Where our numbers come from

The two rules behind every figure

• We round against ourselves. Where a range exists, we take the cautious end — the one that makes you order slightly more rather than slightly less. Running short mid-job costs a second trip, a mismatched batch or a cold joint. Having a little spare costs a little money.
• Your product's label always wins. Every figure here is a default for when you don't have better information. Every calculator lets you type in the number printed on your tin, bag or box, and when you do, ours is ignored entirely.

Source and reasoning. Manufacturers quote 12–16 m²/L for standard emulsion on smooth surfaces — Dulux Easycare and Crown Matt both state 12–14 m²/L, and premium lines claim up to 16. We default to 10, roughly 20–30% below the claimed figure.

That gap is deliberate. Published spread rates are measured on ideal, sealed, smooth substrates by people who paint for a living. Real walls are more porous, real rollers hold more paint, and cutting in wastes more than a lab does. Painting to a manufacturer's headline figure is the single most common way people run out one wall short.

Masonry is the widest-ranging of all: manufacturers quote 12–14 m²/L on smooth low-porosity render, but a first coat on unpainted rough render or pebbledash can be less than half that. We default to 9 and let the surface selector reduce it further.

Note on markets. UK and US labels quote differently — a paint sold as 13 m²/L in Britain is often labelled 350–400 sq ft/gal in America, which is 8.6–9.8 m²/L. That is optimism versus caution, not chemistry. We use the cautious convention in both.

Source. Quikrete Technical Data Sheet #1101 states an 80 lb bag of concrete mix yields approximately 0.60 ft³ (17 L); Sakrete's data sheet agrees. Both are anchored to ASTM C387, the American standard governing packaged dry concrete materials. The yield follows from 80 lb ÷ 133 lb/cu ft, which is where our 2,130 kg/m³ bagged figure comes from — and it gives the familiar 45 bags to the cubic yard.

Why bagged density differs from cured density. Cured concrete is about 2,400 kg/m³, but a bag of dry mix yields more volume than that implies, because you add water and the mix takes up more space wet. Using the cured density to count bags would leave you roughly 11% short. Our calculators use 2,130 for bag counts and 2,400 for weight.

Source and reasoning. UK aggregate suppliers publish MOT Type 1 at 1.45 t/m³ loose and 1.85 t/m³ compacted, with 2.0–2.2 t/m³ commonly quoted as a working figure and 2.1 the usual middle. We use 2.1.

Sand is quoted at 1.60 t/m³ dry, 1.75–1.80 damp as delivered from the quarry, and 1.90 saturated. We use the dry figure and warn on the page that damp material weighs more — because yards sell by weight, a wet load delivers less material for the same tonnage.

Gravel spans a genuine range: decorative rounded gravel is quoted around 1.35 t/m³ and pea gravel around 1.50, while angular crushed stone runs higher. We use 1.6, which suits crushed stone; if you're buying decorative rounded gravel, your supplier's lower figure is the better one.

Topsoil was our worst figure and has been corrected. We previously used 1.3 t/m³. Published figures put loose dry topsoil at 0.9–1.2 t/m³, rising to 1.4–1.5 once settled and partly compacted. Since you order it loose, we now use 1.2 — the top of the loose range, so the tonnage errs high rather than low.

Pack sizes vary far more than most people expect, which is why every calculator here asks you for yours rather than picking one.

The bulk bag is the one that catches people. Builders Merchants Federation guidance agreed with Trading Standards is that bulk bags are filled to around 900 kg and marked with a minimum of 850 kg — not the tonne almost everyone assumes from the name "tonne bag". Order by weight and count the bags accordingly, or you'll be short by roughly one bag in five.

A US 94 lb cement bag is 42.6 kg because it is defined as one cubic foot of cement — a volumetric convention rather than a weight one, and a genuine trap for anyone converting between markets.

What we still don't know well enough

• Sod pallet coverage. Varies by supplier and roll size with no standard at all, which is why the sod calculator asks rather than defaults. Ask your grower before ordering.
• Tile adhesive and grout coverage. Depends on tile size, trowel notch and substrate flatness to a degree that makes any single figure misleading. We don't publish one.
• Composite decking and vinyl siding movement. Manufacturer-specific and not interchangeable between brands. We point you at the product's own figure rather than inventing one.
• Regional variation generally. Densities move with moisture, stone type and how the yard stores its stockpiles. Every figure above is a reasonable default, not a measurement of your material.