DPI explained — what resolution you actually need

DPI only means something once a physical size is involved. An image on its own has pixel dimensions — 1200 × 800 — and nothing else. The dpi value is the bridge between those pixels and centimetres of paper, and it is stored in the file as a label that print software reads and browsers ignore.

pixels = physical size in inches × dpi

A 6 × 4 inch print at 300 dpi needs 1800 × 1200 pixels. The same file printed at 150 dpi covers 12 × 8 inches, at half the sharpness. Nothing about the file changed; only the size it is spread over.

The quickest route here: Resize image sets the pixel dimensions the calculation gives you, and PDF to JPG lets you pick a dpi directly when a printer asks for a page at a specific resolution.

DPI, PPI and why the two words get mixed up

Strictly, dpi is dots per inch — a property of a printer, which lays down ink dots and may use several of them to make one colour pixel. PPI is pixels per inch, a property of an image or a screen. In everyday use, form fields, scanner dialogs and photo software all say "dpi" when they mean ppi, and so does this guide from here on. It only matters when a print shop talks about their machine's dpi, which is usually a much higher number than the resolution they want from you.

On a screen, dpi is irrelevant

This is the part that causes the most confusion. A screen displays pixels, so what matters is the pixel dimensions, not the dpi tag stored in the file. Saving an image "at 72 dpi" for the web does nothing at all: browsers ignore the value entirely. The number itself is a leftover from early Macintosh screens, which really did show about 72 pixels per inch. CSS since fixed its own reference at 96 px per inch, which is a unit of measurement rather than a description of your monitor.

What actually matters for the web:

Use	Sensible width
Full-width banner	1920 px
Article illustration	1200 px
Thumbnail	400–600 px
Logo in a header	2× its displayed size

The "2×" for logos and icons is the one place where screen density comes back. High-density displays render two or three physical pixels for each CSS pixel, so a logo shown at 150 px looks crisper if the file is 300 px wide. Beyond 2× the gain is hard to see and the extra weight is real.

For print, 300 dpi is the usual target

Commercial printing generally asks for 300 dpi, which is roughly where the eye stops resolving individual dots at reading distance. You can go lower as the viewing distance grows: 150 dpi is fine for a poster read from two metres, and large-format banners are often produced at 72 to 100 dpi because nobody stands close to them.

Format	Pixels at 300 dpi
A3 (297 × 420 mm)	3508 × 4961
A4 (210 × 297 mm)	2480 × 3508
A5 (148 × 210 mm)	1748 × 2480
A6 (105 × 148 mm)	1240 × 1748
10 × 15 cm photo	1181 × 1772
4 × 6 in photo	1200 × 1800
Business card (85 × 55 mm)	1004 × 650

Those figures round to the nearest pixel; a printer's template may differ by one or two, plus whatever bleed it asks for.

Raising the dpi does not add detail

Changing a file's dpi tag without changing its pixels only changes the size it will print at. In a JPEG the value lives in a small density field in the header; in a PNG it is an optional pHYs chunk. Editing it moves no pixels.

Software that "upscales to 300 dpi" is doing something different: it invents the extra pixels by interpolation. The file gets bigger, the picture does not get sharper, and enlarging past about 150% usually looks worse than the original at its own size. If a printer says your file is too low-resolution, the only real fix is a larger original — reshoot it, rescan at a higher setting, or use the vector version if one exists.

The dpi tag does have one visible effect: Word, Pages and similar programs read it to decide how big to place an image when you insert it. A 3000 px photo tagged 300 dpi arrives as a 10-inch picture; the same file tagged 72 dpi arrives absurdly large. The pixels are identical in both cases.

Scanning: what setting to choose

• 300 dpi — text documents, contracts, anything that will go through OCR.
• 600 dpi — small originals you will enlarge, fine line art, or photographs you want to reprint larger.
• 150 dpi — documents you only need to read on screen; the files stay much smaller.

A colour A4 page at 600 dpi produces roughly four times the data of the same page at 300 dpi, because doubling the resolution quadruples the pixel count. That is why scanned PDFs get so heavy so quickly. Resize image brings an over-scanned image back to a sensible size, and Compress image handles the rest.

What the built-in apps can do

• Windows Photos and Paint resize in pixels or percent only; neither exposes a dpi field. The Windows Fax and Scan and manufacturer scanner apps do, at scan time.
• macOS Preview has both, in Tools → Adjust Size: a Resolution field in pixels/inch and a "Resample image" checkbox. With resampling off, changing the resolution only changes the print size; with it on, pixels are added or removed.
• iPhone Photos has no concept of dpi. Photos are pixel dimensions only, and what a print service does with them is its own calculation.
• Android Google Photos is the same: no resolution setting, no print size.

Questions people ask

How do I make an image 300 dpi? Decide the printed size first, multiply by 300, then resize to those pixels. Setting the tag alone changes nothing you can see.

Is 72 dpi enough for a website? The question does not apply. Match the pixel width to the space the image occupies on the page, and ignore dpi.

Does dpi affect file size? Only indirectly. Pixels cost bytes; the tag costs a few.

A form asks for 200 dpi — what do I enter? Work out the pixels for the stated physical size and resize to them. Portals check pixel dimensions far more often than the tag.

How do I turn a PDF page into an image at a set resolution? Use PDF to JPG, or PDF to PNG for pages of sharp text and line art, and choose the dpi before converting.

Next steps

• Hit exact pixel dimensions: Resize image
• Convert a page at a chosen resolution: PDF to JPG
• Prepare an identity photo from a millimetre requirement: How to prepare a photo for an online application form
• Deal with the size an over-scanned file ends up at: Your file is too big for the upload form