Room Area Calculator
Floor, ceiling, walls, skirting and volume, all at once.
Quick answer
With the default inputs: Room length 6 · Room width 4 · Clear height 2.8 etc.Floor area 24.00 m²
Edit the inputs below to recalculate.Inputs
Result
Floor area
24.00m²
| Ceiling area | 24.00m² |
| Perimeter | 20.00m |
| Gross wall area | 56.00m² |
| Openings deducted | 4.00m² |
| Net wall area | 52.00m² |
| Skirting length | 19.10m |
| Room volume | 67.20m³ |
| Wall to floor ratio | 2.17 |
| Walls share of painted area | 52% |
These are clear dimensions between finished faces, not grid lines. Floor and ceiling are equal only in the ideal case; deduct beams, service shafts and boxing where present. Skirting is perimeter less door widths, without deducting architraves.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Floor / ceiling | A = length × width |
| Perimeter | P = 2 × (length + width) |
| Gross wall area | A = P × clear height |
| Net wall area | A = gross − openings |
| Skirting | L = P − total door width |
| Volume | V = length × width × clear height |
A 6 × 4 m room at 2.8 m clear height with 4 m² of openings and a 0.9 m door: 24.00 m² floor and ceiling, 52.00 m² of net wall, 19.10 m of skirting, 67.20 m³ of volume and a wall-to-floor ratio of 2.17.
How to work it out
- Measure clear internal dimensions rather than reading them off drawings. In a shell-and-core flat, wait until plastering is complete — blockwork clear sizes typically run 2-3 cm small per wall, which is 0.24 m² on a 4 m wide room.
- Floor and ceiling areas are identical, both length × width. Where the ceiling has bulkheads or downstand beams, add the developed area separately.
- Gross wall area = perimeter × clear height. Perimeter is 2 × (length + width) and clear height runs from finished floor to ceiling soffit, not structural storey height.
- Deduct openings to get net area. Bay windows, full-height glazing and door openings all come off, but niches and built-in recesses do not — they still get skimmed and painted.
- Skirting length = perimeter − total door opening width. No skirting runs across a doorway, and this is where two to four metres is routinely over-ordered.
- Room volume = length × width × clear height, used for cooling load, fresh-air rate and coating VOC release.
Where it goes wrong
- Using structural storey height as clear height. A 3.0 m storey loses 120 mm of slab, 50 mm of floor build-up and 200-300 mm of ceiling, leaving about 2.5 m. Using 3.0 m overstates wall area by 15-20%.
- Not deducting openings. In a 4×6 m room, one door at 1.8 m² and two windows at 3.6 m² total 5.4 m² — nearly 10% of the 56 m² gross wall — and that is paint bought for nothing.
- Computing skirting on the full perimeter. At 0.9 m per doorway, six doors in a three-bedroom flat is 5.4 m; at 12 CNY/m that is 65 CNY, and far more on larger jobs.
- Deriving room area from floor area. Floor area includes common-area apportionment (20-25% in high-rise housing), and usable area then loses wall thickness, so actual room area is over 30% below floor area.
Material and cost
These figures underpin every subsequent quantity: floors (tiling, timber, self-levelling), walls (skim, paint, wallpaper), ceilings (suspension, emulsion) and skirting. A 10% error here propagates 10% through all four.
On site
- Sketch each room during the survey with opening positions and sizes marked. It is far more useful than a list of numbers, because wall calculations later can be traced directly.
- For irregular rooms with columns, service shafts or niches, compute the bounding rectangle and subtract the projections — the error is smaller than splitting into several small rectangles.
- Distinguish flat ceilings from stepped bulkheads. A stepped ceiling is measured on its developed area, typically 15-25% more than the plan area.
- Rooms above 3.5 m need a working-at-height premium on painting labour, an item that is easily missed in a quote.
Basis
- GB/T 50353-2013 Standard measurement for construction area of building
- GB 50854-2013 Standard method of measurement for building and decorative works, wall and ceiling finishes
- GB 50210-2018 Code for acceptance of construction quality of building decoration
FAQ
Why not just multiply floor area by 3 for walls?
The “times 3” rule is a fit-out shortcut that only holds for roughly square rooms at 2.7-2.8 m. The longer the room, the worse it gets: a 6 × 4 m room has a wall-to-floor ratio of 2.17 while 3 × 8 m gives 1.83 — nearly 20% apart. Wall area drives paint and filler quantities, and running short mid-job risks a colour mismatch.
Should openings be deducted?
Yes for finishes such as paint, wallpaper and wall tiles. For filler and primer either is accepted and most estimators leave them in as margin. But the reveals — the inside faces of the opening — must be added: a 1.5 × 1.5 m window in a 200 mm wall has about 1.2 m² of reveal, so four windows is close to 5 m² that is easily forgotten.
How do clear height and storey height differ?
Storey height runs between structural floors; clear height runs from finished floor to underside of ceiling, the difference being slab thickness plus floor build-up. A 3.0 m storey usually gives 2.7-2.8 m clear. Wall areas must use clear height — using storey height overstates by about 10%, two extra tubs of paint across ten rooms.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Room Area Calculator. 2026-10-25. https://jianzhujisuan.com/en/room-area
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.