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.

Volume tools (concrete, gravel, sand, mulch)

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.

Weight from volume (gravel, sand)

Suppliers price aggregate by the ton or tonne. We multiply volume by a typical compacted density — about 1.4 tons per cubic yard for gravel, lower for sand. Your quarry can give an exact figure for the stone you are ordering.

Count tools (drywall, tile, decking, fence)

Area or run length is divided by the size of one unit (sheet, tile, board, panel), rounded up, then waste is added. Partial units still count as a full purchase.

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).

Cost lines

Default material prices are planning placeholders you can override. They are not live quotes. Labour and delivery are never guessed — you enter them or leave them at zero.

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error is reported with reproducible inputs, or when a default assumption (coverage rate, bag yield, density) is clearly out of date.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.

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.