Every calculator here rests on a handful of assumed figures — how far a litre of paint goes, what a bag of concrete makes, what a cubic metre of gravel weighs. Most calculators keep those hidden. Ours are listed below, with the source and the reason we picked the number we did.
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 has been corrected twice, and the second correction went the other way. We used 1.3 t/m³, then 1.2. Both were wrong, and the reason is more interesting than the number. There are two populations of published figure, not one range: earthwork tables such as Caterpillar's give around 0.95 t/m³ for dry topsoil, while British merchants supplying damp screened loam imply about 1.4. Those describe different material in different conditions, and averaging them produces a figure that describes neither. We now use 1.4 t/m³, because what arrives on a lorry is damp screened loam and because a higher density yields more tonnes for the same volume — which errs long rather than short.
Fill dirt has no material standard in either market. It is uncharacterised excavation arisings, and its density moves with soil type and moisture more than with anything a supplier controls. Our figures come from published earthwork tables — 1,510 kg/m³ for dry packed earth, 1,600 for wet excavated. Treat the tonnage as a planning figure and the volume as the real answer.
Sand: we now publish three figures instead of one, because moisture moves it nearly 30%. Dry loose sand is about 1,420 kg/m³, damp 1,690 and saturated 1,840. Our calculators use the damp figure, because sand essentially never arrives dry. The consequence is worth stating plainly: where sand is sold by weight, a wet load delivers less material for the same tonnage. That is water, not sharp practice — but it is a reason to order by volume where a yard will allow it.
Source. BS 3882:2015 Specification for topsoil, Annex A. The standard says spread topsoil should not normally exceed 300 mm. The rooting depths above are totals, and the depth below the top 300 mm is made up with loosened subsoil to BS 8601:2013 — not with more topsoil.
Why this figure earns its place here. It is the most consequential number on this page, because getting it wrong triples an order. Calculators that offer "900 mm — trees" as a topsoil depth are quoting a rooting depth as though it were a soil order. A user who follows that buys three times what the standard asks for, of the more expensive of the two materials. We have no reason to let that happen, since nobody pays us per tonne.
Settlement figures are field practice, not measurement, and they vary with soil type more than with product. We take the cautious end.
Why we publish two densities for the same stone, and why it matters more than any other figure on this page. Aggregate is delivered loose and laid compacted, and those are different densities. A tonnage answer is right if you use either pair consistently:
compacted volume × compacted density = 1.0 m³ × 2.1 = 2.1 t
loose volume × loose density = 1.25 m³ × 1.68 = 2.1 t
It is wrong if you mix them. Multiplying the volume by a 1.25 compaction factor and then converting at 2.1 t/m³ gives 2.6 t — because the compaction factor is itself the ratio between the two densities, so using both applies the same correction twice. That is roughly a fifth too much stone, and it errs in the seller's favour every time.
This is not a hypothetical. It is the method used by at least one of the largest UK material-calculator sites, on pages carrying affiliate links to the merchants selling that stone. Our calculators are configured with the loose density so the allowance is applied exactly once, and each page shows the arithmetic so you can check it.
Where these figures are weak, which is most of them. The #57 numbers are measured, from a US Federal Highway Administration study of open-graded aggregates — the best-grounded aggregate figures we publish. Everything else here is trade-typical. There is no primary measured loose density for dense-graded aggregate anywhere, and a state highway authority states plainly that no universal value can exist, because maximum dry density depends on the source rock. Treat our dense-graded densities as planning figures and your quarry's as correct.
Source. Specification for Highway Works, Series 800, Clause 803 (11/21 amendment, current). It requires the mixture to meet BS EN 13285 and Table 8/1, with the grading summarised in Table 8/6a — reproduced above.
Cite both, in that order. BS EN 13285 alone is the wrong reference: it describes how unbound mixtures are specified and contains no material called "Type 1". The numerical envelope lives in the SHW table. We mention this because it is a common citation error, including on sites that otherwise take their standards pages seriously.
Type 1 F is not a general substitute. Table 8/6b covers a 0/20 version restricted to trench reinstatements and widenings under a metre. It should not be offered as an alternative for a driveway.
Source. The 100 mm and 150 mm figures are the CMHA/ICPI published minimums for paving over well-drained soils. British practice via Pavingexpert agrees closely — around 75–100 mm for a light-duty patio and 100–150 mm for a domestic drive. Anything carrying commercial traffic is engineer-determined and the figures above do not apply.
The lift thickness is the figure most often quoted wrongly. SHW permits up to 225 mm compacted in a single layer — but only with highway plant and a specified number of passes. That number gets repeated at domestic readers with hired wacker plates, and it is how bases end up soft in the middle. We deliberately do not offer it, and our pages state the domestic figure instead.
Every depth here assumes the ground beneath drains. Over clay, peat or made ground the base wants to be thicker and wants a geotextile separator under it, or the stone works down into the subgrade and the depth you paid for quietly disappears.
Source. ICPI Tech Spec 17 Bedding Sand Selection for Interlocking Concrete Pavements and the ICPI/CMHA construction specification, both of which require the sand to meet the ASTM C33 concrete-sand grading and cap the laying course at 1.5 in.
