When planning any excavation job, one of the earliest questions is how much material will need to be removed. Builders, groundworkers and homeowners all face this challenge. You need to know the size of the excavation, convert that into a volume of muck, then estimate how many lorry loads it will take to cart that muck away. Getting this right helps with budgeting, scheduling, and avoiding delays on site. It also helps for choosing the right digger for the job.

This simple 5-step guide shows you how to approach the calculation with confidence.

Step 1. Measure the excavation area

Start by measuring the length, width and depth of the area. Use metres for consistency. If the shape is irregular, break it down into smaller sections and measure each part. Accuracy at this stage leads to better calculations later.

You’ll need:

• Length in metres
• Width in metres
• Depth in metres

If the depth varies across the site, use a sensible average.

Step 2. Calculate the excavation volume

If the excavation is a simple rectangular shape, the formula is straightforward:

Volume = Length × Width × Depth

This gives the volume in cubic metres.

Example:

• Length: 10 metres
• Width: 5 metres
• Depth: 0.8 metres

Volume = 10 × 5 × 0.8 = 40 cubic metres

If the excavation has more than one section, calculate each part and add the volumes together.

This same method is used to calculate concrete volume.

Step 3. Allow for bulking of materials

Once soil comes out of the ground, it loosens and expands. This is called bulking. The amount of bulking depends on the soil type.

Typical bulking factors:

• Clay: around 1.2 to 1.3
• Sand: around 1.1 to 1.2
• Gravel: around 1.1 to 1.2
• Topsoil: around 1.2 to 1.4

Multiply your in-ground volume by the bulking factor to get the loose volume that will be loaded into lorries.

Example using clay:

40 cubic metres × 1.25 = 50 cubic metres of loose material

Step 4. Work out how many lorry loads you need

Lorry payloads vary. Different operators use different vehicles, and the tare weight of each vehicle affects how much muck can be legally carried. Soil type and moisture content also affect how quickly you reach the weight limit.

The common ranges are:

• 8-wheel tipper lorry: usually between 18 and 20 tonnes
• 8-wheel grab lorry: usually between 14 and 16 tonnes

Loose soil often works out at around 1.2 to 1.4 tonnes per cubic metre, depending on type and moisture. Because of this, a full load by volume does not always equal the stated payload in tonnes. Wet clay, for example, is heavy and can hit the weight limit before the body is fully filled.

To keep the example simple, we will convert using a typical loose soil density of 1.3 tonnes per cubic metre.

Example:

Loose volume: 50 cubic metres

Estimated weight: 50 × 1.3 = 65 tonnes

If you are using an 18-tonne tipper: Number of loads = 65 ÷ 18 = about 3.6

Round up to 4 loads

If you are using a 16-tonne grab lorry: Number of loads = 65 ÷ 16 = about 4.1

Round up to 5 loads

Step 5. Always check with your muck away contractor

This is essential. Actual payload depends on:

• The tare weight of the specific vehicle
• The soil type and moisture
• Local road weight regulations
• The rules for waste material versus aggregate deliveries

Two firms may give completely different load capacities even for the same type of lorry. Your contractor will tell you the real figures for the vehicles they run, which allows you to refine your calculations.

Final thoughts

Calculating excavation volume and estimating lorry loads becomes straightforward once you understand the method. Use accurate measurements, allow for bulking and apply realistic payload ranges rather than fixed values.

Construction Training

Click on images for details