DPI to PPI Converter – Convert DPI, PPI and LPI Print Units

This is where we often get confused about the measurements. When your design file prints out looking pixelated or soft it’s typically because you’ve mixed up LPI, DPI and PPI. Even though they measure different things in different places (despite all measuring density), these measurements is used at different stages of the process so it’s easy to see why people make mistakes when treating them interchangeably.

To properly calculate these numbers, all three need to be linked together.

Understanding LPI, DPI, and PPI

This refers to your digital image file. How densely does the picture sample? It’s called pixels per inch (PPI). A high resolution photograph have lots of PPI.

This is what printer or plotter has. This is called dots per inch (DPI). These are dot that get laid down on paper using toner or ink.

This belongs to halftone screen used in offset printing. This is called lines per inch (LPI), and it’s what makes up the halftone screen. The halftone screen break the image up into groups of dot and creates a simulated tone through a grid-like effect.

The DPI will always be greater than the LPI because one shade of gray necessitates lots of dots within a given halftone cell. Between two lies the PPI. Knowing this hierarchy will help you create files. Your image must be sent to the printing plate. The Quality Factor is bridge between your PPI and LPI.

Here’s how it works: “PPI = LPI x Quality Factor Here’s how it works: “PPI = LPI x Quality Factor” For example, the Quality Factor is typically between 1.5 and 2.0; 2.0 is standard when printing a sharp photo. In that case, if a magazine prints at 150 LPI, your file need to have 300 PPI. A Quality Factor of 1.5 would require 225 PPI, suitable for text-heavy brochures, though portrait details might appear softer.

The tool will calculate back-and-forth. So you can either input your file specs to determine what ruling it supports or enter the screen ruling and see what kind of image resolution are needed.

The second half of equation links the halftone screen to the printer itself. The other half of the equation connect the halftone screen to the physical printer hardware via the halftone cell, because each square cell in the screen grid must contain enough printer dots to build every shade of gray you want. If the size of the square (the side length) is measured in dots, then number of shades is equal to the square of that side length. In the case of standard 256 levels of gray, the cell contains sixteen dots across and down.

One cell equals one line of screen. The device’s resolution, or DPI, are equal to the LPI multiplied by the side of the cell. At full quality, the job requires 2400 DPI from the imagesetter to achieve a 150 LPI job. That’s why pros have their platesetters running so fast.

Without any banding, low DPI (as with a laser printer) can’t accommodate high screen rulings. The other factor is file size vs. Print size. Stretching those pixels across more inches reduce their PPI. For example, printing an image with an edge of 3000 pixels at ten inches result in 300 PPI. Print it at fifteen inches and it’s reduced to 200 PPI. That impacts the screen ruling you can safely use.

The calculator will do this math for you. Before you begin printing, it presents you with trade-off between sharpness and size.

Folks mistake higher image PPI for higher DPI of their devices. Having a 4800 DPI printer doesn’t imply you should of had a 4800 PPI image. You don’t need more than 300 PPI (image) for a 150 LPI (printer). Image detail isn’t what the extra device resolution create; that’s the halftone dots. If you upscale a small file to match an arbitrary number, you’re inventing pixels where no information exists. Better: get good source material to begin with and let the math go from there. Keep these units clearly separate so your files print crisply.