Last Modified: April 4, 2024

Sub-Base Calculator

DIY estimator for sub-base Construction

Sub-base calculator

By Ben Parrot - Landscape Gardener & Amateur Geologist


A sub-base is an essential part of many landscaping projects, particularly those built for traffic, whether foot, equine or vehicular. It provides a solid foundation for stability of the top layer and drainage to provide durability. Our sub-base calculator can help you plan your project efficiently and make sure you have the right amount of subbase materials for your needs. It calculates the exact amount of material you need in both metric and imperial units and returns the results in cubic metres as well as the number of standard bulk bags. Just input your project's dimensions, and our calculator gives you an accurate estimate. You can also input the price per unit of material to estimate your project's total cost.

Just a quick note about our new tool, called the 'Pebble Picker', in the sidebar and menu. It allows you to effortlessly browse and choose from every decorative aggregate available based on their colour and type.

Calculating garden gnome

Key Takeaways

  • Simplicity: Utilising a sub base calculator streamlines the planning process for any landscaping project.
  • Accuracy: Precise calculations reduce waste and additional costs.
  • Efficiency: Saves time by quickly providing estimates for your sub-base materials.
  • Imperial or Metric: Our calculator works with both feet and metres.
Pen ruler and plan drawing. Jpg

Type 1 Sub Base Calculator

In the following sections we provide you with step-by-step methodology for using the sub-base calculator for different kinds of crushed aggregate materials. We begin with using metric measurements and then follow with imperial measurements.

We also have a comprehensive article all about sub-base and it's applications, called What is Sub Base.

Using Metric Measurements

This section will guide you through the calculator using metric measurements, with a straightforward method to estimate the volume of gravel needed in cubic metres and bulk bags.

  1. 1
    Measure the Area: Measure the length and width of the area for laying the sub-base. Record the dimensions in metres (m).
  2. 2
    Depth: Decide how deep you want the crushed aggregate layer to be. Measure the depth in centimetres (cm).
  3. 3
    Input the Measurements: Enter the length, width and depth of the area into the correct boxes in the calculator.
  4. 4
    Bulk Bag Cost: Input the price of the sub-base, per bulk bag, for an approximate cost, excluding delivery. (Optional)
  5. 5
    Calculate Volume: Click the calculate button. If you make a mistake, just reset and start again.

Next, we go through imperial measurements, using feet and inches.

Tape measure on a lawn. Jpg

Using Imperial Measurements

This section will guide you through using the calculator with imperial measurements so that you can confidently proceed to calculate the amount of sub-base you'll need in feet and inches before a result is also returned in cubic metres and bulk bags.

  1. 1
    Switch: Click the button at the top to switch to Imperial.
  2. 2
    Measure the Area: Measure the length and width of the area in feet (ft).
  3. 3
    Depth: Determine the desired depth of your foundation layer, measured in inches (in).
  4. 4
    Input the Measurements: Enter the length and width in feet and the depth in inches into the calculator.
  5. 5
    Bulk Bag Cost: Input the price of the sub-base, per bulk bag, for an approximate cost, excluding delivery. (Optional)
  6. 6
    Calculate Volume: Click the calculate button. If you make a mistake, just reset and start again.

That's it then. You know how much sub-base materials you need. Next are some additional tips for measuring accurately and determining the optimal sub-base.

Measuring a bulk bag on a shippings pallet. Jpg

Installation Tips

In this section, we provide additional tips and insights to help you measure-up complex shapes and how to determine the appropriate depth for some common sub-base applications. We also provide some information about the different types of crushed aggregates available for your project.

Complex Shapes

If the area you're working with is not a simple rectangle or square, divide it into manageable geometric shapes, calculate the volume for each, and then sum them up.

Gravel garden irregular shape

Depth Guidelines

In climates where the soil seldom freezes, if you are using a 50mm (2 inches) thick top layer, you will need to excavate 150mm (6 inches) deep - 100mm (4 inches) for the sub-base and 50mm (2 inches) for the top layer.

Where the soil freezes more than 150mm (6 inches) deep, the foundation needs to be 300mm (12 inches) below the surface, which will generally require a thicker layer of subgrade ~200mm compacted and a thicker layer of surface materials, ~100mm.

Layers and Compaction

When building the base, each layer of the sub-base material should be no thicker than 150mm (6 inches) before compacting to minimise the risks of settlement.

