Flooring
Room dimensions plus a waste margin for cuts and offcuts.
Paint
Wall perimeter, height, coats and the paint's coverage rate.
Last updated 23 August 2026
Working out the perimeter
For a rectangular room, perimeter is 2 × (length + width). A 5m × 4m room has an 18m perimeter — which is where the default figures in the paint section come from, matching the flooring section's room size.
Common questions
Why add a waste margin for flooring?
Cuts, offcuts and pattern-matching all use more material than the bare floor area. 10% is a common default for straightforward rooms; irregular layouts or diagonal patterns often need more.
What coverage figure should I use for paint?
Check the tin — coverage varies by paint type and surface, but roughly 10 square metres per litre is a common figure for a smooth wall with standard emulsion. Textured or porous surfaces usually need more.
Does the paint calculation subtract doors and windows?
No — it uses the full wall area from the perimeter and height you enter. For a room with large windows or doors, subtract their combined area from your perimeter-times-height figure before entering it, or simply treat the result as a safe overestimate.