PPI

/piː piː aɪ/

General Definition

PPI (Pixels Per Inch) is a metric that measures the pixel density within a digital image or input file. This unit indicates the number of pixels contained within one inch (2.54 cm) of the image. PPI directly relates to the quality of the input file and the ability to reproduce details in graphic design software. If an image's PPI is low, the image will suffer from pixelation when enlarged, and the final quality in the Lithography process will be compromised.

Real-World Usage

PPI is a critical decision in the Packaging Graphic Design stage. Designers must set their file's PPI based on the LPI (Lines Per Inch) required for the print output. As a rule of thumb, the input resolution (PPI) should be approximately 1.5 to 2 times the LPI selected for the final print. For example, for offset printing with 150 LPI, the input image resolution must be set to 300 PPI. Failing to observe this ratio will either lead to irreversible quality loss or unnecessarily inflate the file size.

In practice, increasing the PPI excessively beyond a certain threshold (typically 350 PPI) for offset printing no longer results in a noticeable quality increase and only increases file volume. This slows down the software, extends the RIP processing time, and increases the load on Prepress systems. Therefore, optimizing PPI means finding the balance between preserving sufficient detail and efficiently managing files throughout the production chain.

Consultant's Note

Our specialized advice is that, consider PPI as the first and most crucial control point in the Resolution Chain. Any deficiency in quality at this stage is irreversible. Before starting the design, the Prepress team must determine the required LPI based on the printing standard (such as ISO 12647) and the final substrate type. This reference LPI is then used as the basis for the accurate calculation of the input file's PPI (by multiplying by the 1.5 to 2 factor), preventing reliance on arbitrary upscaling tools.

Furthermore, design teams should internally standardize the PPI based on their final press type and substrate. For instance, the PPI required for printing on matte packaging board with 120 LPI is much lower than the PPI required for fine art printing on coated paper with 200 LPI. Establishing this alignment between the Packaging Graphic Design and Prepress departments prevents time wastage on file corrections and re-runs in lithography.

Aligned with: Adobe / ISO 12647 / Kodak