Complete Guide to Resolution and DPI
When converting a PDF to an image, the most common question is "What DPI should I use?" Your DPI choice directly affects the output image's quality and file size. This guide covers everything from basic concepts to practical application.
What Is DPI?
DPI stands for Dots Per Inch. It describes how many pixel dots exist within one inch (2.54 cm) of length. Higher DPI means richer detail in the image, but also larger file sizes.
A common confusion is between DPI and PPI (Pixels Per Inch). Strictly speaking, DPI refers to print while PPI refers to screens, but in everyday use the terms are often used interchangeably.
DPI and Image Dimensions
Using a standard A4-sized PDF page (8.27 x 11.69 inches) as an example:
| DPI | Output Width | Output Height | Approx. File Size |
|---|---|---|---|
| 72 DPI | 595 px | 842 px | ~100 KB |
| 150 DPI | 1240 px | 1754 px | ~400 KB |
| 200 DPI | 1654 px | 2338 px | ~700 KB |
| 300 DPI | 2481 px | 3507 px | ~1.5 MB |
| 600 DPI | 4962 px | 7014 px | ~5 MB |
Key Takeaway: Doubling the DPI quadruples the pixel count (both width and height double), and file size increases proportionally. Choose DPI based on the balance between quality and file size your use case requires.
DPI Recommendations by Use Case
Screen Display (72-150 DPI)
Traditional screens have a resolution of 72-96 DPI. Even for Retina displays, 150 DPI images are sufficient to render crisp results. For screen-only use, there is no need to exceed 150 DPI.
General Printing (150-300 DPI)
Office document printing typically requires only 150-200 DPI. For higher-quality prints (such as marketing materials), 300 DPI is recommended. The ISO standard recommends 300 DPI as the minimum for high-quality printing.
Professional Printing (300-600 DPI)
Books, magazines, posters, and other professional print materials require 300 DPI or above. Documents with fine lines or small text may need 600 DPI.
Retina Screens and HiDPI
Apple's Retina displays use 2x pixel density, meaning 1 CSS pixel consists of 4 physical pixels. Google's Material Design guidelines also account for different pixel density levels (mdpi, hdpi, xhdpi, xxhdpi).
On Retina screens:
- 72 DPI images may appear blurry
- 150 DPI images are sharp enough for most cases
- 300 DPI images look crisp on any screen
DPI Selection Strategy for PDF Conversion
- Determine your purpose — Screen display, office printing, or professional printing?
- Consider file size — Higher DPI means larger files; factor in storage and transfer costs
- Check source PDF quality — If images within the PDF are already low-resolution, high-DPI conversion cannot improve them
- Test and compare — Convert a single page at several different DPI settings to find the optimal one
Conclusion
Higher DPI is not always better — it should be chosen based on actual usage. Use 150 DPI for screens, 200-300 DPI for general printing, and 300+ DPI for professional printing. With a solid understanding of DPI concepts, you can make the optimal choice every time you convert a PDF to an image.
References
- Apple. "Human Interface Guidelines: Image Size and Resolution." Apple Developer, 2024. https://developer.apple.com/design/human-interface-guidelines/images
- Google. "Material Design: Pixel Density." Material Design, 2024. https://m3.material.io/foundations/layout/understanding-layout/overview
- ISO. "ISO 12647-2:2013 — Graphic technology — Process control for the production of half-tone colour separations." International Organization for Standardization, 2013. https://www.iso.org/standard/57470.html