How these estimates are calculated
Nothing here is a black box. This page describes the rules that apply to every calculator, and each calculator page describes what it does on top of them.
Assumption set last reviewed 2026-07-01.
One set of base units
Every calculation happens in metres, square metres, cubic metres and kilograms, regardless of what you typed in. Your input is converted once at the start and the result is converted once at the end.
This matters more than it sounds. Repeatedly converting between metric and imperial partway through a calculation is how cheap calculators end up with totals that do not reconcile — a slab that is 4.00 m² in one place and 43.06 ft² (4.001 m²) in another.
| Dimension | Base unit | Exact conversions used |
|---|---|---|
| Length | metre | 1 yd = 0.9144 m exactly; 1 ft = 0.3048 m; 1 in = 0.0254 m |
| Area | square metre | Derived from the length conversions |
| Volume | cubic metre | 1 yd³ = 0.764555 m³; 1 ft³ = 0.0283168 m³; 1 L = 0.001 m³ |
| Mass | kilogram | 1 lb = 0.45359237 kg exactly; 1 US ton = 907.18474 kg; 1 tonne = 1,000 kg |
Waste comes before rounding
The order of operations is fixed: calculate the theoretical quantity, add the waste allowance, then round up to a whole purchase unit. Rounding first and then adding waste produces a different — and wrong — answer.
27.40 boards → +10% waste → 30.14 → rounded up → 31 boards
Every rounded figure in a result carries this sentence next to it, so you can see exactly where the number came from.
Floating-point arithmetic is handled explicitly. A value within one part per billion of a whole number is treated as that whole number, so a calculation that should give exactly 3 bags never rounds up to 4 because of binary representation.
Rounding rules
| Material | Rule |
|---|---|
| Boards, slabs, bricks, blocks, sheets | Rounded up to a whole unit |
| Bags of any kind | Rounded up to a whole bag |
| Bulk bags | Rounded up to a whole bag |
| Loose aggregate (UK) | Rounded up to whole tonnes, minimum one tonne |
| Bulk aggregate (US) | Rounded up to half cubic yards, minimum one |
| Ready-mix (UK) | Rounded up to 0.1 m³, plant minimum applied |
| Ready-mix (US) | Rounded up to 0.25 yd³, plant minimum applied |
| Packs and pallets | Reported alongside the unit count, not instead of it |
| Timber lengths | Set by a cutting plan that reuses off-cuts |
Cutting plans, not just division
For linear materials, dividing total length by stock length is not enough. A 2 m run cut from a 3.6 m board yields one usable piece and a 1.6 m off-cut that is too short for another row.
The decking module runs a first-fit-decreasing cutting plan over the required pieces and takes the higher of the two figures — the linear-metre calculation with waste, or the number of stock lengths the plan actually consumes. It also warns you when a different stock length would waste far less.
Regional data
There is one calculation engine, not two. What changes between regions is the configuration: units, currency, tax, terminology, product sizes and the pack sizes suppliers actually sell.
United Kingdom
- Currency
- GBP
- Tax
- VAT, default 20%
- Default units
- Metric
- Paving called
- paving slabs
- Timber called
- timber
- Sub-base called
- MOT Type 1 sub-base
- Waste disposal
- skip hire
United States
- Currency
- USD
- Tax
- Sales tax, default 0%
- Default units
- Imperial
- Paving called
- pavers
- Timber called
- lumber
- Sub-base called
- gravel base
- Waste disposal
- dumpster rental
Costs
This site does not hold, scrape or estimate supplier prices. Every cost figure comes from a price you typed in, and the wording says so wherever a total is shown.
Money is calculated in integer pence or cents rather than in floating point, so a total is never a penny out. The cost panel separates the cost of the theoretical quantity from the cost of the waste allowance and purchase rounding, because that difference is usually larger than people expect.
Per-calculator method
Decking calculator
- Board rows, not area: A deck is covered by whole board widths separated by gaps. The number of rows is the span across the boards, plus one gap, divided by the module of one board plus one gap, rounded up.rows = ⌈(span + gap) ÷ (board width + gap)⌉
- Board direction: Which way the boards run changes everything downstream: the number of rows, the length of each run, the number of joists and how much cutting there is. Both options are offered because the answer is genuinely different.
- Cutting plan: The board quantity is the greater of two numbers: total linear length divided by the stock length plus waste, and the number of stock lengths a first-fit-decreasing cutting plan actually consumes when off-cuts are reused.
- Frame: Joists run perpendicular to the boards and are counted along the board run at the spacing you set, plus one for the final joist. Bearers run parallel to the boards beneath the joists, and posts are counted along each bearer.joists = ⌊run length ÷ spacing⌋ + 1
- Fixings: Face-fixed decking takes two screws per board at every joist crossing. Hidden fastener systems take one clip per crossing, though some need two at board ends — check the system before ordering.screws = rows × joists × 2
- Post-hole concrete: Each hole is treated as a cylinder: π × radius² × depth, using 300 mm × 600 mm in the UK and 12 in × 30 in in the US. No deduction is made for the post itself, which leaves a small margin.
