Short answer: the floor is length × width. The walls are the distance round the room (the perimeter) × the ceiling height, less doors and windows. Skirting is the perimeter less the doorways. A 12 × 10 ft room with an 8 ft ceiling, one door and one window has 120 ft² of floor, about 320 ft² of wall to paint and 41 ft of skirting. A 4 × 3 m room with a 2.5 m ceiling has 12 m² of floor, about 31.7 m² of wall and 13.1 m of skirting.
What you need
A tape measure (5 m or 16 ft is enough for most rooms) or a laser measure, a pencil and a quick sketch of the room. Write each measurement on the sketch as you take it.
Step 1: the floor
Measure wall to wall at floor level, the longest way and the widest way. Walls are rarely perfectly straight, so measure in two or three places and use the biggest figure.
- L-shaped room: split it into rectangles, work out each one and add them up.
- Bay window or alcove: measure it as a rectangle of its own and add it.
- Fixed kitchen units: take off the floor under them for a floating floor such as laminate, which must not be trapped under units.
In feet and inches, turn the inches into part of a foot first: 12 ft 6 in is 12.5 ft, because 6 ÷ 12 = 0.5.
Step 2: the walls
Add up the length of all the walls (for a rectangle, 2 × length + 2 × width) and multiply by the ceiling height. Then take off the doors and windows. Measure your own; the RoomCalcs calculators take off 900 × 2100 mm (36 × 80 in) per door and 1200 × 1200 mm (36 × 48 in) per window unless you enter other sizes.
The ceiling of a flat room has the same area as the floor.
Step 3: skirting and trims
Skirting runs round the room except across the doorways: perimeter less the door widths. Buy it in whole lengths and allow a little extra for the mitres in the corners. The baseboard calculator turns the metres or feet into lengths to buy.
Two rooms worked out
| 4 × 3 m, 2.5 m ceiling | 12 × 10 ft, 8 ft ceiling | |
|---|---|---|
| Floor | 12 m² | 120 ft² |
| Perimeter | 14 m | 44 ft |
| Walls before openings | 35 m² | 352 ft² |
| Less 1 door | − 1.89 m² | − 20 ft² |
| Less 1 window | − 1.44 m² | − 12 ft² |
| Walls to paint | 31.67 m² | 320 ft² |
| Ceiling | 12 m² | 120 ft² |
| Skirting (less one door) | 13.1 m | 41 ft |
From measurements to materials
The measured area is what gets covered. What you buy is more, because of cuts, pattern and coats:
- Laminate and vinyl planks: add about 8 % for a straight lay and 15 % for diagonal or herringbone. See how much laminate flooring you need.
- Tiles: add about 5–10 %, or plan the actual cuts with the tile layout planner, which reuses offcuts and finds the start line with no thin cuts.
- Paint: walls × the number of coats ÷ the coverage on the tin. See how much paint you need.
- Wallpaper: rolls depend on the pattern repeat. See how many rolls of wallpaper you need.
Measure once, use it everywhere
In My renovation you save each room once: length, width, ceiling height, doors and windows. Every calculator with a length and width then offers Use a saved room and fills its boxes in its own units. My renovation also keeps your saved calculations, a shopping list grouped by shop, a budget with payments, and a checklist. It all stays in your browser. You can download a backup, or send a link to a partner or builder.
The checklist follows the usual order of a renovation:
- Plan: measure every room, set a budget and get quotes for the labour.
- Buy: choose materials (see samples in the room's own light) and order everything with spare, from one batch.
- Prepare: clear the room, remove the old floor, tiles or wallpaper, then repair, level and prime.
- Lay: set out the start lines, lay the tiles or floor, grout and seal.
- Finish: paint ceilings before walls, fit skirting and trims, clean up, and keep spare tiles and paint labelled.
The order can change with the job. A carpet usually goes in after the painting, and many decorators paint before a new wood floor is laid. Add or tick off steps as your job needs.
Frequently asked questions
How do I measure a room in square feet?
Multiply the length by the width, both in feet. Turn inches into feet first by dividing by 12: a room of 12 ft 6 in × 10 ft 3 in is 12.5 × 10.25 = 128.1 ft².
How do I measure an L-shaped room?
Split it into two rectangles, measure each one, and add the two areas. Make sure the part where they meet is counted only once.
Should I subtract doors and windows when working out paint?
For an accurate amount, yes, especially on walls with big windows or a patio door. Small openings make little difference, and many decorators leave them in as a little spare.
Do I measure to the wall or to the skirting?
For flooring, measure wall to wall. The expansion gap at the edge is so small it makes no real difference, and the skirting covers it.
How many square metres is a 4 × 3 m room?
12 m² of floor. With a 2.5 m ceiling, the walls are 35 m² before you take off doors and windows.
Calculators in this guide
Enter your own sizes for an exact answer.
- Square footage calculatorArea of any room made of rectangles, triangles and circles.
- Paint calculatorLitres, gallons and tins of paint for walls and ceiling, plus primer and putty.
- Flooring calculatorPacks of laminate, vinyl or wood flooring, plus underlay.
- Baseboard / skirting calculatorLength and pieces of baseboard or skirting board.
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