Your hardware store or paint stockist will be able to tell you how much paint you need if you give them your room or exterior measurements, or you can work it out yourself using the information below. Approximate measurements are near enough. Remember to err on the high side. Not allowing enough paint to do the job properly is a common mistake of DIY painters.

EXTERIOR

1. First, work out how many square metres you need to paint by measuring the distance around your house (the perimeter). Multiply that number by the height of your house up to the roof line. If you have a particularly large area of glass, or other surfaces you do not wish to paint, work out the area by multiplying its length by its height and subtract the total from the figure above.

2. Next, determine the area above the roof line. For each pitched roof or gable, multiply the height of the peak as measured from the roof baseline by half the length of the baseline. For any other areas, like second storey additions, work out the total area by dividing it into rectangles or triangles and adding their individual areas.

3. Add the area above the roof line to the area below the roof line to get the total area to be painted.

To estimate the volume of paint you'll need look at the label on a can of the paint type you've chosen. On a panel next to 'coverage' it will tell you how many square metres, on average, each litre will cover. Divide your total area by this figure to determine the number of litres you will need for each coat. Then multiply the result by the number of coats.

Remember this figure is a guide only. The actual amount you use may vary according to the type of surface you are painting and the condition it is in.

As a rule of thumb you can add 20% for rough and porous surfaces, 10% for narrow weatherboards, 33% for corrugated metal and 10% extra for the first coat on concrete block masonry walls.

INTERIOR

If you'd like a rough estimate of how much paint you'll need, follow these general rules. For a more accurate estimate use the table below.

• General: One litre of paint will cover approximately 16 square metres, an area 4 metres x 4 metres.

• Ceilings: 2-3 litres will provide 2 coats in an "average" sized room.

• Walls: 4 litres will provide two coats in an "average" sized room with "average" door and window areas.

• Bathrooms: 3 litres is enough for two coats on walls and ceiling.

• Doors: 1 litre will cover both sides of two doors with one coat.

This chart uses the distance around the room and the ceiling height to estimate the paint quantities, rounded up to the nearest half litre. For instance; a room that has a circumference of 16 metres and a 3.1 metre ceiling would need around 3.5 litres of paint per coat on the walls and 1.5 Iitres on the ceiling.

Note: For porous or rough surfaces such as bare plasterboard, add approximately 20% to the first coat quantity. Complicated mouldings and plaster work will require extra paint.