How to use these calculators

Every result page is the same four things, in the same order. It's worth knowing what each is for.

• 1 · The green block is the answer. The number you act on — bags, tins, pallets, tonnes, boards. If you read nothing else, read this. It already includes the waste allowance.
• 2 · The coloured panels are the judgement calls. Green means you're fine. Amber means something is easy to get wrong here. Red means the tool thinks you're about to make an expensive or unsafe mistake. These are the parts a supplier's calculator would never show you.
• 3 · "How that number was reached" is the audit trail. Every step, in order, so you can check our arithmetic against your own. If a figure looks wrong, this is where you'll find out why.
• 4 · "Worth knowing before you order" is the trade knowledge. Things that aren't in the maths but will cost you a second trip — batch numbers, delivery access, what settles, what doesn't wait.

• Depth is where the error lives. On a slab or a driveway, being 25 mm out on depth changes the order far more than being 100 mm out on length. Measure depth carefully, and measure it in several places — the ground is never as flat as it looks.
• Measure every wall at both ends. Rooms are rarely square. Take each wall at floor and ceiling, use the larger figure, and accept that you'll have a little spare. The alternative is being 300 mm short at the far corner.
• Finished floor to finished floor, not subfloor to subfloor. Especially for stairs and skirting. Forgetting the thickness of the flooring at one end is the single most common measuring mistake in the trade.
• L-shaped and irregular rooms: split them into rectangles. Run the calculator once per rectangle and add the results. Don't try to average an odd shape into one — you'll under-order on the long leg.
• Round measurements up, not to the nearest. A 4.15 m wall is a 4.2 m wall for ordering purposes.
• Measure the thing that exists, not the drawing. Once the frame is up or the ground is dug, the real dimensions are what matter.

The first thing every calculator asks is metric or US imperial, because the answer isn't a conversion — it changes what you're offered. Metric markets sell 20 kg bags and 10 litre tins; the US sells 80 lb bags and gallons. Converting one into the other gives you a number you can't buy.

• Your choice is remembered on your device. Pick once, and every calculator on the site opens in your units. Nothing is sent anywhere — it's stored in your own browser.
• Switching converts your measurements. It doesn't reset them. A 4 m wall becomes a 13.12 ft wall, not a blank field.
• Canada is a special case. Canadian trades are officially metric but work largely in imperial — framing in feet and inches, concrete by the cubic yard, but paint in 3.78 litre cans. Pick whichever suits the job in front of you, and use the pack-size field to enter what your yard actually sells. Both paths work.

This is the assumption people most often misunderstand, so it's worth being precise. The percentage is not a safety margin bolted on at the end. It's an estimate of material that gets used but doesn't end up on the surface — and it means different things for different materials.

• For paint it's roller and tray absorption, spills and cutting in. Around 10%.
• For tile, flooring and decking it's offcuts — the pieces cut to fit an edge whose remainder is too small to use anywhere else. This is why the layout matters so much: straight lay wastes about 10%, diagonal 15%, herringbone 20%.
• For gravel, road base and topsoil it is not waste at all — it's compaction. Spread and plated, these materials lose real depth. A 20% allowance on gravel means the loose load settles to the finished depth you asked for. Removing it to save money leaves you with a shallow driveway, not a saving.
• For concrete it's spillage, an uneven subgrade and what stays in the barrow.
• It's a percentage of area or volume, not of cuts. A room with twenty small alcoves genuinely wastes more than the percentage suggests. If your job is unusually cut up, raise it.

In plain terms, so you can judge whether any of them don't fit your job.

• Rooms are rectangular. Every area calculation assumes straight walls and square corners. Split odd shapes and run it more than once.
• A door is 1.5 m² / 16 sq ft and a window is 1.4 m² / 15 sq ft. Standard sizes, used for deductions. If yours are patio doors or a bay, use the "I know the area" mode and work the area out yourself.
• Deducting openings is not always right. For paint it is. For siding and tile, material is cut around an opening and the offcuts are usually unusable — so many trades order the gross area and treat openings as waste. The siding calculator says so on the page. Never do both: either deduct openings or raise the waste allowance, not the two together.
• Bag and pack counts are only right for the size you enter. A "bag" spans 18 kg to 36 kg across markets. That's why every result also shows the total weight or volume — so you can divide it by whatever your yard actually stocks.
• Coverage figures assume one product over the whole surface. Different paint on the ceiling, or a different tile in the border, means running it again.
• Counts along a run include both ends. Studs, posts, joists and balusters are positions, not divisions — there's always one more piece than there are gaps. This is deliberate and it is the number most calculators get wrong.
• The first coat on bare or porous material takes more. New plaster needs a mist coat, bare wood needs primer, unpainted render drinks paint. Each calculator flags this where it applies, but it is on top of the figure given, not included in it.
• We round up, never down. Every displayed quantity errs high. Where we've had to choose a figure from a published range, we take the cautious end. You will occasionally have a little spare. You should not run short.

We never quote a price. Not a national average, not an estimate, not a "typical" figure. Prices move, they vary by 30–50% between regions, and a stale number is worse than none.

• Type in what you were quoted, or what's on the shelf edge. We'll total it properly, including the minimum charge that nobody mentions until the invoice arrives.
• Currency is just a label. Type £, $, €, A$ or nothing at all. No conversion happens — it's your money in your units.
• The crossover figure is the useful one. Give us a bag price and a bulk price and we'll tell you the exact volume at which bulk delivery overtakes bags for your supplier. That answer can't go stale, because it's built from your numbers today.
• Cost per square metre is what to compare between quotes. It survives different pack sizes, which raw prices don't.

• Press print, and you get a one-page order sheet — not a printout of the web page. Quantities, the assumptions behind them, blank lines for brand and colour, and an empty price column.
• Fill in the blanks before you go. Brand, colour, batch, delivery date. The sheet is designed to be completed at the counter, not admired.
• Ask them to price every option. Where there's more than one way to make up the quantity, all of them are on the sheet. Let the supplier tell you which is cheaper — that's the one thing they genuinely know better than we do.
• Ask for one batch. The sheet has a line for the batch number on anything where shade drifts: paint, tile, siding, carpet, laminate.

• Anything structural. Beam sizes, lintels, joist spans, footing depths, rebar specification. We count what a drawing or an engineer specifies. We don't decide it, and no calculator should.
• Anything with a code limit. Stair riser heights, baluster gaps, pool barrier spacing, fire-rated construction. We use common limits and flag when you're outside them, but building codes vary by country, state and council. Check the rule that applies where you're building.
• Anything where being wrong is dangerous rather than expensive. Retaining walls above about a metre, anything holding back water or earth, anything supporting people.
• Very large or commercial jobs. These are built for domestic and small trade work. At scale, the supplier's own take-off is worth more than any web tool.
• Products with unusual coverage. Textured coatings, self-levelling compounds, specialist adhesives. Use the manufacturer's figure — every calculator lets you override ours.