Monday, December 11, 2023

How to calculate the paint we need to paint?

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Calculate the paint needed To perfectly cover a wall is not a complex task, but it requires having some tools at hand, a table with the yields of paints according to the surface and quality, and of course, some knowledge of mathematics.

Fear not! In this note we will look at some common cases and how to solve them using laser measurement tools to simplify the task.

Calculate the paint: a matter of three steps

1. Measurement of the total area to be painted (or measurement of partial cloths)

For calculate the paint needed To properly cover all the walls (and the ceiling if necessary) some measurements are required.

The area is measured in square meters (m²). This unit arises from multiplying the sides of a rectangle. Thus, a 1 m by 1 m square will give us 1 m² of surface, while another 2 m square will have 4 m²; 1 x 1 = 1; 2 x 2 = 4. In turn, a 3 m by 4 m rectangle will give us an area of ​​12 m².

Now, not always the walls can be perfectly described as rectangles: it will happen on many occasions that we will find cuts (projections, depressions and openings) that will make the task difficult. For these cases it is necessary to know how to add or subtract.

It may also happen that we want to paint the wall in two colors, in two stripes. In this case, it is necessary to calculate the partial areas and write the result along with the desired color on a piece of paper.

calculate paint

How to approach the task of measuring? Formerly it was done with a tape measure, pencil, a square and a lot of patience. Fortunately, with the invention of distance meters and the laser line levels this work is greatly simplified.

Line laser levels (which can be either red or green lines) help to project perpendicular lines onto walls. Those that are capable of projecting lines around (360 °) are the most suitable for complex painting tasks (where it is necessary to paint in stripes, for example); Although the most basic models also help enormously if the paint job is easier.

In turn, laser distance meters are used to take measurements quickly, without having to reach the other end of the wall or gap that we need to measure.

Basically, we have to divide the main surfaces into indivisible rectangles. If there were to be an arc or any other curved cut, it would be enough to draw a rectangle that is as inclusive as possible; in this way we make sure that it is overpainted and we do not miss it.

The partial results are summative as long as they correspond to the same type of paint. For example, if we have a light blue and a white strip, each divided into three sections, the results of the three sections of each painting are added separately. Thus, if we have three panels of 10 m², 4 m² and 6.5 m² for the white stripes, the total area to be covered with that painting will be 20.5 m².

2. Calculation of paint performance

calculate paint

The next step is to determine the type of surface to paint. The most common is to have a smooth surface (walls with fine plaster) although it is also possible to find walls with spatter, textured and even with exposed brick if it is exterior walls.

In the table seen above (or to the left on mobile phones and tablets) we see two tables. The first corresponds to the performance per liter according to the type of paint. The second indicates a coefficient according to the type of surface.

Suppose we have a textured wall of exactly 10 m². We choose a top quality paint. In the table we see that in principle we need 1 liter. Next we look for the coefficient for the type of surface: 0.55.

To know the total amount of paint needed, we must use the following equation:

((surface to be painted in square meters) / (performance)) / (coefficient s) = total liters

First we must divide the total square meters to paint by the performance found in the table. In this case, it gives us exactly 1. We now divide that result by the coefficient s. It gives us an approximate value of 1.81. In this way we obtain the total amount of paint needed: 1.81 liters.

3. Other factors to consider

calculate paint

It must be taken into account that the previous calculation is an approximation, and that it does not take into account secondary factors such as the tone of the previous paint or if the wall has deficiencies repaired with filler.

The previous painting is darker

calculate paint

Many times it happens: the previous paint is darker than the one we want to apply.

In cases where the previous paint tone is darker than the one we intend to use, two coats of paint will have to be applied. In this case, the value obtained must be multiplied by two.

You have to paint a wall whose ceiling is sloping

calculate paint

Laser distance measurer with built-in inclinometer. They are very useful measuring instruments for the painter.

This is common in chalet-type constructions, where the roof (made of machimbre) presents an inclination to drain the waters.

It is in this case where laser measurement tools play a starring role. In this case we can use a laser distance meter with a built-in inclinometer. It will allow us to measure the exact inclination of the roof and, if the model is advanced, it will even give us the total area without having to calculate it by hand.

calculate paint


In case you do not have one, we can also make use of digital angle meters, also called goniometers. These instruments allow any angle to be accurately measured, be it concave or convex.

calculate paint

Let’s look at a concrete example. Suppose that two of the walls of the house to be painted have the geometry shown above. It is a gabled roof, with uneven angles.

For this case it is not necessary to measure the angles; It will be enough to know the measures h, p and q.

We know that the area of ​​a right triangle is equal to half the product of its base and its height. With this in mind, the total area of ​​that wall will be equivalent to the sum of its two constituent right triangles:

Surface r = (hxp) / 2

Surface s = (hxq) / 2

Total area = area r + area s

Tiles and ceramics

calculate paint

It may happen that we have a wall that we want to paint, but we know that later on we will place ceramic tiles or tiles on the wall, up to a predetermined height. As the surface where these ceramic or tiles will be placed does not require painting, we must calculate the free surface.

Now, we only have a point at a certain height on the wall, indicating how far the ceramics will go. With a laser line level it is possible to project this point to both sides. Even if the wall will have a ceramic or tile cloth in a single quadrant of the wall (with which the free space will be up and to one side), the laser line level will allow us to perfectly project the perpendicular lines.

For calculate the surface to be painted We must first calculate the two areas, and subtract the free surface that will have the ceramics. Something very simple!

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