Pixels, DPI and Resolution: What Actually Matters
Ask five people what “high resolution” means and you will get five answers. Most of the confusion comes from mixing up three separate ideas: how many pixels an image has, how tightly those pixels are packed when it is printed (DPI), and how big the file is. Once they are apart, sizing an image for any purpose takes one line of arithmetic.
By the CrispPic team4 min read
Pixels are the only thing a screen cares about
A digital image is a grid of coloured squares. A 1920 × 1080 image has 2,073,600 of them, and a screen shows them at one image pixel per screen pixel — or more on high-density displays. The DPI number stored in the file is ignored on a screen: a 1200 × 800 image tagged 72 DPI and the same image tagged 300 DPI look identical on a website.
DPI and PPI only matter when you print
DPI means “dots per inch” on paper, and PPI (“pixels per inch”) is the same idea measured on the image. Together they decide how large a print will be: print size = pixels ÷ DPI. About 300 DPI is the usual standard for a sharp print viewed close up; large posters seen from a distance are fine at much less.
The formula you need
pixels = inches × DPI
inches = millimetres ÷ 25.4| Print size | Pixels needed at 300 DPI |
|---|---|
| 4 × 6 in | 1200 × 1800 |
| 5 × 7 in | 1500 × 2100 |
| 8 × 10 in | 2400 × 3000 |
| A4 (8.27 × 11.69 in) | 2480 × 3508 |
| 35 × 45 mm passport photo | 413 × 531 |
What megapixels tell you
Megapixels are just width × height ÷ 1,000,000. A 4000 × 3000 photo is 12 MP, and at 300 DPI it prints about 13 × 10 inches. More megapixels give you more room to crop and print large, not automatically a better picture. A 4K screen is only about 8.3 MP, so anything beyond that is never fully shown on one.
Aspect ratio: the shape matters as much as the size
- 1:1 — square. Profile pictures and classic social posts.
- 4:5 — tall portrait, popular for social feeds.
- 4:3 and 3:2 — most cameras, tablets and 35 mm-style prints.
- 16:9 — video, thumbnails and widescreen displays.
- 9:16 — vertical full-screen Stories and Reels.
Changing the ratio means cropping. Resizing with the aspect ratio locked keeps the shape and just changes the size, which is why a form that wants an exact width and height often needs a crop first and a resize second.
Why enlarging doesn’t add detail
Scaling an image up has to invent new pixels, and it does so by blending their neighbours. A 400-pixel photo stretched to 1080 looks soft because no new information exists to fill it. Start from the largest original you have and scale down; the reverse never works.
A practical checklist
- For a website or social post: think in pixels only, and ignore DPI.
- For a print: work out the pixels with the formula above, and check the original has at least that many.
- For an upload form: follow the pixel and file-size rule it states, exactly.
- Always: keep the original, and export sized copies from it.
Frequently asked questions
Do I need 300 DPI for images on the web?
No. DPI has no effect on how an image displays on a screen. What matters is the pixel width and height.
What is the difference between DPI and PPI?
PPI counts pixels per inch of an image or screen; DPI counts printed dots per inch. In everyday use the two terms are used interchangeably.
Does changing the DPI make a file smaller?
Not meaningfully. DPI is a small note stored in the file. Size comes from the number of pixels and the compression, so to shrink a file you change those.
How many pixels do I need for a passport photo?
A 35 × 45 mm print at 300 DPI is 413 × 531 pixels. Online portals set their own rules though, so follow the one you are using — see the passport and visa photo guide.
Try it with the right settings already applied
These pages open the CrispPic tool pre-configured for this exact case. Your image is processed in your own browser — it is never uploaded anywhere.
Free, no account, no watermark.
Keep reading
- How to Reduce Image Size Without Losing QualityWhy photos are so big, and the four steps that shrink them by 70–90% with no visible difference: resize, pick the right format, set a sensible quality, trim the rest.
- How to Prepare a Passport or Visa Photo for Online UploadDigital passport and visa photos have strict pixel and file-size rules. What the US DS-160 and UK passport portals ask for, and how to meet them step by step.
- How to Make Website Images Load Faster (Without Blur)Images are usually the heaviest part of a web page. The sizes, formats and HTML settings that speed them up and improve Core Web Vitals without visible quality loss.