What is the total wing surface area of the A320? I was trying to calculate the total area of an A320, and in this link I found the wing area and tail area. But here I read that the wing area is just the 2D projection of the wing. So if I wanted to calculate the total surface of the wing I should consider the wing area (122.4 m) and multiply by two, which would be an approximation.

By doing this, and considering the fuselage to be a cylinder, I get a total area of 847 square meters for an Airbus A320, which is close to the 900 square meters reference I found in this article.

So, is it correct to say that the surface area of a wing is approximately the wing area multiplied by two?
As I'm not sure whether you were only looking for the methodological answers provided earlier, this is a more detailed answer to the calculation problem you were trying to solve in the first place, providing an answer to the question posed in the title of your question. It is based on the A320 Airport and Maintenance Planning Manual, which provides approximate dimensions of the aircraft.

For the wings and wing-like surfaces, details are as follows:


Wing top surface: $99.7 m^2$ (excluding wingtips devices)
Wingtip devices (normal, non-sharklet A320, both sides): $1.8m^2$
Horizontal tailplane top surface: $27 m^2$
Vertical tailplane (both sides): $43 m^2$


Counting horizontal wing and tailplane surfaces twice (upper and lower), that yields a total of $298.2m^2$. This does not properly account for thickness (using the factor $1.07$ mentioned by Daniel yields $319.074 m^2$) and movables (high-lift devices, control surfaces, etc).

For the fuselage and nacelles, only top surfaces are listed. Modelling these as simple cylinders therefore seems the best way to go. Based on a fuselage diameter of $4.14 m$ and a length of $37.57m$, that yields a surface area of about $488m^2$. Of course, the fuselage is not strictly cylindrical, but I'm assuming here that is offset by the fact that I'm not explicitly including the front and rear surfaces (of a closed cylinder). Nacelle dimensions are not specified (probably as that somewhat depends on engine type), but based on drawings, I'm estimating a length of $5 m$ and a diameter of $2.5m$, resulting in a total surface (again: cylindrical approximation) of $78.5 m^2$ for the two engines. For the pylons, lets add another $5m^2$ per side.

In total, that adds up to $872 m^2$ (or $902.874 m^2$ with the 1.07 thickness approximation). That corresponds nicely with the $900m^2$ you found online - and also shows your own calculation wasn't too far off (some 3%).