Search for “300 DPI in pixels” and the literal answer is one line: 300 pixels per linear inch — 300 in each direction, so a square inch holds 90,000 of them. Everything else about the number — why it’s the standard, what it costs in megapixels, when it’s the wrong target — follows from a bit of geometry and a bit of printing history.

300 DPI, converted to pixels

The conversion is pure scaling, with 1 inch = 25.4 mm doing the metric work:

Span	Pixels at 300 DPI
1 inch	300 px
1 cm	≈ 118.1 px (300 ÷ 2.54)
10 mm	≈ 118.1 px
1 mm	≈ 11.8 px
Size of one pixel	1/300 in ≈ 0.0847 mm

That last row is worth pausing on: at 300 DPI each image pixel occupies a square ~0.085 mm across — about the width of a fine human hair, and roughly the smallest mark a good printing process can place repeatably.

Why 300 became the standard — two converging reasons

The eye’s limit. 20/20 vision resolves detail down to about one arc-minute. At a normal reading distance of 28–30 cm, one arc-minute spans:

300 mm × tan(1/60°) ≈ 300 × 0.00029 ≈ 0.087 mm

A pixel pitch of ~0.087 mm means ~290 pixels per inch — anything finer is detail the eye physically cannot retrieve at that distance. 300 is a tidy round number just above the limit.

The halftone rule. Offset presses fake continuous tone with a screen of variable-size dots, typically 150 lines per inch for magazine work. The old prepress rule of thumb is ~2 image pixels per halftone line, so 2 × 150 = 300 ppi. Not a sharpness threshold — a data ceiling. Past it, the press can’t use more.

Two independent derivations landing on the same number is why 300 stuck. It’s also why the number is a default, not a law: change the viewing distance or the content type and the right answer moves — the full sliding scale is in how much resolution print really needs.

The pixels every size needs at 300 DPI

pixels = inches × 300, per dimension:

Print size	Pixels at 300 DPI	Megapixels
Business card (3.5×2 in)	1050×600	0.6 MP
4×6 photo	1200×1800	2.2 MP
10×15 cm photo	1181×1772	2.1 MP
5×7 photo	1500×2100	3.2 MP
A5	1748×2480	4.3 MP
8×10 photo	2400×3000	7.2 MP
US Letter	2550×3300	8.4 MP
A4	2480×3508	8.7 MP
12×12 square	3600×3600	13.0 MP
11×14	3300×4200	13.9 MP
A3	3508×4961	17.4 MP
16×20	4800×6000	28.8 MP
24×36 poster	7200×10800	77.8 MP

Read the last row carefully: it exists to show when 300 is the wrong answer. A 24×36 poster demands 78 MP at 300 DPI — which is why nobody prints posters at 300. The calculator’s print-size tab runs this same arithmetic in both directions for arbitrary sizes and units.

When 150 is the better number

Anything viewed from arm’s length onward. The eye resolves angle, so doubling the viewing distance halves the density you need: a wall print seen from a metre is fully served by ~150 ppi, and a banner across a room by 60–100. Staying at 300 for these jobs doesn’t buy sharpness — it buys file size, slower RIP times and upload rejections from print portals. 150 is also a sane ceiling for proof prints and for content that is mostly flat colour or big type.

When 600 earns its keep

Two cases, and they’re really the same case: hard-edged content and capture.

• Print: line art, maps, sheet music, small type and fine-art reproduction. Halftone screens render a photo’s soft gradients at ~300 ppi, but a crisp vector edge benefits from 600 or even 1200 on processes that support it — book interiors and technical drawings live here.
• Scan: “600 DPI” on a scanner means 600 samples per inch of original — real data, not a target density. Archiving a 4×6 photo at 600 yields 2400×3600 px, leaving headroom to reprint at twice the size. (Why a scanner’s number is really PPI wearing a DPI label: PPI vs DPI.)

For ordinary photographs on ordinary paper, 600 halves your workflow speed for a difference no press can express.

The one-line version

300 DPI = 300 pixels per inch, because ~290 ppi is what an arc-minute-resolving eye can use at reading distance, and because 150-lpi halftones want ~2 pixels per line. Feed hand-held work 300, wall work 150, across-the-room work 60–100, and line art or archival scans 600 — and let pixels = inches × DPI do the rest. The print-size tab and the viewing-distance guide cover every case between.

Frequently asked questions

How many pixels is 300 DPI?

Per linear inch: exactly 300. Per centimetre: ~118.1 (300 ÷ 2.54). Per millimetre: ~11.8. So a one-inch-square patch holds 300 × 300 = 90,000 px, and an A4 sheet at 300 DPI needs 2480×3508 px. To go the other way — pixels ÷ 300 = inches — use the calculator's print-size tab.

Is 300 DPI the same as 300 PPI?

For an image file, effectively yes — the 'DPI' field in file metadata is really a pixels-per-inch figure wearing a printer's name. For a printer spec it's different: 300 dpi of ink dots is a coarser spray than 300 ppi of image data, because each pixel is painted by many dots. The full distinction is in PPI vs DPI.

How many megapixels do I need to print at 300 DPI?

Multiply each print dimension in inches by 300, then multiply the results: an 8×10 needs 2400×3000 = 7.2 MP, an A4 needs 8.7 MP, a 16×20 needs 28.8 MP. Any modern phone or camera covers everything up to about A3.

When is 150 DPI good enough?

Whenever the print is viewed from about a metre or more — wall prints, posters, banner stands. The eye resolves angle, not absolute density: 150 ppi at 1 m subtends the same arc as 300 ppi at 50 cm. The viewing-distance math is in how much resolution print really needs.

When should I use 600 DPI instead?

For content with hard edges the process can render — line art, small type, maps, fine-art reproduction — and for archival scanning, where 600 samples/inch of a 4×6 photo yields a real 2400×3600 px of data. For continuous-tone photos, 600 doubles the file for a gain the paper can't hold.

Does changing the DPI number in Photoshop add detail?

No — with 'resample' unticked it only rewrites a metadata label and changes the suggested print size; the pixel count is untouched. To actually gain pixels you must resample, which synthesizes detail rather than recovering it.