What is DPI (Dots Per Inch)?

In computer graphics, we used many terms like pixel, dot, resolution, sharpness, Quality of picture. All these terms are used during soft copy and hard copy output. When we talk about soft copy output, we generally deal with pixels. Hard copy output considers the term dots pitch for picture quality. Quality of printed media depends upon the distance of dots. Density means; how many dots are located under per inch area. Greater the DPI, more enhancement and focus on the quality of printed media.

During the discussion of DPI, there was confusion between DPI and PPI. ARE they similar term? Are they having the same meaning? Are they used for the same purpose? And biggest question if they are not similar, not having the same meaning, not used for the same purpose Then what is the difference? Now we will deal with the concept and each term. All questions having single answer; Dpi measure how many dots in vertical (column) and horizontal (row) in printed media. PPI measure how many pixels are in the digital image.
DPI: The resolution of a printer or a scanner is measured in dots per inch, meaning “dpi.”  If a printer has a resolution of, say, 300 dots per inch, that means in one inch there are 300 dots in a row across, and 300 dots in a row down. More dots per inch, space will get less between dots, more smoothly color droplets get to mix with each other and quality of printed media is sharp and acceptable.  A color droplet is a drop coming out from printer during printing. In DPI, we are generally talking about the distance between two dots. If fewer dots are in one-inch clarity in printed media get reduced, printed media look like faded. Actually, when these tiny dots are not far from each other they get to mix with each other properly on paper and give good impact. By default, 300Dpi is set to get a good and smooth result. If we talk about the previous printer inkjet printer, drum printer, chain printer they all have different dpi. They don’t provide a high-quality printout.

PPI: You may hear the resolution of a Macintosh screen, or monitor, measured in pixels per inch, or “ppi”. Screen and Monitor are made up of pixels. These are tiny dots of light in RGB color combination. More the pixels in row and column of screen more will the sharp and good image quality will be given by monitor. These ppi convert to dpi during press printing.
Let’s we consider an example to understand it:
1. Suppose if the size of any picture is 10×12 means to say that width=10 and height=12.According to given by default value of Dpi=300 dots per inch. In 10”x12”, we have 3000 dots in width, 3600 dots in height and total 3000X3600=1, 08, 00,000 pixels.
2. If we reduce the rate of DPI from 300 dpi to 150 dpi for same size picture 10×12.let’s see what happened. We will follow the same procedure, now we have 1500 dots in width,1800 dots in height and 1500×1800=27,00,000 pixels. In this, we can calculate the pixel for the image. Now consider the result, Which had come out after all this small discussion?
Result:     in first point, dpi=300,                pixel =1, 08, 00,000 pixels
                 In second point dpi=150            pixel=27, 00,000 pixels    
Due to a reduction in the rate of DPI, the number of pixels gets reduced, size get increased by 4 multiple. That means pixel will look big, blur, foggy and unfocused.
How to calculate DPI from PPi and Dimension:
Let us consider a simple example of Digital image having dimension 5.75 x 8.62 or having  pixels 552 x 828
Calculate DPI=(552/5.75) x (828/8.62) =96 or 96.05
We can check an image ppi from properties tab option in the simple paint also.
Similarly, we can calculate dimension by dividing ppi and dpi.
Dimension of image=(552/96) x (828/96.05) =5.75 X 8.62