Wall Area and Paint Calculator
Net wall area with openings deducted, plus paint and filler.
Quick answer
With the default inputs: Wall length 10 · Wall height 3 · Number of openings 2 pcs etc.Net wall area 26.40 m²
Edit the inputs below to recalculate.Inputs
Result
Net wall area
26.40m²
| Gross wall area | 30.00m² |
| Total openings | 3.60m² |
| Opening share | 12.0% |
| Paint required | 5.28L |
| Filler (1.5 kg/m²) | 39.6kg |
| Bonding agent (0.25 kg/m²) | 6.6kg |
| Buckets at 5 L | 2cans |
| Paint per coat | 2.64L |
Use the manufacturer's spreading rate; dark or saturated colours and rough substrates consume 20-30% more, so reduce coverage accordingly. Filler is for two coats over a fair substrate. Beyond 30% openings, price on net area — a labour-and-materials rate on gross area works against you.
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Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Gross area | A = length × height |
| Openings | A = count × area each |
| Net area | A = gross − openings |
| Paint | V = net × coats ÷ coverage |
| Filler | W = net × 1.5 kg/m² |
| Buckets | N = ceil(paint ÷ 5) |
A 10 m wall at 3 m with two 1.8 m² openings, 10 m²/L coverage and two coats: 30.00 m² gross, 26.40 m² net after deducting 3.60 m², openings at 12%, 5.28 L of paint — two 5 L tubs.
How to work it out
- Measure wall length and height. Length is clear between corners; height is clear from finished floor to ceiling soffit or beam underside.
- Gross area = length × height, measured as one whole wall rather than in horizontal bands.
- Deduct openings: count × area each. Doors, windows, air-conditioning sleeves and distribution boards all come off, though openings under 0.3 m² are conventionally ignored.
- Net area = gross − openings. This is the area that governs paint, skim and wallpaper pricing.
- Paint volume = net area × coats ÷ spreading rate. Take 85% of the published rate: thinning, roller retention and substrate absorption all take a cut.
- Skim coat: the tool uses 1.5 kg/m², but two-coat work actually needs 2-3 kg/m², so multiply by the number of coats. Round container counts up.
Where it goes wrong
- Estimating wall area as floor area × 3. That coefficient only works for regular rooms at 2.8 m storey height; large living rooms, double-height spaces and corridors break it by over 30%.
- Deducting nothing, or deducting doors but not windows. Window openings are usually larger — one 1.5×1.8 m window is 2.7 m², and three of them equal a small wall.
- Using the theoretical spreading rate on the tin. That figure comes from ideal laboratory substrates; real concrete and skim absorb heavily and the first coat can take half the stated coverage.
- Treating 1.5 kg/m² of skim as the total. That is a single coat; two coats double it, and buying to 1.5 always falls short.
Material and cost
Project-grade emulsion runs 15-40 CNY/L, branded domestic 80-150. Skim powder is 1.5-3 CNY/kg, or 3-5 CNY/m² for two coats. Labour: skimming 12-18 CNY/m², painting 8-12 CNY/m² per coat. All-in with substrate preparation, 35-60 CNY/m².
On site
- On older walls with poor flatness, skim consumption can double. Rake a torch across the surface before quoting — it reveals hollows over 3 mm.
- Absorption varies by wall: the outside face of a bathroom wall or the flue area in a kitchen traps moisture and needs damp treatment before skimming, or it will blow within six months.
- Dark and saturated colours have poor opacity, normally needing three coats and no thinning — roughly 40% more material than light colours.
- Buy against coverage rather than wall area: roller application loses 5-10%, spraying 15-20%.
Basis
- GB 50210-2018 Code for acceptance of construction quality of building decoration, plastering and painting
- JGJ/T 29-2015 Specification for construction and acceptance of building coating works
- GB 18582-2020 Limit of harmful substances in wall coatings for buildings
FAQ
Why is coverage a range rather than a fixed number?
Spreading rate varies with substrate absorption, paint viscosity and application method. The same tub covers 12 m² on a fair plaster surface and 6 m² on bare cement render. Manufacturers quote theoretical figures from standard panels, so take 20-30% off in practice. Estimate on the pessimistic side and buy an extra tub — colour matching depends on the same batch.
Does filler need two coats?
Yes. The first fills and levels and is sanded when dry; the second closes the surface and is sanded to within 2 mm. A single coat leaves local hollows over 3 mm thick, which crack and shell off. Keep total filler thickness at 2-3 mm — beyond 5 mm it debonds, because filler is weaker than the render beneath.
How does new work differ from repainting?
New render needs a bonding agent to bind dust and improve adhesion before filling. Repainting requires removing every hollow and flaking patch first, or the new coat leaves with the old. Where the previous finish was oil-based, abrade it or use a bridging primer — water-based paint straight on top will lift and wrinkle.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Wall Area and Paint Calculator. 2026-10-25. https://jianzhujisuan.com/en/wall-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.