How we calculate: landscaping
Every formula and default figure our landscaping calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Gravel calculator
- Volume (m³) = area (m²) × depth (m) × (1 + waste)
- Weight (t) = volume (m³) × density (t/m³)
- Bulk bags = weight (t) ÷ bulk bag size (t), rounded up
- Small bags = weight (t) × 1,000 ÷ small bag size (kg), rounded up
Densities differ between sources, so the range is shown. Your supplier's figure takes precedence.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Density: Gravel or pea shingle | 1.6 t/m3 Sources range 1.6–1.75 t/m3 | Typical value Not yet checked against the source | agg.directory calculator; aggregatecalculator.co.uk density table (checked 19 October 2026) Gravel and pea shingle. Sources differ, so the range is shown. |
| Density: Decorative stone | 1.5 t/m3 Sources range 1.3–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Decorative stone varies with stone type. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Spreading and settling; compaction varies. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
| Small bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common small bag. |
MOT Type 1 calculator
Open the mot type 1 calculator
- Volume (m³) = area (m²) × depth (m) × (1 + waste)
- Loose depth: compacted volume = loose volume ÷ compaction factor; loose volume = compacted volume × compaction factor
- Weight (t) = volume (m³) × density (t/m³)
- Bulk bags = weight (t) ÷ bulk bag size (t), rounded up
- Small bags = weight (t) × 1,000 ÷ small bag size (kg), rounded up
Enter the depth your design or installer specifies. We don't recommend a depth.
The densities are for compacted material, so the tonnes are what you order whichever depth you enter. If you measure a loose depth (as spread, before the roller or plate), we turn it into a compacted volume with your supplier's compaction factor.
Recycled Type 1 is made from crushed concrete and masonry. Its density differs from quarried Type 1, so the tonnes differ: use your supplier's figure.
Compaction varies, so order by weight and check with your supplier.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Compaction factor: loose volume ÷ compacted volume | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 §2 (sub-base: compacted vs loose toggle). Loose volume ÷ compacted volume for MOT Type 1. Researcher to source from an aggregate supplier or laying guide (e.g. Brett, Tarmac, Marshalls); until then the user enters their supplier's figure. |
| Density: MOT Type 1 (compacted) | 2.1 t/m3 Sources range 2–2.2 t/m3 | Typical value Not yet checked against the source | agg.directory calculator (worked 4.2 t example) (checked 19 October 2026) Compacted. Supplier's figure takes precedence. |
| Density: Recycled Type 1 (compacted) | 1.9 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Compacted. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Compaction varies. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
| Small bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common small bag. |
Patio calculator
- Slabs per m² = 1 ÷ ((slab length + joint) × (slab width + joint)), all in metres
- Slabs = area × slabs per m² × (1 + waste), rounded up; project packs = area × (1 + waste) ÷ m² per pack, rounded up
- Mortar bed (wet) = area × bed depth × (1 + mortar waste); dry volume = wet × dry-volume factor
- Cement (kg) = dry ÷ (1 + r) × cement density; sharp sand (t) = dry × r ÷ (1 + r) × sand density, for r parts sand to 1 cement
- Mortar joints (wet) = area × joint share × slab thickness × (1 + mortar waste); joint share = 1 − L·W ÷ ((L + j)(W + j))
- Jointing compound (kg) = area × kg per m² from the tub; tubs rounded up
- Drop for the fall = distance the water runs ÷ n, for a fall of 1 in n
A full mortar bed and the fall away from the house are laying details: follow the slab maker's laying guide and BS 7533.
