Room Paint Calculator
Estimate the wall paint a room needs from its length, width and height, minus doors and windows, with gallons or litres to buy and the ceiling.
Work out how much wall paint a room needs from its length, width and wall height. The calculator finds the wall area, takes off doors, windows and any other openings, multiplies by the number of coats and tells you how many gallons or litres to buy. Tick the ceiling box for a separate ceiling-paint figure.
How to use this tool
- Choose feet and gallons, or metres and litres, then enter the room’s length, width and wall height.
- Count the doors and windows, and add any large unpainted areas such as patio doors or a fireplace.
- Set the number of coats, the coverage printed on the paint and an extra allowance for touch-ups.
- Tick “Ceiling too” if you are painting the ceiling as well.
Formula
Wall area = 2 × (L + W) × H − openings; Paint = wall area × coats × (1 + extra %) ÷ coverage
Each door takes off 21 sq ft (a 3 × 7 ft door, 1.95 m²) and each window 15 sq ft (3 × 5 ft, 1.39 m²). For larger openings, add only the excess in “Other unpainted area”, or leave them out of the count and enter their full area there. Coverage is per coat, often 350–400 sq ft per US gallon or around 10–13 m² per litre. Deep colours, fresh plaster and rough walls cover less, so check the label.
Example
A 14 × 12 ft room with 8 ft walls has 2 × (14 + 12) × 8 = 416 sq ft of wall. Taking off one door (21 sq ft) and two windows (30 sq ft) leaves 365 sq ft. Two coats at 350 sq ft per gallon, with no extra allowance, need 365 × 2 ÷ 350 = 2.09 gallons: buy 2 gallons and a quart. Ticking the ceiling adds 0.96 gallons of ceiling paint for its 168 sq ft.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, the walls come to 333 sq ft. Two coats at 350 sq ft per gallon need about 1.9 gallons before any extra allowance, so two gallons covers it.
Should I subtract doors and windows?
Yes. With few openings it makes little difference, but patio doors or big windows can save you a can.
Do I need primer first?
Use primer on new drywall or plaster, patched areas, stains, glossy surfaces, or when covering a dark colour with a light one. Over sound paint of a similar colour you can usually go straight to the top coat.
Is trim included?
No. Skirting boards, door frames and window frames normally take a separate gloss or semi-gloss paint, and a room’s trim rarely needs more than a quart or a litre.
Last updated: 2026-10-30