How much paint do you need for a room or a house?

Short answer: one litre covers about 16 square metres, per coat

One litre of standard wall paint covers roughly 16 square metres for one coat. Dulux says, as a general guide, 1 litre of its wall, trim or ceiling paint covers about 16 square metres, while 1 litre of its primer covers about 14. Taubmans' wall painting guide says the same thing: as a guide, 1 litre covers roughly 16 square metres.

Most jobs need two coats, so in practice 1 litre does about 8 square metres of finished wall. Dulux's calculator assumes 2 coats of topcoat and calls that the ideal number for most Dulux paints. Turned around, 1 square metre of wall takes about 0.125 litres for two coats.

Treat 16 as the top of the range. Dulux says to check the spread rate on the back of the can, because a lower spread rate means more coats may be needed. Taubmans says its own calculator is only a guide, and the real amount moves with how porous the surface is, the surface type and the colour underneath.

Measure the walls: the four-step method

To work out paint for a room, find the wall area, subtract the openings, multiply by the number of coats and divide by the spread rate. Dulux's calculator uses the same core sum: surface area, divided by the coverage rate per litre, multiplied by the number of coats. All you need is a tape measure.

1. Measure around the room. Add up the length of every wall to get the perimeter. Taubmans' calculator works the same way, from the length, width and height of a 4-wall room.
2. Multiply by the ceiling height. A 3 by 3.5 metre room has a 13 metre perimeter. At 2.4 metres high, that's 31.2 square metres of wall.
3. Take off doors and windows. Measure each one, width times height, and subtract it. Leave small things like light switches in.
4. Multiply by the coats, then divide by the spread rate. Use two coats and the rate printed on your can. Round up to the next whole litre.

Do the ceiling as a separate sum, since it usually takes its own paint. Length times width gives the ceiling area. Trims like doors and skirting are a separate product again, and a small one. Our guide to interior paint finishes covers which sheen goes on which surface.

Spread rates by product, from the makers' own pages

Wall and exterior topcoats from Dulux and Taubmans list up to 16 square metres per litre, and primers sit lower at 12 to 14. The words "up to" matter. Dulux's Weathershield instructions say the amount you need varies with the surface area, texture, porosity and how you apply it. Here's what each maker publishes for the products in this guide.

The Dulux calculator uses two different primer figures in different places: 14 for its general primer guide and 12 for prep products inside the calculator itself. If you're priming, using 12 gives you a little in hand.

Worked examples: a bedroom, a lounge and a 3-bedroom interior

A 3 by 3.5 metre bedroom with a 2.4 metre ceiling needs about 4 litres of wall paint and 2 litres of ceiling paint for two coats. The table shows the working at 16 square metres per litre. The room sizes and openings are examples. Swap in your own measurements.

The 3-bedroom figure leaves out the kitchen, bathrooms and laundry, so add those rooms the same way. Dulux's own project guides list more paint than this sum. Its small bedroom guide lists 6 litres of Wash&Wear and 2 litres of ceiling paint, plus 6 litres of 1Step Prep and 2 litres of Aquanamel. Its lounge room guide lists 10 litres of Wash&Wear and 8 litres of ceiling paint. Those guides don't give a room size, so measure yours rather than copy them.

Ready reckoner by wall area

Once you know your wall area, read the litres straight off this table. It covers two coats at 16 square metres per litre, the general guide from Dulux and Taubmans, and at 12 square metres per litre for a can with a lower spread rate.

Taubmans rounds its calculator results up to the nearest litre, and that's the right habit. Running short halfway through a wall means stopping for a second trip to the shop.

Allow more paint for texture, new surfaces and dark colours

Rough, porous or dark surfaces take more paint than the spread rate on the can, so buy extra for them. Dulux says the amount of Weathershield needed varies with surface area, texture, porosity and application method. Here's where the makers say to allow more.

• Textured walls and render. Dulux's Weathershield instructions say to use a longer roller, 18mm nap or more, on textured surfaces and to allow more paint for them. Our guide to painting rendered walls covers the prep.
• New, unpainted surfaces. Haymes says primer seals new surfaces so the paint doesn't soak in, which would otherwise take extra coats.
• New plasterboard. Dulux says paper-faced plasterboard with jointing compound may need a second coat of 1Step Prep for an even finish. Allow primer for two coats there.
• Patched walls and stains. Dulux recommends primer for patched or repaired areas and for stains.
• Going light over dark. Haymes says an undercoat is essential to cover dark colours effectively. Dulux says some colours need more than two coats of Ceiling White, especially over dark colours.

When the job needs a prep coat, Taubmans recommends one coat of prepcoat followed by two topcoats. So you're buying primer for one coat as well as topcoat for two.

House exteriors: the same sum, with more allowance for texture

An exterior uses the same method as a room: wall area, minus openings, times two coats, divided by the spread rate. Dulux Weathershield Low Sheen and Taubmans All Weather both list up to 16 square metres per litre. Measure each wall of the house at its full height, including gable ends, and subtract the windows and doors.

For a feel of the scale, Dulux's brick exterior guide lists 20 litres of Weathershield for four brick walls, and another 2 litres for 2 doors and 8 windows. Brick, render and bagged walls are textured, so allow more paint than the flat sum gives, as Dulux advises.

Exterior quantities are the easy part to get right. Height is the hard part. For what an exterior repaint costs on the Sunshine Coast, by wall type and storeys, see our exterior house painting cost guide, or talk to our exterior house painters.

Stairwells and high walls: measure them, then leave them to a painter

You can measure a stairwell or a two-storey wall from the ground, but painting it from a ladder is a job for a professional. The Queensland Government says over 1,000 Queenslanders are hospitalised each year after falling off ladders, and most of those injuries happen around the home.

For the ladder work you do take on, the Queensland Government's advice is not to stand above the second step from the top of a step ladder, or above the third rung of an extension ladder.

If reaching the top of a wall means a ladder on a stair or a roof, stop there and call a painter with the right access gear.

We've painted Sunshine Coast homes since 1985 and hold QBCC licence 1281728. Our interior house painters work across the suburbs on our locations page, and our interior painting cost guide shows what a room or a whole house costs.