due to its flat, sedimentary shape. They come as 10mm slate mini mulch and right up to 40mm slate chippings, with even larger paddlestones also available.
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Self-binding Gravel: A blend of fines (dust), gravel and sometimes clay, that looks like traditional gravel but has the stability of a bonded base. It is available in various sizes and is ideal for outdoor applications like paths and outdoor living areas.
Ultimately, choosing the right size of gravel is important to getting the best outcome for your landscaping project. If you work within the guidelines set out above and choose a type of gravel that you like - whether it's a particular colour or texture - according to the size guide, you should be well on your way to a successful outcome.
Summary
Gravels come in a variety of sizes, ranging from small pebbles to larger stones, and choosing the appropriate size for your landscaping project is crucial for achieving many of the specific practical benefits. It's a versatile material that can enhance the beauty and functionality of your outdoor space.
From the fine gravels ideal for resin-bonded applications to coarse gravel for foundations and decorative borders, understanding and choosing the right gravel size is key to component to achieving your landscaping goals. Now that you have the knowledge, it should be clear that sometimes 'size does matter', at least in landscaping!
Just Before You Go..
In case you weren't aware, the Pebble Picker is a new search filter that allows you to choose decorative aggregates based on their colours. The filter provides access to the largest range of colours and types of aggregates available in the UK, making it easy to find the ideal shades or materials for any project. Whether it's a garden design or an architectural project, the search filter guarantees access to the biggest selection quickly and easily. You can also get started quickly by heading to the sidebar.
FAQs
What are the different sizes of gravel available?
Gravel sizes range widely, typically classified by the Udden-Wentworth scale, from fine gravel (2mm to 6.3mm) to medium (6.3mm to 20mm) and coarse gravel (20mm to 63mm), extending to boulders over 256mm in diameter.
What are the common uses for small-sized gravel?
Small-sized gravel, often fine pea gravel, is ideal for garden paths due to its comfort underfoot, decorative borders, and as a top layer in plant beds for moisture retention and weed deterrence. The sizes of pea gravel range from 2mm-14mm in diameter.
How does gravel size impact drainage?
Gravel size significantly impacts drainage; coarser gravel provides better drainage due to larger spaces between the rocks, making it ideal for French drains and areas needing efficient water runoff.
How does the size of gravel affect stability in walkways?
For walkways, medium-sized, angular gravel offers stability and comfort underfoot, preventing excessive movement and providing a solid walking surface. However, medium, round pea gravel is more comfortable and perfectly suited to pathways.
Can I use large-sized gravel for garden pathways?
Large-sized gravel is generally unsuitable for garden pathways due to its bulk and discomfort underfoot, making it better suited for foundational purposes or as decorative accents in landscaping.
What size gravel for a driveway surface is best?
The ideal gravel size for driveways is medium gravel, particularly around 20mm, which provides a balance between stability and ease of walking while also preventing significant displacement.
We have every type of gravel available to choose from in the UK. Check them out!
Ben Parrot is a landscape gardener and amateur geologist. He is the founder of Outdoor Aggregates, with the mission to create the first encyclopaedia for decorative aggregates. From natural stone through to decorative bark, these aggregates are critical to garden and landscape design, universally.
The problem: There isn't any place where all the different types of decorative landscaping materials and construction aggregates can be found, nor are the specifications of each type readily available. So, enriched by his extensive experience, and inspired by the timeless elegance and raw beauty of stone and rock, etched by nature's artistry over millennia, Outdoor Aggregates was born.