Gravel calculator
- Volume: Every section you add is converted to square metres, subtracted sections are removed, and the total is multiplied by the finished depth to give a volume.volume = total area × finished depth
- Compaction: Granular materials lose volume when they are compacted. If you tick the compaction option, the loose volume ordered is increased by the material's compaction factor — 1.3 for MOT Type 1 and dense graded base, 1.1 for angular chippings and sharp sand.loose volume = finished volume × compaction factor
- Weight: Weight is volume multiplied by the material's loose bulk density. The engine stores a range for each material and shows it alongside the result, because moisture content alone can move the delivered weight by 10%.weight = loose volume × bulk density
- Purchase units: Bags and bulk bags are defined by weight in the UK and by volume in the US, which is why the same project produces a different pack count in each region. Loose deliveries are rounded to whole tonnes or half cubic yards to match how suppliers actually sell.
Concrete slab calculator
- Volume: Volume is length × width × thickness, multiplied by the number of identical slabs. Thickness is entered in millimetres or inches because that is how it is specified, and converted internally to metres.volume = length × width × thickness × number of slabs
- Bag counts: Bagged concrete is sold by dry weight but used by volume. The engine uses a yield of 0.00047 m³ per kilogram of dry mix, derived from the published figure of 0.60 ft³ from an 80 lb bag.bags = volume with waste ÷ yield per bag, rounded up
- Ready-mix: Ready-mix is rounded to the increment the plant sells in — 0.1 m³ in the UK and 0.25 yd³ in the US — with a supplier minimum applied. Short loads normally attract a surcharge.
- Sub-base: Sub-base volume is the slab area multiplied by the sub-base depth and by a 1.3 compaction factor, because granular material loses volume as it is whacked down.sub-base = area × depth × 1.3
- Reinforcement: Mesh is estimated by effective coverage after lapping, not by sheet size. A UK A142 sheet is 11.52 m² gross but only about 10.13 m² once a 300 mm lap is allowed on two edges.
Topsoil calculator
- Volume: Bed areas are summed, subtracted sections removed, and the total multiplied by the finished depth.volume = total area × finished depth
- Settlement: Tipped soil consolidates as it is watered and worked. The settlement allowance increases the ordered volume so the bed still reaches the depth you wanted once it has settled.ordered volume = finished volume × (1 + settlement %)
- Litres and bags: UK topsoil bags are labelled in litres, so litres are shown alongside cubic metres. One cubic metre is 1,000 litres — which is why a 35 L bag covers so little ground and why bulk bags dominate above about a cubic metre.
- Weight: Screened topsoil is taken as 1,300 kg/m³, blended topsoil 1,150 kg/m³ and soil/compost blends 800 kg/m³. Very wet soil can exceed these figures by a fifth, which matters when you are checking whether a delivery can cross a driveway.
Patio and paving calculator
- Unit count: Paving is counted by module, not by slab. A 600 × 600 mm slab with a 10 mm joint occupies a 610 × 610 mm module, so it covers 0.3721 m² rather than 0.36 m².units = area ÷ ((unit length + joint) × (unit width + joint))
- Pattern and waste: Each pattern carries a recommended minimum waste allowance: 5% for a straight grid, 7% for running bond, 10% for basket weave and 15% for herringbone, where every perimeter unit is a diagonal cut.
- Random-size packs: Mixed-size packs are estimated from the coverage printed on the pack, because the size mix differs between suppliers and no calculator can predict a layout for them. The estimate says this plainly instead of inventing a plan.
- Bedding: A full mortar bed is converted to sand and cement at a 1:5 ratio, using 1.6 tonnes of sand and 320 kg of cement per cubic metre of mortar.
- Jointing: Joint volume is derived from geometry: the proportion of the surface that is joint rather than slab, multiplied by the depth of the unit.joint volume = area × (1 − unit area ÷ module area) × unit thickness
Brick wall calculator
- Co-ordinating size: A brick does not occupy its own size in a wall — it occupies its work size plus one joint in each direction. A UK brick is 215 × 65 mm, but with 10 mm joints it occupies 225 × 75 mm.units per m² = 1 ÷ ((unit length + joint) × (unit height + joint))
- Openings: Each opening's area is multiplied by how many of them there are and deducted from the gross wall area. Openings still need lintels sized for the load above, which are a design item and are not estimated here.net area = (length × height) − Σ(opening width × height × count)
- Mortar: Mortar volume is derived from geometry: the proportion of the wall face taken up by joints, multiplied by the skin thickness, with a 1.2× practical allowance for frogs, perp waste and mixing losses.mortar = net area × joint fraction × skin thickness × 1.2 × skins
- Sand and cement: Mortar volume is converted to materials using 1.6 tonnes of sand per cubic metre, plus cement according to the mix: 480 kg/m³ for 1:3, 400 for 1:4, 320 for 1:5 and 270 for 1:6.
- Wall ties: Cavity walls are estimated at 2.5 ties per square metre, from the standard 900 mm horizontal by 450 mm vertical spacing. Additional ties are required at openings and movement joints.
Safety notice: Limits of these estimates