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A wall being painted orange with a paint roller
You can save yourself time and money by working out the volume of paint you need before you buy. This will ensure you don't run out of paint in the middle of your painting job, and also limit wastage from buying too much.

Before you start

Top quality paints not only give you easier and better coverage but are longer lasting and easier to clean than low grade paints. Budget for the best – it pays in the long run.

Similarly, good quality paint brushes and rollers will give you a better finish.  

Calculate the wall area in a rectangular room

To calculate the wall area in a rectangular room with average size windows and doors, do the following:

1. Measure the length and width of the room. Round off each figure to the nearest half metre and add them together.

2. Multiply the total by the room height, then double that result. The final figure is the total area of your walls in square metres.

3. From this total, subtract 1.5 square metres for each window and 2 square metres for each door.

4. The ceiling area is simply the length of the room multiplied by the width.

Calculate the wall area of an unusually shaped room

If you need to calculate the area of an odd-shaped room, simply follow these steps:

1. First measure the height and width of each rectangular section to be painted. Multiply the height by the width to get the area of each section, then add them together for the total area.

2. A stairwell often forms a triangular shape. For the area of a triangle, multiply the height by the width and divide by 2.

What about the trims and skirting boards?

If you plan to paint the trim a different colour, calculate these areas separately. Simply multiply the length by width of each trim then add your totals together. Remember, your measurements need to be in metres (1cm = 0.01m).

Skirting board area is the height of the board multiplied by its length (which is generally the same as the perimeter of the room).

How much paint do I need for a door or window frame?

Allow 2 square metres for each side of a door and 1.5 square metres for a single size window frame.

How does coverage affect the total?

When you know the total area you want to cover, you have half the information you need to order paint. The rest depends on the covering capacity of your paint and the surface it must coat.

If your walls are smooth, estimate on covering 14–16 square metres with 1 litre of matt or semi-gloss paint. Divide your total area figure by that amount to arrive at the number of litres you need for a first coat. If you're using finishing paint on porous, rough or previously unpainted walls, a litre will cover considerably less than that. You can count on more coverage for the second coat.

Always check the coverage information on the label of the paint can, as different types of paint have different spreading capacities.

As an example, 5 litres of paint is generally ample for a 4m x 5m room with a 3m ceiling and smooth, previously painted walls. The ceiling of the 4m x 5m room would take 1.5 litres. Professional painters often allow for one quarter as much trim paint as wall paint but you can make a more accurate and economical purchase if you calculate the trim area as suggested above.

How long will it take?

Estimating time brings in a new set of variables. In one hour you are likely to cover about 7 square metres of an ordinary surface, or about 5 square metres of bare wood or plaster.

You will probably be able to paint that 4m x 5m room with one coat in 56 hours. If you apply two coats, you must allow time for the first coat to dry. Allow 4 hours for water-based paint and overnight for oil-based paint (check the paint can label). You can safely estimate 2 or 3 days to paint a room and clean up afterwards.

Don't guess, use our free paint calculator

Also, check out our free paint calculator to help calculate how much paint you'll need for your next painting job.


Health & Safety

Please make sure you use all equipment appropriately and safely when following the advice in these D.I.Y. videos. You need to be familiar with how to use equipment safely and follow the instructions that came with the equipment. If you are unsure, you may feel it is safest to consult an expert, such as the manufacturer or an expert Bunnings Team Member.

Grave health hazards are linked to asbestos, which may be in homes built up to 1990. Health hazards may result from exposure to lead-based paints in older materials and copper chromium arsenic (CCA) treated timber. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer. You can also use a simple test kit from Bunnings to indicate the presence of lead-based paint.