How to Calculate Sub Base for Patio Foundations

To calculate the sub-base required for patio foundations, first measure the area of the patio by multiplying its length by its width. Next, determine the desired depth of the sub-base, typically recommended to be at least 100mm fully compacted. Finally, convert this volume into cubic metres (length x width x depth in metres) or bulk bags, using our calculator, below. For more detailed guidance, please see our page Sub Base Depth for Patio Installation

Garden excavation in preparation for gravel installation scaled. Jpg

Need to work out how much gravel your project needs? Try our free gravel calculator.

Sub-Base Aggregate Types

Different types of gravel or aggregate may settle or spread differently, so it's wise to consult with a supplier if you're unsure about the depth or volume needed. Here is a list of commonly used sub-base aggregates available in the UK:

Grano Dust

One of the most common sub-base materials is crushed granite stone, often referred to as grano dust or 4-6mm dust. This type of base aggregate is well-suited to the laying of astroturf or as an artificial grass base.

Grano dust close up

MOT Type 1 (Ministry of Transport Type 1)

MOT Type 1 sub-base, also known as 'hardcore', is a high-quality unbound aggregate used to provide a stable foundation for roads, pathways and patios. It has great load-bearing and compaction properties, making it ideal for heavy traffic areas.

  • MOT Type 1 is a strong foundation material used for roads and driveways.
  • It's made of crushed rock, slag, or concrete, ranging in size from 60mm to dust for compacting.

MOT Type 2 (Ministry of Transport Type 2)

MOT Type 2 sub-base is an unbound aggregate that is also used as a sub-base material but is generally considered to be slightly lower quality than Type 1. MOT Type 2 is a primary sub-base material suitable for less critical applications such as paths or base layers where lower traffic volumes are expected.

Grey slate chippings around artificial grass

MOT Type 3 (Ministry of Transport Type 3)

MOT Type 3 sub-base is a lower grade specifically designed for use in areas requiring effective drainage, such as sustainable urban drainage systems (SUDS), permeable pavements, and in landscaping projects where water accumulation can be an issue.

  • Known for its permeability, featuring a more open gradation that allows water to pass through more easily than MOT Type 1 or 2.
  • It's made from crushed aggregate with sizes from 38mm down to dust.

Sub-Base Calculator

Outdoor Aggregates Sub-Base Calculator



(excluding delivery)


In conclusion, planning and executing a landscaping project can be daunting, but with the right tools and resources, it can be a smooth process that yields great rewards. Our sub-base calculator is one such tool designed to simplify your planning by helping you order the right amount of sub-base for your project. So go ahead and give it a try, and happy gardening!

Black limestone patio. Jpg

Commercial photography by StudioRoost


How do you calculate sub-base?

To calculate the sub-base quantity needed for your project, measure the length and width of the area, multiply to get the area in square metres, decide on the desired depth, convert to metres, multiply the area by the depth to find the volume in cubic metres, and then multiply by the density of the material to get the required tonnes.

How much sub-base per square metre?

For a driveway project, approximately 250kg of Type 1 sub-base is required per square metre, assuming a standard depth of 15cm (150mm). This calculation is based on the typical compaction and coverage needs for a durable and stable driveway foundation. Type 1 sub-base is widely regarded for its optimal blend of crushed stone and fines, ensuring a solid base that facilitates effective drainage and load distribution.

How much sub base do I need for patio?

To build a patio, it's important to excavate to a depth that accommodates the layers, including sub-base and mortar, and the thickness of paving slabs. This ensures a strong foundation. Also, when building close to a property, ensure the patio surface is at least 150mm below the damp proof course to avoid moisture ingress. This meticulous approach to depth and layering is crucial for a functional and long-lasting patio.

How deep should a sub base be for a patio?

When constructing a patio, excavate to a depth that accommodates the layers and thickness of the slabs for a strong foundation. Keep the finished surface at least 150mm below the damp proof course to avoid moisture ingress. A meticulous approach to depth and layering is crucial for a functional and long-lasting patio.

How much sub-base do I need for driveway?

A strong sub-base layer is essential for driveways to bear vehicular traffic and prevent sinking or cracking. It should be at least 150mm deep to ensure stability, longevity, and durability. Meticulous planning and execution are necessary for proper sub-base installation in driveway construction.

How do you calculate base material?

To calculate the amount of base material needed for your project, measure the length and width of the area, determine the desired depth, and multiply them to find the volume. Base materials are usually sold by weight, so convert the volume to tonnes using the material's specific gravity. This method ensures accurate material estimation and reduces waste.

We have every type of gravel available to choose from in the UK. Check them out!

About the author 

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.