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How to Calculate Excavation Volume and Spoil Removal: Bulking, Skips and Tipper Loads

A dig that measures 20 m³ in the ground does not produce 20 m³ of spoil. Learn how to calculate excavation volume, apply soil bulking, work out skip and tipper loads and estimate disposal costs, with worked examples and a free calculator.

Last updated 7 November 2026

Why the dig volume is not the spoil volume

Ground in its natural state is compacted. Once you dig it, it loosens and takes up more space. This increase is called bulking or swell.

So a dig that measures 20 m³ in situ can easily become 24 m³ of loose spoil to cart away if bulking is 20%. Skips, tippers and tip charges all deal with the loose volume, so planning on the in-situ figure means you under-order.

KaelSite’s free Excavation and Spoil Removal Calculator converts the dig volume to loose spoil, counts the loads and journeys, and can estimate disposal cost.

The basic formulas

Work in metres so volumes come out in cubic metres (m³).

  • In-situ volume for trenches, pits, foundation digs and basement digs: length × width × depth
  • Circular pit or sump: π × (diameter ÷ 2)² × depth
  • Loose spoil: in-situ volume × (1 + bulking %)
  • Number of loads: loose spoil ÷ usable vehicle capacity, rounded up

For several identical pits, calculate one and multiply by the quantity.

Choosing a bulking percentage

The right figure depends on the ground. As a rough UK guide:

Ground Typical bulking
Sand and gravel 10–15%
Mixed ground about 20%
Topsoil about 25%
Clay 25–40%
Chalk 30–40% or more

 

These are guides only. Your site investigation and previous tip tickets are better evidence than generic figures. The calculator offers preset soil types and a custom percentage.

Worked examples

1. Service trench
A trench 30 m long, 0.6 m wide and 1.0 m deep in mixed ground (20% bulking):
30 × 0.6 × 1.0 = 18 m³ in situ
Loose spoil: 18 × 1.20 = 21.6 m³
With an 8 m³ tipper: 21.6 ÷ 8 = 2.7, so 3 loads.
As an example only, at £30 per m³ disposal the cost would be about £648. Use your own rates.

2. Extension foundation dig in clay
A dig 5 m × 4 m × 1.2 m deep in clay (30% bulking):
5 × 4 × 1.2 = 24 m³ in situ
Loose spoil: 24 × 1.30 = 31.2 m³
With 6 m³ skips: 31.2 ÷ 6 = 5.2, so 6 skips.

3. Circular sump in chalk
A sump 2 m in diameter and 3 m deep in chalk (35% bulking):
π × 1² × 3 = 9.42 m³ in situ
Loose spoil: 9.42 × 1.35 = 12.7 m³
With a 10 m³ grab wagon: 1.27 loads, so 2 loads.

Use the capacity you actually achieve

A 6 m³ skip rarely takes a full 6 m³ of wet clay, and some vehicles cannot be filled to the brim safely or legally. Enter the usable capacity you really achieve on site, not the headline size. This gives a more honest number of loads.

Loads, journeys and vehicles

Each filled skip or tipper normally needs one tip journey. If you have more than one vehicle, divide the journeys between them to plan the day. Remember the round trip time to the tip, queuing and loading time, not just the number of loads.

Common mistakes

  • Planning on in-situ volume. This under-orders skips and tippers every time.
  • Using one bulking figure for all ground. A trench through topsoil, clay and gravel needs a mixed or layer-by-layer approach.
  • Ignoring over-dig. Machine buckets and unstable sides mean you dig more than the drawing shows.
  • Forgetting wet ground. Wet soil is heavier, bulks differently and fills skips less efficiently.
  • Leaving out disposal extras. Permits, waiting time and contaminated spoil can add significant cost.
  • Not allowing for reuse. If you backfill or re-use material on site, only the surplus needs removing.

UK waste and safety points

Once excavated material leaves the site it is generally treated as waste, so the duty of care applies. Use a registered waste carrier, take it to a suitably authorised site and keep your waste transfer paperwork. Contaminated soil must be assessed and classified, and hazardous spoil needs separate handling. Reusing soil on site under a recognised protocol can cut both haulage and disposal.

Before you dig, find and mark underground services and follow safe digging practice. Trenches and deep pits can collapse, so support the sides or batter them back, and keep people and plant clear of the edge.

Frequently asked questions

What is soil bulking?
It is the increase in volume when compacted ground is dug and becomes loose. Plan haulage on the loose volume.

What bulking percentage should I use?
Around 10–15% for sand and gravel, 20% for mixed ground, 25% for topsoil, 25–40% for clay and 30–40% or more for chalk. Use site data if you have it.

How do I calculate excavation volume?
Multiply length by width by depth in metres, or use π × radius² × depth for a circular pit.

How many skips or tipper loads will I need?
Divide the loose spoil by the usable capacity and round up.

How are disposal costs estimated?
Multiply loose volume by a price per m³, or use a price per load. Add haulage per tip journey if needed. These are estimates and exclude permits, waiting time, contaminated spoil charges and VAT unless built into your rates.

Related calculators

Digging is the first step of many groundworks tasks. These tools help with what comes next:

See all of them in our complete guide to free construction calculators.

Keep your groundworks records in one place

Excavation inspections, services checks, photos and waste paperwork all need recording. KaelSite helps UK site teams manage inspections, safety and reporting from one mobile-first app. Explore KaelSite features.

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