How do I calculate PPI?

How do I calculate PPI?

Ppi monitor

What is the right resolution for printing an image? How do you know if an image has sufficient resolution? These are questions that we are asked every day, but to answer them we must clarify some basic concepts necessary for their understanding.

These acronyms: SPI, PPI, DPI, LPI, may seem familiar to us, but we often confuse the concepts they contain, in this article, we will try to dispel doubts by explaining in each case what they define, and their most representative values.

Technically the terms are not interchangeable, however, in daily use, when talking about digital images we use the terms considering that they are the same thing. Again, in everyday use, when talking about digital images, PPI, DPI, LPI or SPI can be understood as the same thing.

The term SPI samples per inch refers to the resolution of a scanner, i.e. the physical capacity of the reader to acquire an image in so many samples per inch (e.g. 300 SPI).

Calculate dpi

A high PPI means a screen that will display a much sharper image. Many laptop models are sold with the high resolutions they support and both pixel density and resolution are a major selling point advertised by laptop manufacturers. Even if it is not advertised, some manufacturers will mention it in the laptop or monitor specifications. Assuming you can’t find this information on the manufacturer’s official website or in the manuals that come with your laptop, here’s an easy way to calculate it with reasonable accuracy.

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Next, find the diagonal height of your screen. This should be fairly easy to find in your laptop’s manual or on the manufacturer’s specifications page. Once you have both the bottom root value and the diagonal length of your screen, you just need to divide the bottom root value by the horizontal length and the answer will give you an approximate PPI value.

How to calculate the pixels per inch of an image

We need to get that of the same line and for this we will resort to the Pythagorean Theorem, which tells us that we can know the distance of a hypotenuse in terms of its two legs, in the case of a right triangle, as it happens in the case of mobile screens.

The Pythagorean Theorem states that the square of the hypotenuse is equal to the sum of the squares of each of the legs. To clarify a little, the hypotenuse of a right triangle is the side opposite the right angle, or in other words, the longest side. The legs are the other two sides.

The above picture makes it clear that the hypotenuse squared is equal to the sum of the squares of the legs. We can clear the hypotenuse, which would be equal to the square root of the sum of the squares of each of the legs.

In this case, since we already know how many inches the diagonal is, we want to find out how many pixels are on that diagonal. This diagonal is the hypotenuse of our triangle, and the legs are the height and width of the screen in pixels. In the example, we are using a Samsung Galaxy Note 2. Its screen resolution is 1280 by 720 pixels.

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How to calculate ppi

PPI (or ppi) stands for Pixels per inch and is a measure of the spatial pixel density. It indicates how many individual pixels can be placed in a 2.54 cm (1 inch) line.

Pixels are clearly defined colored dots on a fixed grid. Depending on the product (sensor / light source), each individual pixel can take color values from 1 bit (2 colors) to 48 bits (280 billion colors). The terms PPI and DPI are often used synonymously due to their similar meaning.