Each dig is measured, then we work out what comes out, what goes back in, how many loads move the spoil and what it all costs. The worked example is a block-paved driveway 8 m by 5 m, dug 250 mm deep in mixed soil, with 50 mm of sharp sand on 150 mm of Type 1 going back in.
Step 1 - Area and Volume
An area is its length times its width; a round dig is π times the radius squared. That is multiplied by how many identical digs there are, then by the depth. Where the depth changes along the length, we use the average of the two ends.
Volume (m³) = length × width × average depth × how many
Example: 8 × 5 = 40 m², × 0.25 m = 10 m³ dug out.
Step 2 - What Goes Back In
Each layer is the dig's area times its thickness. Dug soil put back, such as a drainage trench's backfill, is taken off what leaves the dig. A build-up layer that "takes the rest", like a trench fill, is sized to the shallow end of the dig so it never comes out too deep.
Example: sand 40 × 0.05 = 2 m³; Type 1 40 × 0.15 = 6 m³. Nothing dug is put back, so all 10 m³ leaves the dig, with 50 mm left at the top for the blocks.
Step 3 - Bulking and Weight
Soil swells once it is dug, so it takes up more room in a lorry or dumper than it did in the ground. Its weight doesn't change, so the tonnes come from the volume in the ground.
Loose volume = volume dug × (1 + bulking %)
Weight (t) = volume dug × weight per m³
Example: mixed soil bulks by 25% and weighs 1.9 t/m³: 10 × 1.25 = 12.5 m³ loose, 10 × 1.9 = 19 tonnes.
Step 4 - Loads
A lorry, skip or dumper is full at its volume or its weight, whichever it reaches first. We work out both and round the larger up to whole loads.
Loads = the larger of (loose volume ÷ what it holds) and (weight ÷ its weight limit), rounded up
Example: a grab lorry holding 12 m³ and 16 t: 12.5 ÷ 12 = 1.04 and 19 ÷ 16 = 1.19, so 2 loads, full by weight. A 1 tonne dumper holding 0.5 m³: 12.5 ÷ 0.5 = 25 and 19 ÷ 1 = 19, so 25 dumper loads.
Step 5 - Materials to Order
Stone, sand, tarmac and topsoil are ordered in tonnes: each layer's volume times the material's compacted weight, plus your wastage. Bulk bags are counted at about 850 kg each. Ready-mix is ordered in m³ and rounded up to the half cubic metres small loads are delivered and charged in. Membrane covers the dig's area plus wastage.
Tonnes = layer volume × weight per m³ × (1 + wastage %)
Example, at 10% wastage: Type 1 6 × 2.1 × 1.1 = 13.86 t, about 17 bulk bags; sand 2 × 1.6 × 1.1 = 3.52 t, about 5 bags; membrane 40 × 1.1 = 44 m². 0.44 m³ of ready-mix is charged as 0.5 m³.
Step 6 - Days and Price
The days on site are the volume dug divided by what the machine digs and loads in a day, rounded up to the half day, plus any extra days you add. A dig narrower than it is deep is dug to line in a tight space, so the machine manages less of it in a day. Narrower than its average depth one way, it is a trench, and its volume is divided by the machine's trench rate. Narrower both ways, or round and narrower across than it is deep, it is a pit, and the pit rate is slower again. Anything else is open digging at the full output. The machine and driver, the dumper and each labourer are charged for every day on site. Muck-away is the loads times the price per load; spoil dumpered on site has no muck-away charge. Your markup is added to the materials, the muck-away and any extras. Extras go in at the cost you enter. VAT at 20% is only added to the customer estimate, and only if you choose it.
Digging days = open volume ÷ output + trench volume ÷ (output × trench %) + pit volume ÷ (output × pit %), rounded up to the half day
Example: a mini digger at 30 m³ a day, 85% in trenches and 55% in pits. The driveway is open digging: 10 m³ ÷ 30 = 0.333. Footings 0.6 m wide and 1 m deep are a trench: 20 m³ ÷ 25.5 = 0.784. Two soakaways 1 × 1 m and 1.5 m deep are pits: 3 m³ ÷ 16.5 = 0.182. That makes 1.30 days, rounded up to 1.5 days. The estimate shows this sum for your job under "Where the Money Goes".