Why we treat 40 mm as a hard limit rather than a guideline. A bedding course thicker than that keeps consolidating under traffic, unevenly, and the paving goes wavy in its first year. It cannot be corrected without lifting it. It is a defect, not a tolerance — and when someone enters a thicker screed, the honest answer is to sell them less sand and tell them to fix the base.
Soft building sand and mason sand are graded for mortar (ASTM C144, BS EN 13139) and are the wrong sand here. Rounded grains do not interlock.
The coordinating size is the unit plus one mortar joint, and it is the number that belongs in a calculator. 215 + 10 = 225 long, 65 + 10 = 75 high. Enter the bare brick size instead and you get 71.6 bricks per m² rather than 59.3 — a 21% over-order. For blocks the same mistake costs 7%. It is the commonest error in this category and it always errs upward.
Both of the famous numbers are rounded up. 225 × 75 mm is 0.016875 m², which is 59.26 bricks to the square metre — not 60. 450 × 225 is 0.10125 m², which is 9.88 blocks — not 10. The trade rounds up because rounding down leaves you short, and we keep the rounding for the same reason. But a rule of thumb with a 1.2% cushion in it is a different kind of thing from a measurement, and this site would rather say which is which than let a tidy number pass as a fact.
Block weight is a specification, not a detail. The same 440 × 215 block ranges from about 8 kg in aircrete to over 25 kg in solid dense. UK guidance treats 20 kg as the point at which repeated one-person handling becomes a real risk. Ask for the weight per block on the data sheet — it decides whether a wall is one person's work.
Mortar we do not publish a figure for. Roughly a tonne of sand and six to eight bags of cement per thousand bricks is a planning figure only; joint thickness, how flat the units are and how much gets dropped vary far more than the arithmetic. Your bricklayer knows their own rate and it beats ours.
Why there is a maximum, which is unusual on this site. Almost every figure here is a minimum you should not go below. Mulch depth is the opposite: past about 100 mm it starts doing harm, because it stops air and water reaching the soil, keeps the surface permanently wet, and encourages roots to grow up into the mulch where they die in the first dry spell. 50–75 mm suppresses weeds perfectly well.
This is the clearest case on the site of a figure a supplier's calculator would never volunteer, because the honest answer sells less mulch.
Mulch against a trunk rots the bark. Keep it 75–100 mm clear and spread it wide and thin instead. The "mulch volcano" kills established trees slowly enough that nobody connects the two.
Order mulch by volume, not weight. It is light enough that moisture swings the weight enormously — wet bark can arrive half as heavy again as it left the yard. Volume is the honest unit even in markets that normally sell by the tonne.
There is no coverage default on the insulation calculator, and that is deliberate. Pack coverage varies by thickness, product and brand — a pack of 100 mm loft roll covers roughly twice the area of the same pack in 200 mm — so any figure we published would be wrong for most readers and would be trusted anyway. We ask for the number on the packaging instead.
The figure that actually matters is not one we can supply either. How much insulation you need is a U-value calculation set by building regulations or code, and it depends on the whole construction rather than the thickness alone. More importantly, insulating a wall or roof changes where moisture condenses inside it — ventilation, vapour control and eaves gaps are what stop a well-insulated roof rotting, and none of that is a quantity.
What we will say plainly: a small gap costs far more performance than the same fraction of thickness, because a gap is not slightly worse insulation but a hole with air moving through it. Fitting a thinner product properly beats fitting a thicker one badly.
These four pages publish a formula instead of a figure, because in each case a single figure would be wrong for most readers. Grout coverage swings by a factor of five between a large floor tile with a thin joint and a mosaic with a wide one. Mortar depends on the unit size, the joint and the wall thickness. Wallpaper depends on whole drops per roll, which is a floor() and therefore jumps in steps that no coverage figure can model — a 600 mm straight-match repeat takes a standard roll from four drops to three, which is 33% more rolls for the same wall.
Every one of them shows its working on the page, so you can check the arithmetic rather than trust it. That is the whole reason for publishing the formula rather than the answer.
Two of these figures are ours and are assumptions. The 1.25 dry-materials bulking factor is the usual trade allowance and varies with how damp the sand is. The 1,440 kg/m³ loose cement density is a standard value. A bricklayer's own measured rate beats both.
Tile adhesive has no figure here at all, deliberately. Coverage is set by trowel notch, tile size and substrate flatness — a 6 mm notch covers roughly twice what a 12 mm does. The bag states it per notch size, measured on that product. Ours would be a guess.
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, and our previous figure for it was wrong. We said "filled to around 900 kg, marked min 850 kg". That describes neither bag actually on sale. There are two, and they are not interchangeable: suppliers who declare a volume sell around 750 litres (0.75 m³), while merchants who declare a weight sell a bag with a minimum fill of 800 kg occupying roughly 0.5 m³. The gap between them is about a third of the material — far bigger than the shortfall we previously warned about. Ask which one you are buying. It is not a "tonne bag" in either case.
In America the equivalent trap is the retail bag. Aggregate and sand come in 0.5 cubic foot bags, which is 54 bags to the cubic yard. That number is the point at which bagging stops making any sense, and it is worth knowing before you load a trolley.
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.
Found one that's wrong? These figures matter more than anything else on the site — a bad density is a bad answer no matter how good the arithmetic. If your supplier's data sheet says something different from ours, theirs is right for your product, and we'd rather hear about it than keep publishing a figure that sends someone home short. Tell us here.
Last reviewed 2 August 2026.