How much paint do I need?

The sum, in one line

Resene sets it out as simply as anyone: surface area divided by spreading rate, multiplied by the number of coats, equals the litres you need. Everything else is just being honest about those three numbers.

The spreading rate lives on the tin under coverage, and on the product data sheet under theoretical coverage. The word theoretical is doing some work there, and we will come back to it.

How do you measure the area?

For walls, add up the length of every wall in the room to get the perimeter, then multiply by the ceiling height. For a ceiling, length times width. For an outside wall, do each elevation on its own and add them up, because gables and different heights will catch you out otherwise.

Take out the big openings. Resene says to subtract the area of windows and doors from the total, and on a room with sliding doors or a picture window that changes the answer by litres. Small openings are not worth deducting: the paint you lose to cutting in around them roughly makes up for it.

• Walls: perimeter of the room multiplied by the ceiling height.
• Ceiling: length multiplied by width.
• Take off anything larger than a doorway.
• Measure the height to the top of the skirting if the skirting is being done separately.

What spread rate should you use?

Use the product's own figure, not an average. The published rates sit further apart than most people expect, and the gap between a primer and a cabinet paint is nearly double.

Those numbers assume a smooth, sealed, previously painted surface and a painter spreading the paint evenly. Rougher profiles pull the real figure down. Resene puts it plainly: porous surfaces and shapes such as corrugated iron will need more paint.

• Dulux wall, trim and ceiling paints: around 16 square metres a litre, by Dulux's own calculator.
• Dulux primer: around 14 square metres a litre, and 1Step Prep is listed at exactly that.
• Dulux Weathershield outside: up to 16 square metres a litre.
• Cabot's Timbercolour deck and exterior paint: 16 square metres a litre.
• Dulux Renovation Range cabinet paint: 12 square metres a litre, and the tin says not to stretch it further.

Worked example: one bedroom

Take a bedroom four metres by three and a half, with a two point four metre ceiling. The perimeter is fifteen metres, so the walls are fifteen times two point four, or thirty-six square metres. Knock off a door and a window, call it three and a half square metres, and you are painting thirty-two and a half.

Thirty-two and a half divided by sixteen is a bit over two litres a coat. Two coats makes a bit over four litres, which is why a four litre tin is tighter than it looks and most people walk out with the next size up. The ceiling is four by three and a half, or fourteen square metres, which is under a litre a coat and under two for the pair.

Add a small tin of primer for the filled patches and the water stain in the corner, and that is the whole shopping list.

Worked example: an outside wall

A weatherboard wall ten metres long and three metres high is thirty square metres. At the published sixteen square metres a litre that is under two litres a coat, so under four for two coats, and you would be short every time.

Boards are not a flat wall. Every lap has an edge, every board has ends, and a brush pushed into those edges uses paint the flat area never sees. The same applies to a paling fence, a balustrade or a rendered wall with a coarse texture. Allow more than the sum says, and if the timber is bare, allow for the extra coat that unpainted timber needs on top of its primer.

Why the tin never goes quite as far as it says

Spread rates are measured under ideal conditions. Real walls take more, for reasons that stack up quickly.

The temptation is to stretch it anyway. Do not: paint spread too thin is the main reason a finished wall looks patchy in afternoon light, and the fix is another coat, which costs more than the litre you saved.

• Absorbent surfaces, bare filler and fresh plaster drink the first coat.
• Texture, whether that is a render finish or a weatherboard profile.
• Strong and deep colours, which Dulux notes can need more than two coats, especially over a dark colour.
• Cutting in, which puts paint on thicker than a roller does.
• What stays in the tray, the sleeve and the brush at the end of the day.

Buy a little over, and sort out what is left

Buy the colour in one go if you can, so it is all tinted at once, and keep a labelled bit for touch-ups with the room written on the lid. It will be worth its weight the first time a chair goes through the hallway wall.

What you do not need should not go in the bin or down a drain. Paintback takes leftover paint free, up to 100 litres per visit in containers of up to 20 litres, and covers interior and exterior paint, deck coatings, floor paints, undercoats, sealers, stains and varnishes. The Frankston Regional Recycling and Recovery Centre on Harold Road in Skye is a Paintback site with no fee, and Paintback's own location finder will show you the closest one further down the Peninsula.

When is it worth having someone else measure it?

Measuring one room is a tape measure and two minutes. Measuring a whole house is where people either give up or guess, and a guess on an exterior is an expensive one.

It is worth handing over the measuring when the house is double storey and you cannot safely get a tape to the gable, when there are several different surfaces that each need their own product and coat count, or when the colour change is drastic enough that the coat count is the real question. That is the arithmetic we do at every quote for an exterior repaint or an interior repaint, and the written quote shows what is going on each surface and how many coats it gets. If you are pushing ahead yourself, our step-by-step guide to painting a room covers the order to do it in.

Quick questions

Sources

What this guide relies on. Product advice changes, so check the maker's current instructions for the exact paint you're using.

1. Paint calculator: coverage assumptions — Dulux
2. How much paint do I need? Paint calculator — Resene
3. Weathershield Low Sheen: spread rate — Dulux
4. Timbercolour Deck and Exterior Paint: spread rate — Cabot's
5. Find a collection location and drop-off limits — Paintback