Free materials calculator

Paint Calculator

Enter your room size, the number of doors and windows, and how many coats - and get the exact litres or gallons of paint to buy, plus how many tins. Works in feet/gallons or metres/litres.

Rooms & paint

01. Rooms

Length (ft)

Width (ft)

Wall height (ft)

Doors (count)

Windows (count)

Paint the ceiling too

02. Paint specification

Coverage (ft²/gal)

Tin size (gal)

Number of coats

Calculated requirement

2.4gallons of paint

Paintable area

418.8 sq ft

Across 2 coats

838 sq ft

Tins (1 gal)

Guide & worked example

How this calculator works

The calculator measures the wall area, subtracts your doors and windows, multiplies by the number of coats, then divides by the paint’s coverage (spreading rate):

litres = (wall area − openings) × coats ÷ coverage per litre

Wall area is the room perimeter times the wall height - 2 × (length + width) × height. Tick the ceiling box to add the floor area on top. Because everything runs in exact metric internally, switching between gallons and litres never changes the answer.

Worked example

This example follows the unit system you pick in the calculator above.

A 16 ft × 13 ft room with 8 ft walls, one door and two windows, two coats:

Wall area: 2 × (16 + 13) × 8 = 464 sq ft.
Subtract openings: one door (21 sq ft) + two windows (12 sq ft each) = 45 sq ft → 419 sq ft to paint.
Two coats: 419 × 2 = 838 sq ft of painting.
At 350 sq ft per gallon: 838 ÷ 350 ≈ 2.4 gallons → three 1-gallon tins.

Getting the coverage right

Surface	Effect on coverage	What to do
Smooth, previously painted	Best case	Use the tin’s stated figure
Bare plaster or filler	Soaks up the first coat	Add a mist/primer coat
Textured or rough render	Much lower coverage	Drop coverage 20–30%
Strong colour change	Needs extra coats	Use 3 coats, not 2

Interior, ceiling and exterior paint

Walls, ceilings and outside surfaces all use the same area × coats ÷ coverage math, just with different settings:

Interior walls - two coats over a sound surface of a similar colour; the default coverage figure applies.
Ceilings - tick the ceiling box (or run a separate job with the walls set to 0) and use ceiling-grade paint, which often covers a little less than wall paint.
Exterior walls - masonry and render are porous and textured, so drop the coverage 20–30% and plan on two full coats. Measure the outside wall area the same way: perimeter × height.

Set the coverage from your tin and the number of coats to suit the surface, and the calculator adjusts the litres or gallons to match.

Tips for measuring

Measure the room perimeter and wall height; you don’t need each wall separately for a normal rectangular room.
For a feature wall only, set the length to that wall and width to 0, then untick the ceiling.
Ceilings usually need their own tin of ceiling-grade paint - calculate them as a separate job with the ceiling box ticked and the walls set to 0.

Frequently asked questions

How much paint do I need for a 12x12 room?

A 12ft × 12ft room with 8ft walls has about 384 square feet of wall. After subtracting one door and one window, two coats need roughly 2 gallons (about 7.5 litres) at a typical 350 sq ft per gallon coverage. Enter your exact room size, openings and coats above for a precise figure.

How many coats of paint do I need?

Two coats is standard for a durable, even finish on most walls. Use one coat only when repainting a similar colour over a sound surface, and three when going from dark to light, painting bare plaster, or using a strong colour. This calculator multiplies the wall area by your number of coats.

What is the coverage (spreading rate) of paint?

Most wall paints cover about 10–12 m² per litre (roughly 350–400 sq ft per US gallon) per coat on a smooth, sealed surface. Textured, porous or bare surfaces drink more paint, so they cover less. The figure is printed on the tin - enter it in the calculator to match your specific product.

Should I subtract doors and windows?

Yes, for accuracy. This calculator subtracts a standard size for each door (about 1.8 m² / 19 sq ft) and window (about 1.2 m² / 13 sq ft). For a single feature wall or many large windows the saving is significant; for a normal room it trims roughly half a tin.

Should I buy paint by the tin or get it mixed?

Buy whole tins in a standard size and round up - the calculator already does this. Keep the leftover for touch-ups, since a later mixed batch may not match exactly. For large jobs, buying one larger tin is usually cheaper per litre than several small ones.