How We Calculate

Every calculator on Project Home Calc uses the same pattern: standard geometry, stated assumptions, and a waste line you can change. This page is the site-wide summary — each tool page still shows its own formula and worked example.

Why we show the formula

A single output number without a method is hard to trust. Reviewers, suppliers, and homeowners all ask the same question: what did you assume? We put the method on the page so you can catch a wrong thickness, a missing opening, or a waste factor that does not match a diagonal layout.

Volume tools (concrete, gravel, sand, mulch, topsoil)

Length × width × depth, with depth entered in inches or centimetres because that is how slabs and beds are specified on plans. US results convert cubic feet to cubic yards (÷ 27). UK results stay in cubic metres. Waste covers settlement and uneven grade — you can lower it for a simple rectangle or raise it for soft ground.

Weight from volume (gravel, sand, topsoil)

Suppliers price aggregate and soil by the ton or tonne. We multiply volume by a typical bulk density. Moisture and rock type move the real weight; treat our tonnage as a planning figure and confirm with the yard’s product sheet.

Count tools (drywall, tile, decking, fence, wallpaper rolls)

Area or run length is divided by the size of one unit (sheet, tile, board, panel, or usable roll coverage), rounded up, then waste is added. Partial units still count as a full purchase. Pattern-match wallpaper needs a higher waste percent than plain paint area math.

Paint and coverage

Wall area = room perimeter × ceiling height, minus fixed deductions per door and window. Paint volume = net area × coats ÷ coverage per gallon or litre (about 350 ft² per US gallon on a smooth wall). Rough or porous surfaces need more — override coverage when the tin says so.

Cost lines

Default material prices are planning placeholders you can override. They are not live quotes. Labour and delivery are never guessed into a fake total — you enter them or leave them at zero. See also data sources and calculation assumptions.

What this is not

We do not provide structural design, soil reports, or code compliance. A driveway thickness on a soft clay site is an engineering question, not a volume question. The calculator still helps you compare two proposed thicknesses on materials alone.

When we update a tool

We change a calculator when a formula error is reported with reproducible inputs, or when a default assumption (coverage rate, bag yield, density) is clearly out of date. Each tool page shows a last-updated date. Related guides are updated when the same assumption appears in an article.

Report a problem

Wrong number? Email shoaib.octachat@gmail.com with the tool URL, your inputs, and what you expected. We fix real errors; we do not change results to match a single supplier's quote.