Need a Building Materials Calculator for a Hardware Store Site

We run an online hardware and building-supplies store and want to add an interactive calculator that helps customers figure out how much material they need before ordering. The customer enters room dimensions or total area, picks a finish type — paint, wallpaper, or tile — and the tool returns the recommended quantity, adjusted with a sensible waste allowance for cutting and pattern matching.

The calculation logic differs per material: paint depends on coverage per liter and number of coats; wallpaper depends on roll width, length, pattern repeat and ceiling height; tile depends on piece size, grout width, and layout, since diagonal placement needs a higher margin than straight. We'd like the waste percentage configurable per material rather than hard-coded, since we adjust it by supplier and season.

The output should be a clear, printable list with the exact quantity of each item, rounded to standard package or roll sizes, ideally linking to the matching product category. Open to a lightweight standalone widget or a version integrated with our product feed — whichever is more reliable to maintain.

What we expect:
— mobile-friendly interface with instant recalculation
— separate modules for paint, wallpaper, and tile
— adjustable waste percentage and package rounding
— printable results list
— clear, commented code