Where These Figures Come From
| Value | Used For | Basis | Source |
|---|
| Soil bulking | Loose volume and loads | Clay 25%, mixed soil 25%, topsoil 43%, sand and gravel 12%: from Caterpillar's load factors for each material (bulking = 1 ÷ load factor - 1). Chalk 40% and hardcore 50% sit in the bulking table of BS 6031, the British Standard code of practice for earthworks (chalk about 40%, broken rock and hardcore 20 to 60%). The typical range shown under the field also takes in the wider published spreads, such as clay at 20 to 40%. | Caterpillar: material density tables; BS 6031:2009 (BSI); Engineering ToolBox |
| Soil weight | Tonnes and loads | Weights in the ground: clay 2.0 t/m³, sand and gravel 1.9, mixed soil 1.9, topsoil 1.4 (Caterpillar gives clay 1.84 to 2.08 dry to wet, sand and gravel 1.93 dry, dry packed earth 1.90, topsoil 1.37). Broken-out concrete 2.4 is the weight of concrete itself. Chalk 1.7 is a typical figure. Wet ground is heavier. | Caterpillar: material density tables; BS EN 1991-1-1 for concrete |
| Lorry sizes | Loads | An 8-wheel rigid lorry can weigh at most 32 tonnes loaded. Once the truck itself is taken off, a tipper carries about 18 to 20 tonnes, so we use 19. A grab lorry's crane leaves it about 16 tonnes. Each haulier sets its own limits. | GOV.UK: HGV maximum weights; Truckpages. Grab lorry: typical trade figure |
| Skip sizes | Loads | Volumes are the skip's size in cubic yards in m³ (8 yards is 6.1 m³). Skip firms limit an 8 yard skip to about 8 tonnes. Mini skips take about 2 tonnes of heavy waste. | Mick George; LoveJunk |
| Dumper sizes | Dumper loads | Heaped capacity and payload from the makers' figures: 1 t holds 0.5 m³ (Thwaites 500 litres, JCB 554), 3 t about 1.65 m³ (Thwaites 1,640 to 1,680), 6 t 3.15 m³ (Thwaites 3,150, JCB 3,191) and 9 t 4.3 m³ (JCB 4,300, Thwaites 4,250 to 4,600). The tracked barrow is a typical 500 kg machine. | Thwaites, via Molson Group; JCB site dumpers |
| Material weights | Tonnes to order | Concrete 2.4 t/m³ is the British Standard figure for normal-weight concrete. The rest are compacted weights commonly quoted by UK suppliers: Type 1 2.1 t/m³ (published figures run from 1.7 to 2.2, so check your quarry's), sharp sand and pea shingle 1.6, clean stone 1.5, crushed concrete 1.9, tarmac 2.3, topsoil 1.4. | BS EN 1991-1-1 (Eurocode 1), Table A.1, for concrete. Others: typical supplier figures |
| 1.2 m | Deep dig support note | The depth at which trenches and pits commonly get a trench box, shoring or battered sides. It is a trade threshold, not a legal one: HSE sets no safe depth and says no ground can be relied on to stand unsupported in all circumstances. | HSE: Excavations |
| Services first | Finding services tip | HSE's guidance on buried services: plan the work, find the services with plans and a cable locator, then dig safely, hand digging trial holes to expose them. | HSE: HSG47 Avoiding danger from underground services |
| Narrower than deep | Open, trench or pit | OSHA, the US safety regulator, defines a trench as "a narrow excavation (in relation to its length)... In general, the depth is greater than the width". So a dig narrower than its average depth one way is a trench, and both ways, or round and narrower across than it is deep, a pit. The UK's NRM2 measurement rules class digs by purpose rather than size, so the calculator uses OSHA's size test. | OSHA: 29 CFR 1926.650(b); RICS NRM2 |
| 85% / 70% | Digging days in trenches | Methvin's estimating rates for the same bucket and ground, trench against open digging: trenches dig at 82 to 89% of the open rate with 0.3 and 0.4 m³ buckets, in sand or clay, and 57 to 72% with a 0.5 m³ bucket. So micro, mini and midi diggers use 85% and a 13 tonne machine 70%. These are digging rates only; trench support is extra. International estimating norms, not UK-specific. | Methvin: trench excavation; Methvin: bulk excavation |
| 55% / 30% | Digging days in pits | Methvin's estimating rates for small or deep digs against open digging with the same bucket: 58% in clay and 97% in sand with a 0.3 m³ bucket, 33 to 35% with 0.4 m³ and 26 to 27% with 0.5 m³. So micro, mini and midi diggers use 55%, toward the slower end for the small buckets, and a 13 tonne machine 30%. These are digging rates only; pit support is extra. International estimating norms, not UK-specific. | Methvin: small or deep excavation; Methvin: bulk excavation |
| Digger output | Days on site | Planning figures for digging and loading in average ground. A 3 tonne machine can dig about 50 m³ a day with the spoil close by and much less loading wagons, so we start at 30. Clay, rock, roots and tight access all slow it. | Typical trade figure; no published standard |
| 850 kg | Bulk bags | A typical fill for a builders' bulk bag, which varies from about 800 to 1,000 kg. | Typical supplier figure |
| 0.5 m³ | Ready-mix ordered | Small ready-mix loads have a 0.5 to 1 m³ minimum depending on the supplier (Brett's is 1 m³), usually in half cubic metre steps. Mix-on-site lorries measure what you use, so you may pay for less. | Heidelberg Materials: pricing guide; Heidelberg Materials: small loads; Brett |
| 20% | VAT on the estimate | The UK standard rate. | GOV.UK: VAT rates |
Every one of these can be changed on the page, and should be wherever you know better. "Typical" figures are what the trade commonly works to, with no single published source. Referencing a standard or manufacturer describes where a value comes from. It does not mean this tool is certified against it.
What It Doesn't Allow For
- Over-dig, battered or stepped trench sides, or soft spots dug out and replaced.
- Trench support, pumping out water, or breaking out a hard surface before the dig.
- Finding services, or hand digging around them.
- Extra tip charges for contaminated or mixed waste, waste transfer paperwork, or permits for a skip on the road.
- The surface itself, such as blocks, slabs or paving, unless you add it as a layer.
- A ready-mix supplier's short-load charge on a small order. Add it as an extra.
- Access: tight gateways, long barrow runs or restricted delivery times.
Treat the result as a starting point for your price, not a fixed quote.