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concepts / resolution & dpi

What is Image and Video Resolution?

Pixels are the picture.
DPI is only a note about paper.

An image's resolution is its pixel count: 4032 × 3024 is all the detail it will ever have. The "72 dpi" or "300 dpi" stored beside it changes nothing on screen. It only tells a printer how big to make it. Change it below on a real JPEG and see which bytes move.

01 · ONE PHOTO, TWO LABELS

Same pixels, different printed size

1200 × 900 px · 23,806 bytes · encoded by this browser

Labelled 72 dpidownloadsunset-72dpi.jpg

larger than A4 (white sheet), same scale

prints at423 × 318 mm

on screen1200 × 900 px

FF0D81FF2E030041054A64674984690010011101120113001448150016481700180019

Labelled 300 dpidownloadsunset-300dpi.jpg

white: A4 sheet, same scale

prints at102 × 76 mm

on screen1200 × 900 px

FF0D81FF2E030041054A6467498469001001110112011301142C1501162C1700180019

The first 20 bytes of each file. Purple is the JFIF density field (unit + horizontal + vertical dpi). Amber marks the 4 bytes that differ between the two downloads. Everything else, including every compressed pixel after byte 20, is identical.both files: 23,806 bytes, 1,080,000 pixels

02 · WHAT THE PIXELS CAN FILL

How big can this image print?

What matters for print is pixels per inch of paper: the pixel count divided by the print size. 300 is the photo-lab standard at arm's length; posters seen from a metre away look fine at 150.

Phone photo4032 × 302424 MP camera6000 × 4000Instagram post1080 × 1350Full HD screenshot1920 × 1080Web image800 × 600This page’s JPEG1200 × 900 × px

A6105 × 148

692 ppiPhoto-lab sharp, even up close.

A5148 × 210

488 ppiPhoto-lab sharp, even up close.

A4210 × 297

345 ppiPhoto-lab sharp, even up close.

A3297 × 420

244 ppiSharp at normal viewing distance.

A2420 × 594

172 ppiFine for posters seen from a step back.

A1594 × 841

122 ppiSoft; pixels visible close up.

A0841 × 1189

86 ppiVisibly blocky. Too few pixels.

6 × 4 in102 × 152

672 ppiPhoto-lab sharp, even up close.

10 × 8 in203 × 254

378 ppiPhoto-lab sharp, even up close.

24 × 36 in610 × 914

112 ppiSoft; pixels visible close up.

4032 × 3024 px (12.2 MP) prints at most 256 × 341 mm at 300 ppi, 384 × 512 mm at 200 ppi, or 512 × 683 mm at 150 ppi. Orientation is matched to the paper; the coloured block shows the image filling it.

03 · MAKING IT BIGGER

Raising the pixel count can't add detail

A 40 × 40 pixel icon enlarged by your browser: copying pixels gives blocks, smoothing gives blur. The vector version is redrawn from its shapes at every size, so it stays sharp. That is the real difference between raster (JPG, PNG) and vector (SVG, PDF text).

zoom 4×

Raster, pixels copiedNearest neighbour: each pixel becomes a 4×4 block. Honest, but jagged.

Raster, smoothedBrowser’s high-quality resampling blends neighbours. Edges turn soft; detail that was never there stays missing.

Vector, redrawnSame circle, letter and line, drawn again from their shapes at 160 px. Sharp at any size.

04 · AND ON SCREEN?

Your screen ignores the dpi label entirely

Browsers lay pages out in CSS pixels, defined as 1/96 inch at arm's length, then map each one to however many real pixels the display has. That ratio is what makes phones sharp. The bar on the right is a nominal CSS inch; hold a ruler to it and it won't be one.

one CSS inch (96 CSS px)

device pixel ratio1.00real pixels per CSS pixel

screen in CSS px800 × 600what pages lay out against

screen in real pixels800 × 600what the panel actually has

this window1440 × 1000CSS px

high-density media querybelow 2dppxused to pick @2x images

1200 px image here1200 CSS px wideat 1:1 real pixels

Where each format keeps its dpi note

formatfielddefault if missingnotes

JPEGJFIF APP0 units + Xdensity/Ydensity (bytes 13-17)aspect onlyUnits 0 means "no dpi, just pixel shape". EXIF XResolution may also be present and can disagree.

JPEGEXIF XResolution / YResolution72Cameras write 72 or 300 by habit. Photoshop prefers EXIF when both exist.

PNGpHYs chunk, pixels per metrenoneStored per metre: 300 dpi is written as 11,811. Many encoders omit it.

TIFFXResolution, ResolutionUnit (tags 282, 296)72Rational numbers; unit 2 is inch, 3 is centimetre.

WebP / AVIFonly via embedded EXIFnoneNo native field. Most tools assume 72 or 96.

PDFpage size in points (1/72 in)n/aImages are placed at a size; effective ppi = pixels ÷ placed inches.

SVGwidth/height in mm, in, pt96 per inVector, so there’s no pixel count; a CSS inch is 96 px.

Sources: JFIF 1.02 (ITU-T T.871) · PNG 3rd ed. pHYs · TIFF 6.0 XResolution/ResolutionUnit · EXIF 2.32 · CSS Values and Units Level 4 (reference pixel) · ISO 216 paper sizes

photo_size_select_largeImage resizerChange the pixel count, with a choice of resampling filter.4032 px → 1200 px picture_as_pdfImage to PDFPlace photos on A4 or Letter pages at a known size.scan.jpg → scan.pdf screenshot_monitorScreen size calculatorPixel density and viewing distance for any display.27 in 4K → 163 ppi gradientBit depthThe other kind of resolution: how many shades per pixel.8-bit → 10-bit

Frequently Asked Questions

What does 4K mean?

4K refers to a horizontal resolution of approximately 4000 pixels. The standard 4K resolution is 3840x2160 pixels (UHD).

Is higher resolution always better?

Higher resolution means more detail but requires more storage and processing power. Choose resolution based on where content will be displayed.

What resolution should I shoot video at?

For most uses, 1080p is excellent. Use 4K if you plan to crop, stabilize, or future-proof your footage.

Does resolution affect file size?

Yes, directly. Doubling resolution quadruples the number of pixels and roughly quadruples the file size.

What is DPI?

DPI (dots per inch) measures resolution for printed images. 72 DPI is screen resolution. 300 DPI is standard for print quality.