How we calculate: groundworks
Every formula and default figure our groundworks 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.
Ballast calculator
- Loose layer: ballast (t) = area (m²) × depth (m) × ballast density (t/m³) × (1 + waste)
- Concrete: wet volume = area × depth × (1 + waste); dry volume = wet × dry-volume factor
- Concrete: cement (kg) = dry ÷ (1 + r) × cement density; ballast (t) = dry × r ÷ (1 + r) × ballast density, for a 1 : r mix
- Bags = tonnes ÷ bag size, rounded up
Mix ratios are typical mixes quoted by cement makers, not strength classes. For footings or anything structural, follow the engineer's or cement maker's specification.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Waste allowance: Mixing concrete | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Uneven ground and spillage. |
| Waste allowance: A loose layer | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Spreading and settling; compaction varies. |
| Mix: parts ballast to 1 part cement | 5 parts ballast to 1 cement | Our assumption Not yet checked against the source | Our choice: change it to suit your job A mix quoted by cement makers for general concrete (brief v2 G1). Check the cement maker’s or engineer’s spec; this is not a strength class. |
| Dry-volume factor | 1.54 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job Dry materials shrink when water is added (brief v2 G1). Researcher to check against Hanson, Tarmac or Blue Circle mixing guidance. |
| Ballast density | 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) |
| 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. |
| Small bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common small bag. |
Drain fall calculator
Open the drain fall calculator
- Slope = 1 ÷ n for 1 in n; % ÷ 100; mm per m ÷ 1000
- Drop = run length × slope
- From invert levels: drop = top invert − bottom invert; slope = drop ÷ run length
- Degrees = arctan(slope)
Drains need enough fall to be self-cleansing: too flat and solids settle. Approved Document H gives minimum gradients by flow and pipe size.
Invert levels are measured to the bottom inside of the pipe.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Minimum drain gradients (as 1 in n) by use and pipe size | Approved Document H (2015), H1 section 2 (foul drainage: minimum gradients by peak flow and pipe size) | England | 0/6 values entered Not yet checked against the source |
Muck away calculator
- Bank volume (m³) = area × dig depth
- Loose volume (m³) = bank volume × bulking factor
- Weight (t) = bank volume × density in the ground
- Skip capacity (m³) = skip size (yd³) × 0.764555; skips = loose volume ÷ capacity, rounded up
- By weight: skips = tonnes ÷ the firm's weight limit, rounded up; grab loads the same way
Soil and rubble are heavy: many skip firms only allow them in smaller skips, or charge by weight. Ask before you book.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Bulking factor: Topsoil | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6: bulking factor, sourced to aggregate or skip suppliers. Researcher to fill. |
| Density in the ground: Topsoil | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6: in-situ (undug) density, not the loose density of bagged topsoil. Researcher to fill. |
| Bulking factor: Clay | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6. Researcher to fill from an aggregate or skip supplier. |
| Density in the ground: Clay | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6: in-situ density. Researcher to fill. |
| Bulking factor: Sand or gravel | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6. Researcher to fill from an aggregate or skip supplier. |
| Density in the ground: Sand or gravel | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6: in-situ density. Researcher to fill. |
| Bulking factor: Rubble or hardcore | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6. Researcher to fill from an aggregate or skip supplier. |
| Density in the ground: Rubble or hardcore | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 G6: in-situ density. Researcher to fill. |
| Skip size | 8 yd3 | Common UK size Not yet checked against the source | From your product brief v2 G6 (test MA1 uses an 8 yd³ skip). A product size; skip firms' sizes vary. |
Soakaway calculator
- Inflow I (m³) = catchment area (m²) × rainfall depth (m) × (1 + climate allowance), for each storm duration
- Outflow O (m³) = a_s50 × f × D, where a_s50 = pit perimeter × half the storage depth (the sides up to 50% depth), f in m/s and D in seconds
- Storage S (m³) = plan area × storage depth × void ratio; BRE 365 needs S ≥ I − O for every duration
- So the depth each storm needs is d = I ÷ (plan area × void ratio + perimeter × f × D ÷ 2); the deepest is the critical storm
- Time to half-empty t_s50 (s) = 0.5 × S ÷ (a_s50 × f)
BRE Digest 365 is the method; your building control body may ask for the full calculation and a percolation test.
Keep a soakaway at least 5 m from a building, and check Approved Document H and building control for your site.
We leave the base out of the outflow area because it tends to clog; switch it on under More settings if your designer counts it.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Void ratio of the fill: Crates | 0.95 fraction | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G8: crates void ratio about 0.95 from the maker. Use the crate maker's figure. |
| Void ratio of the fill: Rubble | 0.3 fraction | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G8: rubble fill void ratio about 0.30. Researcher to source (BRE Digest 365). |
| Half-emptying time to aim for | 24 h | Regulation or HSE figure BRE Digest 365 (2016); Approved Document H, H3 (half-emptying time) Not yet checked against the source | Source to be added by the Researcher brief v2 G8: time to half-empty within 24 h, shown as Advisory. Researcher confirms the criterion and the nations it covers. |
Cubic metres to tonnes calculator
Open the cubic metres to tonnes calculator
- Tonnes = m³ × density (t/m³)
- m³ = tonnes ÷ density
- Bulk bags = tonnes ÷ bulk bag size, rounded up
Density varies with moisture and grading: your supplier's figure beats ours.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Density: MOT Type 1 sub-base | 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 | 1.9 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) Compacted. |
| Density: All-in 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) |
| 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 (soft) 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: Gravel | 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 aggregate | 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. |
| 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) |
| Density: Concrete | 2.4 t/m3 | Typical value Not yet checked against the source | tradecalculator.co.uk m³-to-tonnes converter (checked 19 October 2026) Supplier's figure takes precedence. |
| 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. |
Percolation test calculator
Open the percolation test calculator
- V_p75–25 (m³) = length × width × (75% − 25%) × effective depth
- a_p50 (m²) = 2 × (length + width) × 50% of the effective depth + length × width (sides to half depth, plus the base)
- f (m/s) = V_p75–25 ÷ (a_p50 × t_p75–25), with t in seconds
- Design f = the lowest f of the fills; a pit that doesn't drain means the ground is unsuitable for a soakaway
BRE Digest 365 sets out the trial pit and filling procedure; your building control body may want to see the test.
Take the slowest fill: soil drains more slowly once it's wet.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Skip size calculator
- Loose volume = Σ volume × bulking factor (1 if measured loose)
- Weight (t) = Σ volume × density, where you enter a density
- Skip capacity (m³) = size (yd³) × 0.764555; the smallest skip whose capacity covers the loose volume
- If nothing fits: skips of the largest size = loose volume ÷ capacity, rounded up
Keep plasterboard separate from general waste: ask your skip firm how they take it.
Skip firms limit weight as well as volume, especially for soil and rubble.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Skip sizes (nominal capacity) brief v2 §7.1 G6 and G7 (Researcher to cite a skip firm or trade body) | Brief v2 G6/G7: skip sizes are product sizes in cubic yards (yd³ × 0.764555 = m³). Researcher to add the other common sizes and cite a skip firm or trade body. | UK-wide | 2/2 values entered Not yet checked against the source |