How to calculate wall area for painting
To calculate wall area for painting, measure each wall separately, multiply length by height, add the walls together and subtract the large openings. Then account for the number of coats and add a small allowance for waste. The arithmetic is simple; the accuracy lives in the details.
Why it is worth doing yourself
Painting is normally quoted per square metre or per hour, and sometimes per object for a whole home. Once you know the surface area, two quotes that looked incomparable become directly comparable. You will also spot immediately when someone has understated the area to look cheap on paper.
The basic calculation
Measure wall by wall. Length multiplied by height gives the gross area.
- A wall 4.20 m wide and 2.80 m high: 4.20 x 2.80 = 11.76 m2.
- Repeat for all four walls and add the results.
- Shortcut for a rectangular room 4.20 x 3.60 m with 2.80 m ceilings: perimeter is (4.20 + 3.60) x 2 = 15.60 m, times 2.80 = 43.68 m2 gross.
Calculate the ceiling separately: length times width, so 4.20 x 3.60 = 15.12 m2. Ceilings and walls are different work, so keep the two figures apart.
Subtracting openings
Deduct doors and windows, but only where it is meaningful. The working rule among decorators is to deduct openings from roughly half a square metre upwards and leave smaller ones in, because cutting in around them costs time that the deduction would wrongly remove.
- Door 0.90 x 2.10 m = 1.89 m2.
- Window 1.40 x 1.50 m = 2.10 m2.
- 43.68 - 1.89 - 2.10 = 39.69 m2 net to be painted.
Coats and waste
A net area is not a material quantity. Two coats means painting the area twice, and a big colour change or a bare, absorbent substrate adds a primer coat on top of that. Add an allowance of roughly five to ten per cent for waste, what stays in the roller, and a later touch-up.
Never work from a coverage figure you remember. Every product states its own spreading rate on the tin, and the substrate decides what you actually achieve: smooth plaster goes further than a heavy texture or a weathered wall.
What changes in an Amsterdam building
The standard sum breaks down in older stock. Ceilings of three metres and more make walls larger than people estimate, and there is usually cornice, a chimney breast, a bay or a plaster border in the way. Those cost time rather than area.
- Measure the height at several points; floors and ceilings in an old building are rarely level.
- Sloped walls in a loft: calculate per plane and measure along the slope, not horizontally.
- Mouldings, glazing bars and panelled doors are usually priced per item or per hour, not per square metre.
- A stairwell is measured per landing and per wall plane; the real cost driver there is working at height.
What the price depends on
Square metres are only part of the story. The rest sits in:
- Condition of the substrate. Cracks, old wallpaper, flaking paint and damage all mean preparation, and preparation is frequently more hours than painting.
- Number of coats. Dark to light rarely covers in two.
- Level of detail. Many frames, skirtings and corners mean a lot of cutting in.
- Working height. Above roughly three metres, access equipment enters the quote.
- Method. Spraying is fast on large planes but masking takes time.
- Access. Occupied home, tight rooms, furniture to move and protect.
Questions to ask the decorator
- How many square metres is the quote based on, and how was that measured?
- Were doors and windows deducted, and from what size?
- How many coats are included, primer as well?
- Is preparation included or a separate item?
- What happens if the substrate turns out worse once sanding starts?
Common mistakes
Using the floor perimeter and ignoring recesses. A bay or a chimney breast adds real metres.
Deducting every opening. Taking out five small panes lowers the quote and raises the real cost.
Forgetting that coats multiply. Net area times one is not a materials calculation.
Trusting a floor plan. Always measure on site, especially where no corner is square.
Frequently asked questions
Should I add the ceiling to the wall total?
No. Keep them separate; ceiling work behaves differently and is often priced on its own.
From what size should I deduct an opening?
In practice from around half a square metre. Leave smaller openings in the total to offset the cutting in.
How much waste allowance is sensible?
Five to ten per cent is normal, more on a rough substrate or where a lot of touching up is expected.
Can I use a laser distance meter?
Yes, and it is faster than a tape. Take readings at several heights, because old walls are rarely plumb.
Who measures for the quote?
The decorator measures on site. Your own calculation is there to keep the conversation and the comparison honest.