Paint Calculator

How the paint calculator works

The calculator finds the area of the walls you are going to paint, removes the openings, and divides what is left by the area one unit of paint covers. It rounds the result up because paint is sold in whole cans.

paint needed = (wall area − windows − doors) × coats ÷ coverageRounded up to a whole gallon or litre.

Choose Internal painting for a room and give its width, length and height; the four walls make up a band of perimeter times height. Choose External painting when you already know the total length of the walls, for example the perimeter of a house: then you enter a total width and a total height and there is no room length.

What to enter

• Room dimensions. Measure width, length and height of the room. Each value has its own unit, so metric and imperial values can be mixed.
• Windows. Enter the width and height of a typical window and how many there are. If the windows differ, enter the average size or run the calculation twice.
• Doors. Doors are counted at an average size of 1.8 m² (about 19.4 ft²) each, so you only enter how many.
• Number of coats. Two coats are normal for a colour change. A single coat may do for repainting a similar shade over a sound surface.
• Paint efficiency. This is the coverage printed on the tin: area per gallon (US units) or per litre (metric). The boxes start at 400 ft² per gallon and 10 m² per litre. It varies by product and surface, so use the figure from the manufacturer.
• Price. Optional. The price per gallon or per litre is multiplied by the rounded-up amount.

Worked example

A room is 12 ft by 15 ft with 8 ft walls, two windows of 3 ft by 4 ft, one door, and will get two coats of paint rated at 400 ft² per gallon.

1. Perimeter: 2 × (12 + 15) = 54 ft. Wall area: 54 × 8 = 432 ft².
2. Windows: 2 × (3 × 4) = 24 ft². Door: 19.375 ft².
3. Paintable area: 432 − 24 − 19.375 = 388.625 ft², which is 43.18 square yards.
4. Two coats: 388.625 × 2 = 777.25 ft². Divided by 400 ft² per gallon that is 1.94, so you buy 2 gallons.

At $35 a gallon the paint costs $70.

In metric units, a room of 4 m by 5 m with 2.5 m walls, two windows of 1.2 m by 0.9 m and one door has 45 − 2.16 − 1.8 = 41.04 m² of wall. Two coats at 10 m² per litre need 8.2 litres, so buy 9.

Choosing a coverage figure

Most interior wall paints cover roughly 350 to 400 square feet per US gallon on a smooth, primed surface, which is about 8.6 to 9.8 m² per litre. Textured, porous or dark surfaces soak up more paint, and deep colours often need an extra coat. A primer has its own, usually similar, coverage.

The coverage box starts at 400 ft² per gallon in US units and 10 m² per litre in metric units. The two defaults are close to each other (10 m² per litre is about 407 ft² per gallon) and sit in the middle of the usual range for a smooth, primed wall. Replace the default with the figure printed on your tin whenever you know it: a rough surface, a dark base colour or a budget paint can cover noticeably less.

What is not included

The internal mode covers walls only. If you are painting the ceiling, calculate it separately: its area is the room's width times its length (180 ft² for the room above, which is about 0.45 gallon per coat at 400 ft² per gallon), and the area calculator helps with other shapes. Skirting boards, window frames, doors and cabinets are not counted either. Add a small allowance for them, and for touch-ups later.

A calculation also assumes the paint goes on evenly. Rolling over rough plaster, brick or unsealed wood uses noticeably more.

Practical painting tips

• Prepare the surface. Wash off grease and dust, fill cracks, sand glossy areas lightly and let everything dry. Paint sticks to clean, dull surfaces.
• Prime when the surface changes. New plaster, bare wood, stains and strong colour changes all benefit from a coat of primer.
• Work top to bottom. Do the ceiling first, then the walls, then the trim, so drips land on surfaces that are still to be painted.
• Keep a wet edge. Roll in overlapping strips of the full wall height and do not let one section dry before the next joins it.
• Mix cans together. If you buy several tins of the same colour, combine them in one bucket so slight batch differences do not show as stripes.
• Buy slightly more than the minimum. Keep the leftover, labelled with the room and date, for touch-ups.

Quantities are estimates. Always follow the manufacturer's instructions for coverage, drying time and safety, and ventilate the room while painting. See the disclaimer.