How To Calculate Bulking Spoil Volume

We show you how to calculate the true value of excavation when digging earthworks projects.

Article By: Tom Ayre | Last Update: November 2020

Calculating bulking factors are an important element in earthworks / excavations projects big and small.

Having removed a small amount of earth in my garden I was surprised at how much volume the soil bulked out to. I had to hire a grab lorry as the volume would have been too much for a skip! When working on a large earth moving project it is even more critical to calculate this correctly.

Fun fact, the deepest hole ever dug was 7.5 miles into the earth, that’s a lot of soil!

Knowing the volume of material after an excavation has taken place is important because:

  • Calculating accurate costs for removing material (muck away).
  • Working out how and where to store the material.
  • Factoring in the right amount of time to deal with the material.

In this blog article I will explain how bulking factors work, how to calculate by hand and give you a free to use calculator tool that will help with:

  • Calculating bulking factor volume.
  • Calculating how many trips to remove the material (muck away).
  • Calculating how much storage you need.

Let’s get started!

Table of Contents

What Are Bulking Factors?

A bulking factor is a number that is multiplied by the volume of material to get a true reflection of the uncompacted volume.

Having removed a small amount of earth in my garden I was surprised at how much volume the soil bulked out to. I had to hire a grab lorry as the volume would have been too much for a skip! When working on a large earth moving project it is even more critical to calculate this correctly.

To use a bulking factor number you have to multiply the bulking factor number by the excavated volume size.

For instance, a bulking factor for Clay and Gravel is typically 1.15

If your excavation was 15 m3.

Your volume size would be 1.15 x 15 = 17.25 m3

You must consider the bulking factor of the material you are digging to ensure you correctly plan for the removal costs and time to do it in.

What Are Diggability Factors?

Each material that you excavate will have different properties in terms of how easy they are to dig.

The most common way of categorising a soils digability or ease of digging is:

  • E = Easy digging – Loose free-running soils eg sands, fine gravels.
  • M = Medium – Denser cohesive soils eg clayey gravel, low PI clays
  • M-H = Medium to Hard – eg broken rock, wet heavy clay, gravel with boulders
  • H = Hard – material requiring blasting and hard high PI clays

In our calculator below, you will see the materials digability using the “E”, “M”, “M-H”,”H” format.

Calculate Bulking Spoil Volume

Use this tool to:

  • Calculate bulked out / excavation volume
  • Calculate how many trips to remove (muck away) the material.
  • Calculate how many storage units to store the material.

How To Calculate Volume Size

When calculating your volume size you will need accurate measurements. Depending on the size of the project it might be worth hiring a topographical survey company to measure the area / volume precisely. 

Rectangular Excavation

Volume = W x H X L
Example = 1.5 x 2 x 8
= 24 m3

rectangular cross section volume calculator

Cutting Excavation

Volume = ((H * W1) / 2) + (W2 * H) + ((H * W2) / 2)) * L
Example = ((0.5 * 0.3) / 2) + (1.5 * 0.5) + ((0.5 * 0.3) / 2)) * 2.1
= 1.89 m3

Cross section excavation diagram

For more complicated volumes, use an online calculator such as the one here.