Earthwork calculators

Cut volume with side slopes and working space, backfill converted from compacted to loose volume, and compaction degree from field dry density and laboratory maximum.

3 tools, most used first

▽Backfill Volume CalculatorBackfill volume, soil required, surplus to remove and truck loads.▽Earthwork CalculatorEnter pit dimensions and slope ratio to get cut volume, backfill and spoil to remove.▬Compaction Degree CalculatorDry density and compaction degree from field density, with a pass/fail check.

What this group solves

Only three tools, but together they cover the whole cut-to-fill chain. Cut volume comes from the pit geometry, side slope and working space in bank cubic metres; backfill converts the required compacted volume into loose volume for buying or hauling; compaction checks whether the field sample passes. All three have to use the same conversion basis, or the argument about why a thousand cubic metres of excavation will not fill the hole never ends.

The key is three states: bank, loose and compacted. A typical soil bulks 1.30 when excavated and compacts to about 1.15 of its bank volume. So 1000 m³ bank comes out as 1300 m³ loose and compacts back to roughly 870 m³ — the difference is loss and settlement. Imported fill is bought by loose volume and accepted by compacted volume; mixing the two bases puts a 30% error straight into the order.

Only three tools here, yet this is the widest error band on the site. Take one pit and change the slope from 1:0.33 to 1:0.75 and the volume moves by more than 30%; leave 300 mm of working space off each side and a long pit loses hundreds of cubic metres. The right order is geometric convention first (does the bottom dimension include working space, where does depth start and end), then the slope, then the cut-and-fill balance for disposal.

Which one for your case

Rectangular pit — excavation and disposal volumes	Earthwork Calculator
Volume already excavated — backfill and compaction	Backfill Volume Calculator
Roller passes or compaction factor	Compaction Degree Calculator
High water table — needs dewatering alongside	Wellpoint Dewatering Calculator

Where this group goes wrong

Measuring the pit bottom without working space. Footings need 300-800 mm each side (formwork face plus 300 where forms are used); on a 20×30 m pit, 500 mm a side is over 4% of the volume.
Running depth from existing ground to the underside of blinding and forgetting blinding plus over-excavation. Permitted over-dig is usually 200-300 mm, while machine excavation often reaches 400 mm — and that is exactly what gets deducted at settlement.
Guessing the slope ratio. Class I soil 1:0.33, class II 1:0.5, class III 1:0.75, class IV 1:1.0 for depths up to 5 m. One class out on a 10 m pit shifts the top width by 4 m.
Setting backfill as cut minus footing volume with no bulking factor. Excavated soil is loose, and compacted backfill needs 10-15% more loose volume than its in-situ equivalent. Disposal is measured in situ, backfill in compacted terms — they cannot be subtracted directly.

FAQ

How is pit excavation volume calculated?

For a regular rectangular pit use the frustum formula V = h/6 × [ab + (a+a₁)(b+b₁) + a₁b₁], where a₁ and b₁ are the bottom dimensions including working space and a, b are the sloped top dimensions. Side slope factors run 0.33 for class I soil, 0.25 for class II, 0.33-0.5 for class III and 0.5-0.75 for class IV. Working space is typically 300-800 mm per side.

How much backfill do I need to buy?

The design backfill volume is compacted; purchased soil is loose. Loose volume is roughly compacted volume × 1.30, and estimating on that basis is usually safe. Deduct the volume occupied by foundations, blinding and buried services — that deduction is the most commonly forgotten step.

How is the degree of compaction calculated?

Field dry density divided by the laboratory Proctor maximum dry density, as a percentage. Field density comes from a sand replacement or core cutter test. Basement backfill usually requires 0.94 or better, road subgrade 0.93-0.96, per the design. If moisture is more than 2% away from optimum, compaction will not reach the target — dry or wet the material first.

How is the slope ratio chosen?

From soil class and depth: class I (sand) 1:0.33, class II (silt, sandy clay) 1:0.5, class III (clay) 1:0.75 and class IV (gravel, hard clay) 1:1.0, all for depths up to 5 m; beyond that a specific method statement is required. The ratio is horizontal offset per metre of depth, not an angle.

How wide should the working space be?

200 mm each side for brick footings, 300 mm for concrete blinding with formwork, formwork face plus 300 mm where forms are used, and up to 800 mm where scaffolding or waterproofing needs room. It is negotiable at settlement but must be fixed before excavation — once backfilled it cannot be recovered.

How is surplus spoil disposal calculated?

Disposal = cut (in-situ) − backfill required (compacted × 1.15). The two sides are in different states: in-situ soil bulks 1.2-1.3 when excavated, and compacted fill needs more loose volume than its compacted volume. Subtracting one from the other directly gets the balance wrong.

What must a pit monitoring regime cover?

At minimum horizontal displacement at the crest, vertical settlement at the crest, and settlement of adjacent buildings. Deep pits add strut loads, groundwater level and base heave. Alarm thresholds are typically 70-80% of design values, and exceedance means stopping work to review.

Other groups

Concrete Rebar Masonry Finishes Temporary Works Construction Costing M&E Material weights Area and volume Unit conversion Code Lookup

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.