Project packs mix several slab sizes; the pack says how many m² it covers.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Slab length | 600 mm | Common UK size Not yet checked against the source | From your product brief v2 L1 (PA1: 600 × 600 slab). Slabs vary; check the product. |
| Slab width | 600 mm | Common UK size Not yet checked against the source | From your product brief v2 L1 (PA1: 600 × 600 slab). Slabs vary; check the product. |
| Joint width | 10 mm | Common UK size Not yet checked against the source | From your product Standard UK mortar joint. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L1 PA1 and L2 BP1 (5%). Researcher to check against a laying guide. |
| Mortar bed depth | 40 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L1 PA2 (40 mm bed). Researcher to confirm against a laying guide (e.g. Marshalls) and BS 7533. |
| Bed mix: parts sharp sand to 1 part cement | 5 parts sand to 1 cement | Our assumption Not yet checked against the source | Our choice: change it to suit your job Check your maker's or builder's spec; the right ratio depends on the job. |
| Jointing compound coverage | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 L1: brush-in jointing compound coverage comes from the maker's tub (null). |
| Fall: 1 in … | 60 1 in n | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L1 PA3 (fall 1:60). Brief says the default comes from a sourced laying guide (e.g. Marshalls): Researcher to source. |
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Droppings and mixing loss. |
| Dry-volume factor | 1.3 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job Dry materials shrink when mixed with water. Researcher to check against cement makers' guidance. |
| Sharp sand density | 1.6 t/m3 Sources range 1.6–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Damp sand bulks; order by weight. |
| Cement density | 1440 kg/m3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Typical planning figure with no source yet. Researcher to source it and reclassify as typical. |
| Cement bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common UK cement bag. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
Sand calculator
- Volume (m³) = area (m²) × depth (m) × (1 + waste)
- Weight (t) = volume (m³) × density (t/m³)
- Bulk bags = weight (t) ÷ bulk bag size (t), rounded up
- Small bags = weight (t) × 1,000 ÷ small bag size (kg), rounded up
Damp sand bulks up, so order by weight, not volume.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Density: Sharp sand | 1.6 t/m3 Sources range 1.6–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Damp sand bulks; order by weight. |
| Density: Building sand | 1.5 t/m3 Sources range 1.4–1.6 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) |
| Density: Ballast | 1.8 t/m3 Sources range 1.7–1.9 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Spreading and settling; compaction varies. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
| Small bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common small bag. |
Topsoil calculator
- Volume (m³) = area (m²) × depth (m) × (1 + waste)
- Weight (t) = volume (m³) × density (t/m³)
- Bulk bags = weight (t) ÷ bulk bag size (t), rounded up
- Small bags = weight (t) × 1,000 ÷ small bag size (kg), rounded up
Settlement: loose topsoil firms down as it's walked on, rolled and watered, so a bed filled to the brim ends up lower. The settlement allowance adds extra on top of the finished depth to cover this; change it if your supplier suggests a different figure.
BS 3882 sets out grades of topsoil. Ask your supplier which grade they sell and for its soil analysis.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Density: Topsoil | 1.4 t/m3 Sources range 1.4–1.5 t/m3 | Typical value Not yet checked against the source | rospower.co.uk tonnage calculator (exact page to confirm) (checked 19 October 2026) |
| Settlement and waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance for settlement. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
| Small bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common small bag. |
Artificial grass calculator
Open the artificial grass calculator
- Strips run one way; the roll widths are combined to cover the other way at least once, choosing the set that buys the fewest m², then the fewest seams
- Each strip is cut to the run length plus the trimming allowance; grass bought = sum of widths × cut length
- Seams = strips − 1; seam length = seams × run length
- Tape rolls = seam length ÷ roll length; adhesive tubes = seam length ÷ metres per tube; both rounded up
- Infill (kg) = lawn area × kg per m²; sand layer (t) = area × depth × density
Roll widths and the pile direction drive the cost: try both directions to see the difference.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Roll widths you can buy | 2, 4 m | Common UK size Not yet checked against the source | From your product brief v2 L4 (2 m and 4 m roll widths). Check the product range. |
| Sharp sand density | 1.6 t/m3 Sources range 1.6–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Damp sand bulks; order by weight. |
Block paving calculator
Open the block paving calculator
- Blocks per m² = 1 ÷ (block length × block width), in metres. Block paving joints are nominal (the blocks have spacer nibs), so no joint is added
- Blocks = area × blocks per m² × (1 + waste), rounded up
- Laying course sharp sand (t) = area × depth × sand density × (1 + sand waste)
- Kiln-dried sand (kg) = area × kg per m²; bags rounded up
- Edging pieces = edge length ÷ piece length, rounded up
Herringbone needs more cutting at the edges than stretcher bond, so allow more waste.
