How much extra paint do I need for rough or porous walls?

Rough, textured or highly porous surfaces absorb significantly more paint than smooth ones. Increase your paint quantity by 15–25% for textured masonry, rough plaster, or bare brick. You may also need an extra coat. The coverage rate field lets you enter a lower number (e.g. 9–10 m²/litre instead of 12) to model this automatically.

Can this calculator be used for exterior paint?

Yes — enter your wall dimensions and set the coverage rate to match your exterior paint's specification (typically 8–12 m²/litre for masonry paint). Uncheck the ceiling option since exterior jobs usually don't include a ceiling. For rendered or smooth render use the standard rate; for rough masonry or brick reduce the rate by 20–30% or add extra coats.

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Paint Calculator

Calculate exactly how many litres of paint you need for any room. Accounts for doors, windows, number of coats, primer and the paint's coverage rate — so you never buy too much or too little.

Door & Window Deductions Primer Included

Room Dimensions

Room Length (m)

Room Width (m)

Room Height (m)

Number of Doors Each door = 1.89 m² (standard 0.9 × 2.1 m)

Number of Windows Each window = 1.4 m² (standard 1.0 × 1.4 m)

Ceiling Include ceiling (length × width)

Paint Details

Number of Coats

Paint Coverage (m²/litre) Check your paint tin — emulsion typically 10–14 m²/L

Paint Can Size (litres)

Price per Litre optional

Primer Apply primer coat (1 coat at 14 m²/litre)

Wall area (before deductions)—

Deductions (doors + windows)—

Net paintable wall area—

Ceiling area—

Total area to paint—

Paint needed—

Cans to buy—

Primer needed—

— L paint · — cans

Coverage varies by surface texture and porosity. Rough, porous or new plaster surfaces typically absorb 20–30% more paint — reduce the coverage rate or add an extra coat to compensate.

How to Calculate Paint for a Room

Painting a room requires knowing the total paintable surface area, then dividing it by your paint's coverage rate and multiplying by the number of coats. The tricky part is accurately accounting for what you are not painting — doors, windows, skirting boards — and remembering that the ceiling is a separate surface with its own area.

This calculator works through each step:

Wall area — the perimeter of the room (2 × length + 2 × width) multiplied by the wall height gives the gross wall area.
Deductions — standard door (1.89 m²) and window (1.4 m²) areas are subtracted from the gross wall area.
Ceiling — if selected, length × width is added to the total paintable area.
Paint litres — total area × coats ÷ coverage rate (m²/litre).
Cans — litres needed divided by your chosen can size, rounded up to whole cans.
Primer — one coat of primer at 14 m²/litre applied to the total area before the finish coats.

The cost estimate uses your price per litre multiplied by the paint litres required (primer is usually priced differently, so it is excluded from the cost estimate unless you enter the same price per litre).

What Is Paint Coverage Rate?

Coverage rate — measured in m² per litre — tells you how far a litre of paint will spread in a single coat on a typical smooth surface. Most manufacturers print this on the tin, and it usually ranges from 10–14 m²/litre for interior emulsion and 8–12 m²/litre for exterior masonry paint.

Coverage varies significantly based on:

Surface texture — smooth plaster achieves the rated coverage; rough, textured or pebbledash surfaces can absorb 20–40% more.
Surface porosity — bare plaster, new drywall and bare masonry are highly absorbent. A sealer coat or PVA dilution before priming reduces this.
Paint type and sheen — matte/flat paints tend to cover better in fewer coats; high-gloss paints often need more coats for solid colour.
Application method — roller application is more efficient than brush; airless spray achieves the highest coverage per coat.
Colour change — going from a dark to a light colour (or vice versa) usually requires an extra coat regardless of coverage rate.

When in doubt, reduce the coverage rate in this calculator (e.g. use 9 or 10 instead of 12) to give yourself a buffer rather than running short mid-job.

Frequently Asked Questions

A 12×12 ft room (roughly 3.65 m × 3.65 m) with 2.4 m ceilings has about 35 m² of wall area. After deducting a door and a window, the net wall area is around 32 m². At 12 m²/litre with two coats you need approximately 5.5 litres of wall paint, plus another 1.1 m² for the ceiling. Enter your exact dimensions into this calculator for a precise figure.

Primer is strongly recommended on new plaster, bare drywall, repaired areas or any surface with stains. On previously painted surfaces in good condition you can often skip primer and add an extra finish coat instead. The calculator adds one primer coat at 14 m²/litre when the option is selected — this is a typical coverage rate for water-based primer; check your primer tin and adjust accordingly.

Rough, textured or highly porous surfaces absorb significantly more paint than the rated coverage suggests. For textured masonry, rough render or bare brick, reduce the coverage rate by 20–30% in the calculator (e.g. enter 8 or 9 instead of 12 m²/litre) or add an extra coat. An additional sealer coat before priming can also improve coverage on very porous surfaces.

Yes — enter your wall dimensions and set the coverage rate to match your exterior paint specification (typically 8–12 m²/litre for masonry paint). Uncheck the ceiling option since exterior jobs do not include a ceiling. For smooth render use the standard rate; for rough masonry or brick reduce the rate by 20–30% or select an additional coat to account for the increased absorption.