Excavation is a volume problem with a twist: soil expands when you dig it up, so the amount you cart away is more than the hole you dug. This guide covers calculating the volume, the all-important bulking factor, why removal costs more than digging, and the safety step you must never skip.
Excavation is estimated by volume — how much soil you are digging out — in cubic metres. As with concrete and gravel, the calculation is length times width times depth, with every measurement in metres.
Excavation volume (m³) = Length (m) × Width (m) × Depth (m)
A pad 5 metres by 4 metres dug 0.5 metres deep is 5 × 4 × 0.5 = 10 cubic metres of soil to remove. That volume drives both the digging and, just as importantly, the cost of getting the soil off site. Our concrete calculator and volume calculator handle the same length-by-width-by-depth maths.
Here is the trap that catches almost everyone. Soil expands when you dig it up. Compacted ground broken loose takes up more space than it did in the ground — often 20 to 30% more, and more still for clay. So 10 cubic metres of in-ground soil becomes perhaps 12 to 13 cubic metres of loose spoil to cart away.
Digging is often the cheap part; removing and disposing of the soil is where the cost lands. Spoil has to be loaded, transported, and tipped, and tipping fees depend on the soil type. Clean fill is cheapest to dispose of; contaminated or mixed soil costs far more, and some sites require testing before removal. This is why an accurate volume — with bulking — matters so much for budgeting.
How the machine and trucks reach the dig determines much of the cost. An open site where an excavator and tipper can drive straight in is cheap to work. A backyard reachable only through a narrow side gate may need a compact machine, wheelbarrows, or even soil carried out by hand — dramatically more labour. Assess access honestly before estimating, because it can multiply the cost of an otherwise simple dig.
Deep excavations can also require shoring to prevent collapse, and near a boundary or building may need engineering advice. A shallow garden dig is straightforward; a deep or steep-sided excavation is a genuine safety matter.
Work out your in-ground volume, add a bulking allowance of around 25% for removal, assess access honestly, and locate services before anything else. Then get quotes that separate excavation from soil disposal, since disposal often costs more than the digging. For anything beyond a modest garden dig, a machine and operator will be faster, cheaper and safer than doing it by hand.
How do I calculate excavation volume?
Multiply length by width by depth, all in metres, to get cubic metres. A pad 5m by 4m dug 0.5m deep is 10 cubic metres of soil. This volume drives both the digging and the cost of carting the soil away.
Why does soil expand when you dig it?
Compacted ground broken loose takes up more space than it did in the ground — often 20 to 30 per cent more, and more for clay. So 10 cubic metres of in-ground soil becomes around 12 to 13 cubic metres of loose spoil to remove. Always add a bulking allowance.
Why is soil removal so expensive?
Digging is often the cheap part. Spoil has to be loaded, transported and tipped, and tipping fees depend on the soil type. Clean fill is cheapest; contaminated or mixed soil costs far more and may require testing. Removal often costs more than the excavation itself.
How does site access affect excavation cost?
Hugely. An open site where machines and trucks drive straight in is cheap. A backyard reachable only through a narrow gate may need a compact machine or soil carried out by hand, which multiplies the labour and cost of an otherwise simple dig.
Do I need to locate services before digging?
Always. Gas, water, electricity and communications lines can run through a site. In Australia, use the free Before You Dig service to have services located. Hitting a live cable or gas main is dangerous and expensive — this step is non-negotiable.