How to Calculate Bedding and Spoil for a Pipe Trench (Formula, Tonnes and Truckloads)

SNZ Plumbing Estimating · 2026-07-12

The takeoff is done. You know you have 25 metres of DN100 here and 40 metres of DN150 there. Now comes the question your takeoff software will not answer: how do you calculate the spoil coming out of that trench, and how many tonnes of bedding do you need to order?

Here is the method, with the actual formulas, and then the shortcut that does all of it in about thirty seconds.

The spoil calculation formula (and the mistake most people make)

First, a correction to how most people think about this, because it is the single most common spoil mistake: spoil is not everything you excavate. Spoil is only what does not go back into the trench.

What you dig is the excavation volume:

Excavation (m³) = trench width × trench depth × trench length

But most of that native material usually goes straight back in as backfill. The spoil, the surplus you actually cart away, is the volume that gets displaced by whatever takes the native material's place:

Spoil (m³) = pipe volume + imported bedding and embedment + imported trench fill or concrete encasement

Think it through with round numbers. Dig 60 m³ and put every bit of it back around a pipe with 1 m³ of volume, and your spoil is 1 m³, not 60. Dig the same trench but import 30 m³ of bedding and fill, and your spoil is 31 m³. Price the whole 60 as spoil and you have priced disposal you will never pay for, and probably lost the job to someone who worked it out properly.

Now the real worked example. A 25 metre run of DN100 in unpaved ground at 600mm to invert, with 75mm of bedding under the pipe and imported embedment to 100mm above it. The trench is about 310mm wide (pipe outside diameter of roughly 110mm plus 100mm clearance each side) and about 0.68m deep, so the excavation is about 5.3 m³. The imported embedment zone works out to roughly 2 m³, and the pipe displaces about 0.24 m³. So:

Spoil = 2.0 m³ embedment + 0.24 m³ pipe = about 2.2 m³, which in clay at 1.4 t/m³ is roughly 3.1 tonnes to dispose of.

The rest of the excavated native goes back in above the embedment. Under paved areas the picture changes: the trench fill there is usually imported road base, so that fill volume displaces native too and joins the spoil. Same again for concrete encasement, its volume is spoil.

Simple enough for one run. Now do it for every pipe size on the job, each with its own outside diameter, trench width, embedment zone and minimum cover under AS/NZS 3500, which changes between paved and unpaved ground. Then add the pits and tanks. This is the point where the notepad method starts eating your evening.

How to calculate bedding and embedment

Bedding is calculated in zones around the pipe, not as one block.

The bedding layer sits under the pipe, commonly 75mm. The embedment wraps the sides and top of the pipe, commonly to 100mm above it. Everything above that, up to the surface, is trench fill.

So for the same DN100 run: the embedment zone is the bedding below, plus the pipe, plus the cover above, which is 75 + 110 + 100 = 285mm deep, across the 310mm trench width, along 25 metres. That is about 2.2 m³, and then you subtract the volume the pipe itself displaces, leaving roughly 2 m³ of embedment material. In crushed rock at 1.8 t/m³, that is about 3.5 tonnes to order.

Trench fill is whatever depth remains above the embedment, times width, times length, and it only applies where the trench continues up under paved areas.

Two things trip people up here. First, every pipe size has different numbers, so a job with four sizes is four sets of calculations. Second, the material matters: coarse sand, crushed rock and DGB20 all have different densities, so the same volume is a different tonnage depending on what you are ordering. If one run is on sand and another is on DGB20, a single blended figure gives you the wrong order quantities for both. If you also need the fixtures broken out for purchasing, that is what our materials order list service covers.

Converting cubic metres to tonnes

Volumes are what you dig. Tonnes are what you order and what you pay to dispose of. The conversion is just volume times density, but the density depends on the material.

Clay spoil sits around 1.4 t/m³. Sandy soils around 1.5 to 1.6. Coarse sand embedment around 1.7. Crushed rock around 1.8. DGB20 road base around 1.9 to 2.0. Flowable fill is the exception: it is ordered by the cubic metre like concrete, never by the tonne.

These are indicative figures and your supplier or geotech can confirm the exact ones, but they are close enough for tender-stage quantities.

Working out truckloads for spoil removal

The last step is getting the spoil off site. Divide your total spoil by the truck capacity. A truck and dog takes roughly 23 m³ per load and suits open-access sites. A rigid truck takes roughly 10 m³ and suits tight sites. It is one or the other depending on your access, not both added together.

So our little 2.2 m³ run is one rigid truck load. A full civil job at 300 m³ of spoil is thirteen truck and dog loads, or thirty rigid loads, and the difference between those two numbers is real money in your haulage line. On spoil-heavy work, this is exactly where our civil and stormwater estimating earns its keep.

The cases that catch people out

Subsoil and agi drains. These do not follow the zones above. The whole trench is backfilled with aggregate around a slotted pipe, so the aggregate quantity is the full trench volume minus the pipe, and the spoil is the full trench volume, because every bit of native material is displaced by imported aggregate. Standard bedding maths gives the wrong answer here.

Pits, tanks and structures. Every pit and below-ground tank spoils its full excavation volume, because the structure occupies the hole and nothing goes back in. Jobs get undercounted because only the pipe runs were calculated.

Paved versus unpaved. Minimum cover is deeper under trafficked and paved areas, which changes the trench depth, which changes every quantity. Assume one depth everywhere and the paved sections come up short.

The shortcut: let the calculator do all of it

Everything above is exactly what our free Bedding & Spoil Calculator automates. Enter each pipe size and its lengths in paved and unpaved ground, and it applies the right trench width, checks AS/NZS 3500 cover automatically, and gives you spoil, embedment and trench fill instantly. One click converts everything to tonnes using the actual density of each material, and each run can carry its own bedding material with a breakdown of order quantities per material. Subsoil drain mode handles agi drains properly, pits and tanks are included, truckloads come out for both truck types, and you can export the whole calculation as a PDF, email it, or share a link.

You can type formulas straight into the cells too, so 23+14.5+8 from your takeoff just works, the same as it does in your estimating software.

And if you would rather not calculate anything

The calculator handles the trench. The rest of the job, the fixtures, the pipework above ground, the labour, the overheads, the inclusions and exclusions, is what our hydraulic estimating service does every day, from single builds to civil and stormwater packages. The case studies show the range.

Figures in this article are indicative and for general guidance only. Densities, trench widths and cover depths vary by material, specification and site conditions. Always confirm against your project drawings, specification and supplier data. Last updated 6 July 2026.