Lay to the block maker's guide and BS 7533; driveways need a deeper sub-base than paths.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Block length | 200 mm | Common UK size Not yet checked against the source | From your product brief v2 L2 (200 × 100 default block). Check the product. |
| Block width | 100 mm | Common UK size Not yet checked against the source | From your product brief v2 L2 (200 × 100 default block). Check the product. |
| Waste allowance: Stretcher bond | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L1 PA1 and L2 BP1 (5%). Researcher to check against a laying guide. |
| Waste allowance: Herringbone | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L2: pattern waste for herringbone. Researcher to source from a block maker's laying guide. |
| Laying course depth | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L2: laying course depth. Researcher to source (BS 7533 / block maker's guide). |
| Kiln-dried sand | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 L2: kiln-dried jointing sand rate is null in the brief; from the bag or block maker. |
| Kiln-dried sand bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common small bag. |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Spreading and settling; compaction varies. |
| Sharp sand density | 1.6 t/m3 Sources range 1.6–1.7 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Damp sand bulks; order by weight. |
| Bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
Pond liner calculator
Open the pond liner calculator
- Liner length = L + 2 × depth + 2 × overlap; liner width = W + 2 × depth + 2 × overlap
- Underlay = the same size as the liner
- Volume (L) ≈ L × W × depth × shape factor × 1,000
- Pump flow (L/h) = volume × turnovers per hour
Shelves don't change the liner size (it is sized from the deepest point), but they reduce the volume.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Overlap at each edge | 0.3 m | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L6 (overlap each edge, assumption 0.3 m). Researcher to check against a liner maker's guide. |
| Volume shape factor | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L6: volume ≈ L × W × D × shape factor (assumption, no value given). Researcher to source. |
Rainwater harvesting tank calculator
Open the rainwater harvesting tank calculator
- Yield (L a year) = roof plan area (m²) × rainfall (mm) × drainage coefficient × filter efficiency
- Demand (L a year) = litres a day × 365
- Tank (L) = 5% (the BS 8515 simplified figure) × the lesser of yield and demand
- Water butts = tank ÷ butt size, rounded up
Without a demand figure the tank is sized on the yield alone, which may be bigger than you need.
A system feeding WCs or washing machines needs a separate non-drinking water supply and backflow protection: a plumber designs it.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Roof drainage coefficient | 0.75 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L7 RH1 (0.75). Researcher to verify against BS 8515 for the roof type. |
| Filter efficiency | 0.9 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L7 RH1 (0.9). Researcher to verify against BS 8515 / filter maker. |
| Tank size as a share of the yearly figure | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L7: tank = 5% of the lesser of yield and demand (BS 8515 simplified method). Researcher to verify. |
Turf calculator
- Rolls = lawn area × (1 + waste) ÷ m² per roll, rounded up
- Topsoil (t) = area × depth × topsoil density × (1 + topsoil waste)
- Fertiliser (kg) = area × grams per m² ÷ 1,000
Lay turf the day it arrives; order for delivery when the ground is ready.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| m² per roll | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product brief v2 L3: roll size from the supplier (null). |
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L3 TU1 (5%). Researcher to check against a turf supplier's guide. |
| Topsoil density | 1.4 t/m3 Sources range 1.4–1.5 t/m3 | Typical value Not yet checked against the source | rospower.co.uk tonnage calculator (exact page to confirm) (checked 19 October 2026) |
| Topsoil waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance for settlement. |
Bark and mulch calculator
Open the bark and mulch calculator
- Volume (m³) = area (m²) × depth (m) × (1 + allowance); litres = m³ × 1,000
- Bags = litres ÷ bag size (L), rounded up; bulk bags = m³ ÷ bulk bag volume, rounded up
Bark and mulch are sold by volume, so no density is needed.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Bag size | 60 L | Common UK size Not yet checked against the source | From your product brief v2 L5 (bags of 60/70/80 L; BM1 uses 60 L). |
| Bag sizes in the table | 60, 70, 80 L | Common UK size Not yet checked against the source | From your product brief v2 L5 (bags of 60/70/80 L). |
Sleeper calculator
- Each course is two long sides and two ends; the ends fit between the long sides
- Pieces are packed into sleeper lengths longest first (first-fit decreasing); a side longer than a sleeper is made from more than one
- Bed height = courses × course height; inside the bed = (long side − 2 × thickness) × end length × height
- Soil (t) = inside volume × topsoil density; screws = 4 corners × courses × screws per corner
For a bed that retains soil higher than 0.6 m, on a slope, or next to a path or boundary, get a structural design: that is out of scope here.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Sleeper length | 2.4 m | Common UK size Not yet checked against the source | From your product brief v2 L8 (2.4 m × 200 × 100 sleeper). Check the product. |
| Height of each course | 200 mm | Common UK size Not yet checked against the source | From your product brief v2 L8 (2.4 m × 200 × 100 sleeper), laid on its 100 mm edge so each course is 200 mm high. |
| Sleeper thickness | 100 mm | Common UK size Not yet checked against the source | From your product brief v2 L8 (2.4 m × 200 × 100 sleeper). |
| Topsoil density | 1.4 t/m3 Sources range 1.4–1.5 t/m3 | Typical value Not yet checked against the source | rospower.co.uk tonnage calculator (exact page to confirm) (checked 19 October 2026) |
| Tallest bed this page covers | 0.6 m | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 L8: retaining over 0.6 m is out of scope (structural). A Plankley scope boundary, not a regulation. |