How We Calculate
Last updated: 11 November 2026
Trust matters when you're spending money on materials. This page lists the formulas and figures our calculators use, where each figure comes from, and how we check our maths.
The basic method
- Area — length × width for rectangles; π × (diameter ÷ 2)² for circles; ½ × base × height for triangles.
- Volume — area × depth.
- Waste allowance — volume or area × (1 + waste %). You can change the waste % on every calculator.
- Weight — volume × density (for loose materials like gravel and topsoil). Because density changes with the type of stone and with moisture, we show weight as a range and suggest ordering the top figure when you buy by weight.
- What to buy — total ÷ amount per bag/box/tin, always rounded up to whole units.
Unit conversions use exact definitions: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 lb = 0.45359237 kg, 1 US gallon = 3.785411784 L, 1 US ton = 2,000 lb, 1 cubic yard = 27 cubic feet.
Material figures and sources
Loose materials vary with moisture and grading, so every calculator lets you enter your supplier's figure instead — and your supplier's figure always replaces ours.
| Material | Default | Typical range | Source |
|---|---|---|---|
| Crushed stone (e.g. #57, 20 mm) | Range | 1.6–1.78 t/m³ | Vertigraph (1.35 tons/yd3); InvoiceOwl (#57 stone about 1.5 tons/yd3) |
| Pea gravel / shingle | Range | 1.54–1.68 t/m³ | Vertigraph (pea gravel 1.3–1.42 tons/yd3) |
| Road base / MOT Type 1 | Range | 1.6–2 t/m³ | MP Moran (Type 1 bulk bag about 0.5 m3 for 800–850 kg ≈ 1.6–1.7 t/m3); Calcforlife UK MOT calculator (loose 1.8–2.0 t/m3); Vertigraph (road base 1.62 tons/yd3 ≈ 1.92 t/m3) |
| Not sure / other gravel | Range | 1.4–1.8 t/m³ | Designing Buildings Wiki; AggregateMarkets (loose gravels 1.4–1.8 t/m3) |
| Wet gravel | × 1.0625 (about 6% heavier) | Turnbull: same bulk bag “800 kg, up to 850 kg depending on moisture” | |
| Bulk bag (UK) | 850 kg | Common UK supplier size (800–850 kg; some sell 1 tonne) | |
| Topsoil — dry | Range | 1.19–1.54 t/m³ | Hellogravel topsoil guide (dry loose soil about 1–1.3 US tons/yd3) |
| Topsoil — moist (typical delivery) | Range | 1.3–1.6 t/m³ | Calcforlife UK topsoil calculator; topsoilfinder guide (1.4–1.6 t/m3 moist) |
| Topsoil — wet (after heavy rain) | Range | 1.78–2.02 t/m³ | Hellogravel topsoil guide (wet or compacted soil about 1.5–1.7 US tons/yd3) |
| Bark mulch | 0.4 t/m³ | 0.3–0.5 t/m³ | Designing Buildings Wiki |
| Bagged concrete yield | 40 lb = 0.3 ft³; 50 lb = 0.375 ft³; 60 lb = 0.45 ft³; 80 lb = 0.6 ft³ | QUIKRETE Concrete Mix data sheet | |
| Concrete, 20 kg bag (UK) | 10 litres per bag | Wickes and Homebase product pages: one 20 kg bag ≈ 0.01 m3 | |
| Concrete, 20 kg bag (Australia) | 9.09 litres per bag | Cement Australia: about 110 × 20 kg Concrete Mix bags per m3 (Bunnings staff quote about 100) | |
| Concrete, 30 kg bag (Canada) | 14 litres per bag | Home Hardware Canada (QUIKRETE 30 kg Concrete Mix yields approx. 0.014 m3) | |
| 20 kg post-mix (UK) | ≈ 0.0169 m³ per bag (derived) | Derived from Sure-Green's bags-per-hole table; both rows agree | |
| Paint (US) | 350 ft²/gal per coat | 350–400 ft²/gal | Sherwin-Williams (we use the lower figure) |
| Paint (metric) | 12 m²/L per coat | 12–14 m²/L for typical UK emulsion (8–10 for some products) | Homebuilding & Renovating; Dulux UK (we use the low end; the tin's figure wins) |
How we test
Every calculator is built on one shared calculation engine with automated tests. Before a calculator goes live, its tests must reproduce known answers — for example, QUIKRETE's 45 × 80 lb bags per cubic yard, and the supplier table of 3 bags of post-mix for a 300 × 600 mm hole. If a figure changes, the tests are updated and re-run.
Change log
- 11 November 2026 — New: Gravel Driveway Planner, Fence Planner and Raised Bed Soil Calculator; a saved project and shopping list (kept only in your browser); and a bags-or-bulk price comparison using prices you type in. Bag sizes come from real products listed on each page.
- 11 November 2026 — Paint: metric default coverage raised from 10 to 12 m² per litre after checking UK emulsion figures (we were over-estimating by about 30–50%); added Australian tin sizes. Concrete: added UK and Australian 20 kg and Canadian 30 kg bags. Gravel: added stone types, wet delivery, separate compaction and waste allowances, weight ranges and UK bulk bags. Topsoil: added dry, moist and wet soil.
- 10 November 2026 — First ten calculators built, with automated tests.
Found a problem?
Please tell us. We fix confirmed errors quickly and note the change on the page.