zacdaddytools

zacdaddytools

Paint & Decorating Calculator

Combine walls and surfaces, deduct openings and estimate how much paint to buy.

Surfaces to paint

Add each wall, ceiling or separate section. Use quantity for matching surfaces.

Private
Measurement unit
1Surface 1
Area19.20 m²
2Surface 2
Area14.40 m²
%

Paint estimate

Your project total, updated live.

Suggested amount to buy

7.0 litres

Rounded up to the next 0.5 litre

Paintable area

31.60 m²

Paint required

6.95 L

Gross surface area
33.60 m²
Doors & windows deducted
− 2.00 m²
Coverage across 2 coats
63.20 m²
Wastage allowance
+ 0.63 L
Also equals
340.14 ft²
Paint coverage varies by product, colour and surface. Check the tin and use its stated coverage for the best estimate.

Build an estimate surface by surface

Add each wall or surface with its width, height and quantity. Repeated walls can share one entry, while rooms with different heights or feature areas can use separate entries. Choose metres or feet and the calculator will keep the dimensions and area-based settings consistent when switching units.

Enter the combined area of doors and windows to remove openings that will not be painted. Then choose the number of coats, the coverage shown on the paint tin and a wastage allowance.

The paint formula explained

The tool adds width × height × quantity for every surface, subtracts the opening area, and multiplies the paintable area by the number of coats. It divides that coated area by the coverage per litre, then adds the selected wastage percentage.

Living-room example

Four walls measuring 4 m, 4 m, 3 m and 3 m by 2.4 m total 33.6 m².

After 2 m² of openings, two coats cover 63.2 m².

At 10 m² per litre plus 10% waste, the estimate is 6.95 litres, rounded to a suggested 7-litre purchase.

Coverage varies in real rooms

Manufacturer coverage is a guide. Porous new plaster, textured walls, dramatic colour changes and application method can all alter paint use. Primer and specialist coatings may need separate calculations. Check the product instructions and buy compatible batch quantities where colour consistency matters.

  • Measure the full wall before subtracting openings.
  • Use the coverage figure for the exact product, not a generic estimate.
  • Keep a little paint for touch-ups when practical.

FAQs

Questions about the paint & decorating calculator

Should I subtract doors and windows?

Yes, if they will not be painted. Enter their combined area, but do not subtract them twice from individual wall measurements.

How many coats should I enter?

Use the number recommended for the paint and surface. Two coats are common, but primer, colour change and surface condition can alter the system.

Where do I find paint coverage?

Look on the tin or technical sheet for square metres per litre. Coverage varies by product and surface.

Why is the suggested purchase rounded?

The result rounds litres up to the nearest half litre to provide a practical buying quantity rather than an exact liquid measure.