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.
Buy more than the calculation says
The figure this returns is the bare coverage for the area you entered. Real jobs need a margin on top, and tradespeople usually add ten per cent for paint and ten to fifteen for flooring. Offcuts, mistakes, awkward corners and rooms that are not quite square all eat material that no area calculation predicts.
Coats matter as much as area. One coat almost never covers properly, especially over a colour change or bare plaster, so a room needing two coats needs roughly twice the paint. Deep colours and strong reds often want three.
For flooring, the direction of the boards changes waste noticeably — running planks along the longer wall generally produces fewer unusable offcuts — and patterned tiles need extra for matching. It is also worth buying all of it at once: paint and tiles are made in batches, and a later batch of the same product can be visibly off against the first.
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.