Image Resolution, Pixels and DPI Explained
By the NextG Design team · Published 17 October 2026 · 3 min read
Quick answer: An image's real resolution is its pixel dimensions, such as 3000×2000. DPI (dots per inch) only matters when printing: it says how many pixels are packed into each printed inch. For sharp prints, aim for about 300 pixels per inch, so a 6×4 inch print needs roughly 1800×1200 pixels. Changing the DPI number without changing the pixels does not change quality on screen.
Pixels are the real resolution
A digital image is a grid of coloured squares called pixels. A photo described as 4000×3000 has 4,000 pixels across and 3,000 down — 12 million pixels, or 12 megapixels. That grid is all the detail the image contains. Everything else — how big it appears on screen, how big it prints — is just a decision about how large to show those pixels.
DPI vs PPI
PPI (pixels per inch) describes how densely an image's pixels are packed when it's printed or displayed. DPI (dots per inch) technically describes how many ink dots a printer lays down, but in everyday use the two terms are used interchangeably, and image software labels the setting "DPI".
The DPI value stored in an image file is just a note: "print me at this density". It doesn't change the pixels. A 3000×2000 image tagged at 72 DPI and the same image tagged at 300 DPI look identical on screen and contain exactly the same detail — the only difference is the size a printer will choose by default (41.7 inches wide versus 10 inches wide).
How many pixels do you need?
The formula is simple:
pixels needed = print size in inches × PPI
And in reverse, to find the largest good-quality print from a photo:
maximum print size in inches = pixels ÷ 300
For centimetres, divide by 2.54 first to get inches.
Pixels needed for common print sizes
| Print size | At 300 PPI (high quality) | At 200 PPI (acceptable) |
|---|---|---|
| 2×2 in (passport) | 600×600 | 400×400 |
| 6×4 in (10×15 cm) | 1800×1200 | 1200×800 |
| 7×5 in (13×18 cm) | 2100×1500 | 1400×1000 |
| 10×8 in (20×25 cm) | 3000×2400 | 2000×1600 |
| A4 (8.27×11.69 in) | 2480×3508 | 1654×2339 |
| A3 (11.69×16.54 in) | 3508×4961 | 2339×3307 |
| 18×12 in poster | 5400×3600 | 3600×2400 |
Viewing distance matters: a poster seen from across a room looks sharp at 150 PPI, while a photo held in your hand benefits from the full 300.
What about screens?
Screens display images pixel-for-pixel (adjusted by the device's scaling), so DPI is irrelevant online. What matters is whether the pixel width is enough for the space it fills. High-density "Retina" screens pack two or three device pixels into each layout pixel, so an image shown 800 pixels wide on a website looks crispest if it's about 1,600 pixels wide. Beyond that, extra pixels only make the file bigger and the page slower.
If you don't have enough pixels
- Find a bigger original. Images sent by messaging apps or downloaded from social media are often heavily downsized; ask for the original file.
- Re-scan at a higher resolution. For prints, scan at 600 DPI or more.
- Upscale carefully. The Image Upscaler can enlarge up to 400% with noise reduction and sharpening. 2× usually looks natural; it won't invent real detail. See our upscaling guide.
- Print smaller. Often the best answer is the honest one.
And if you have far too many pixels for a web page or email, reduce them with the Image Resizer.
Frequently asked questions
Does changing an image from 72 DPI to 300 DPI improve quality?
No. Changing only the DPI setting doesn't add any pixels or detail. It only changes the default print size. To print larger with good quality you need more pixels.
What resolution do I need to print a 6x4 photo?
About 1800×1200 pixels for a sharp print at 300 PPI. Around 1200×800 pixels is still acceptable for casual prints.
What resolution should images be for a website?
Size them to about twice the width they're displayed at to look sharp on high-density screens — for example around 1,600 pixels wide for an 800-pixel column — and then compress them.
What is the difference between DPI and PPI?
PPI is pixels per inch in an image; DPI is ink dots per inch from a printer. In everyday image editing, the terms are used to mean the same thing: how many image pixels go into each printed inch.
Try it free: the Image Resizer runs entirely in your browser — no upload, no account, no watermark.