How the calculator works
Every figure it produces is either geometry, which is exact, or an assumption, which is not. This page says which is which, gives the assumptions with their ranges, and shows the maths. You should be able to check it rather than take our word for it.
What is exact
Areas and volumes are arithmetic on the dimensions you type in. Slab area is the footprint plus the oversail on each side. Slab volume is that area times the thickness. Sub-base volume is the area times the compacted depth. There is no judgement in any of it, and if your dimensions are right those numbers are right.
The cut and fill is exact too, and it is the part most people would not attempt by hand.
The cut and fill maths
Take a site that falls evenly across the working platform, and a finished level set halfway up the slope so the material you dig out fills the low end. Integrate the height of the ground above the finished level across the whole area and the answer comes out surprisingly clean:
cut = fill = area × fall ÷ 8
For a balanced platform. Set the level at the bottom of the slope instead and it is area × fall ÷ 2, all cut. Build the level up and it is area × fall ÷ 2, all fill.
That divide-by-eight is why a balanced platform moves so little material compared with digging in. On a 350m² platform with 400mm of fall it comes to about 17 cubic metres of cut, against 70 if you dig the level in at the low end.
It is also why topsoil usually dominates the muck-away figure rather than the slope. Strip 250mm off that same platform and you have 88 cubic metres before the level has been touched.
What is assumed
These are trade figures, not laws of physics. They are reasonable defaults and they are what the calculator uses — but where a range is given, that range is real, and your supplier or your ground may sit anywhere in it.
| Assumption | Used | Actual range | Why it is there |
|---|---|---|---|
| Concrete waste allowance | 5% | Typically 5–10% | Spillage, over-dig at the edges and what stays in the drum. Order in half-metre steps because that is how plants deliver. |
| Type 1 compacted density | 2.1 t/m³ | Commonly quoted 2.0–2.3 | Converts the volume you need into the tonnage the quarry sells. Varies with the source stone. |
| Type 1 loose-to-compacted | 1.25 | Trade figure, varies with plant | How deep to spread it so it finishes at the depth you wanted. 150mm compacted means spreading about 190mm. |
| Bulking — topsoil | 1.25 | Varies widely by material | Dug material takes up more space than it did in the ground. This is what fills the lorry. |
| Bulking — clay | 1.3 | Varies widely by material | Wet clay bulks more than dry. Treat as indicative. |
| Bulking — rock | 1.5 | Varies widely by material | Broken rock bulks hard. If you are into shillet or granite, expect the muck-away figure to move. |
| Muck-away lorry capacity | 12 m³ loose | Varies by vehicle | Based on an 8-wheel tipper. Grab lorries and smaller tippers carry less — ask your haulier. |
| Mesh sheet size | 4.8 × 2.4 m | Standard UK sheet | Effective coverage allows a 300mm lap on two edges, plus 5% for cutting round edges and openings. |
| DPM allowance | +12% | Rule of thumb | Laps between sheets and the upstand at the perimeter. Rough, but it stops you ordering exactly the slab area and running short. |
| Working platform margin | 1.5 m each side | Job-dependent | You strip and level wider than the slab so there is somewhere to stand and work. This is why the topsoil figure is bigger than people expect. |
What it will not do
It does not size pad foundations. Those come off a structural engineer's design against your span, eaves height, wind loading and what the ground will bear, and no formula on a web page can produce them. Anyone quoting you a pad size without a design has guessed it.
It does not give prices. Concrete, stone and haulage all move, they move differently in different parts of the county, and a price we published in September would be wrong by Christmas. Quantities are geometry and they stay true, so quantities are what it gives you. Take them to your own suppliers.
It does not know your ground. If you hit rock at 300mm, or the field turns out to be made ground, the figures change and no amount of arithmetic will tell you that in advance. Dig a trial hole.
Why publish this at all
Because a calculator that will not show its working is asking to be trusted rather than checked, and there is no reason you should trust a website with a number you are going to order concrete against.
If you think one of these assumptions is wrong for the way things are done round here, we would rather hear it than not. It is a config file, not a conviction.