If you are planning a painting job, you will want to work out how much paint to buy. You should also try to calculate roughly how long the job will take. This will allow you to plan both your total time and when you will finish each coat, so that you can work out the times of breaks.

How much paint will you need?

Work out how much paint to buy before going to the shop. First measure the length and width of the room. It is best to use metric measurements for this since paint is sold in litres. Round up the dimensions of the room to the nearest half metre and add them together. Then multiply the result by the height of the room and multiply the result by two. This will give you the total area of your walls. It is difficult to estimate the sizes of windows without calculating them exactly, but as a rough guide, subtract 2 square metres for a small window, 4 square metres for a medium sized window, and 5 square metres for a large one. You should allow 2 square metres for each door. You should also add on extra for any protruding chimney breasts or similar features.

Stairways often have triangular walls. Work out the area of these by multiplying the length by the height of the wall and dividing by two.

Covering power

The actual quantity of paint that you will need will depend on the covering power of the type you are going to use. It will also depend on the surface you are painting. For example, liquid gloss and emulsion paints cover a larger area per litre than non-drip gloss. Also, bare plaster will be more absorbent, and therefore take more paint, than a surface that has been prepared. To find the exact covering power of a particular brand of paint, look at the details on the can. As a rule of thumb, expect non-drip gloss and silk-finish paints to cover about 12 sq m per litre; solid emulsion to cover about 14 sq m per litre; undercoat, eggshell and vinyl silk to cover about 15 sq m; and liquid gloss to cover about 17 sq m.

How many coats?

Of course, the number of coats you need will also influence how much paint you require. Several factors determine how many coats you should apply. In ideal conditions, follow the instructions on the can. But sometimes you will need more coats than the manufacturer recommends. For example, if you are covering a dark colour with a lighter one, you should allow extra coats to cover up the old colour. The surface itself is also important. When you are painting over bare wood, use a coat of primer and an undercoat before you start applying the gloss. When covering old gloss paint you may also need one or two undercoats to provide a surface that will take the new gloss paint.

How long will it take?

When you are planning a decorating job, remember that you should allow enough time for preparation and cleaning up afterwards. When working out how long the whole job will take, bear in mind the time each coat of paint will take to dry.

Preparation nearly always takes longer than you think it will. The time you allow for filling holes and rubbing down will obviously vary according to the condition of the surfaces, but allow 2 to 3 hours for priming the walls of a room 3m x 4m.

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