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Pixel Conversion Cheat Sheet — px to mm/cm/inch at Every Common DPI

Screen Ruler TeamApril 28, 202610 min read
pixel conversion cheat sheetpx to mm tablepixel to inch tabledpi conversion chart

A practical reference for designers, developers, photographers, and anyone moving content between digital pixels and physical print. Copy these tables, bookmark this page, or open the Pixel Converter when you need a value that's not in the lookup.

The tables below assume the standard 1 inch = 25.4 mm and pixels = inches × DPI. If your medium uses a non-listed DPI, plug the number into the converter's Custom DPI option for an instant answer.

1. Pixels to millimeters at common DPI

The most-asked conversion. How many millimeters does a given pixel count cover at a given output resolution?

Pixels 72 DPI 96 DPI (web) 150 DPI 300 DPI (print) 600 DPI
1 0.35 0.26 0.17 0.085 0.042
10 3.53 2.65 1.69 0.85 0.42
50 17.64 13.23 8.47 4.23 2.12
100 35.28 26.46 16.93 8.47 4.23
250 88.19 66.15 42.33 21.17 10.58
500 176.39 132.29 84.67 42.33 21.17
800 282.22 211.67 135.47 67.73 33.87
1000 352.78 264.58 169.33 84.67 42.33
1920 677.33 508.00 325.12 162.56 81.28
2400 846.67 635.00 406.40 203.20 101.60

Reading the table: at 96 DPI (CSS web pixels), 100 pixels is about 26 mm — roughly the width of a US quarter. At 300 DPI for print, the same 100 pixels is only about 8 mm — about the width of a pinky fingernail. Same pixel count, 3× smaller print because the dots are packed denser.

2. Pixels to inches at common DPI

The same conversions in inches, for US/UK print specs and product packaging.

Pixels 72 DPI 96 DPI 150 DPI 300 DPI 600 DPI
72 1.0000 0.7500 0.4800 0.2400 0.1200
96 1.3333 1.0000 0.6400 0.3200 0.1600
150 2.0833 1.5625 1.0000 0.5000 0.2500
300 4.1667 3.1250 2.0000 1.0000 0.5000
600 8.3333 6.2500 4.0000 2.0000 1.0000
1024 14.2222 10.6667 6.8267 3.4133 1.7067
1920 26.6667 20.0000 12.8000 6.4000 3.2000
3000 41.6667 31.2500 20.0000 10.0000 5.0000
6000 83.3333 62.5000 40.0000 20.0000 10.0000

Diagonal pattern: where the pixel count equals the DPI, you get exactly 1 inch — that's the definition of DPI. 96 px @ 96 DPI = 1 inch; 300 px @ 300 DPI = 1 inch.

3. Millimeters to pixels — print-spec targets

For when the printer's spec is in millimeters and you need to know what pixel size to source.

Target size (mm) 150 DPI 300 DPI 600 DPI
10 59 118 236
25 148 295 591
50 295 591 1181
100 591 1181 2362
150 886 1772 3543
210 1240 2480 4961
297 1754 3508 7016

Why 210 and 297: those are the short and long edges of A4 paper. So a full-page A4 print at 300 DPI needs 2480 × 3508 px — about 8.7 megapixels of source data. Most modern phone cameras have you covered.

4. Inches to pixels — common print sizes

Standard photo and print product sizes converted at the typical 300 DPI press standard.

Size (inches) Pixels (300 DPI)
4 × 6 (postcard) 1200 × 1800
5 × 7 1500 × 2100
8 × 10 2400 × 3000
8.5 × 11 (US Letter) 2550 × 3300
11 × 14 3300 × 4200
13 × 19 3900 × 5700
16 × 20 4800 × 6000
20 × 30 6000 × 9000
24 × 36 (poster) 7200 × 10800
30 × 40 9000 × 12000

Camera reality check: a 24 MP camera (6000 × 4000 px) prints natively at 300 DPI up to about 20 × 13.3 inches. Beyond that, you're upsampling — acceptable for prints viewed from a distance, but not for sharp gallery work.

5. Common screen resolutions in physical units (96 DPI)

For when you're comparing browser viewport sizes against real-world distances.

Resolution Width (mm) Width (inches) Use case
320 px 84.67 3.33 Old phone portrait viewport (CSS reference)
375 px 99.22 3.91 iPhone X-style portrait viewport
768 px 203.20 8.00 iPad portrait viewport
1024 px 270.93 10.67 iPad landscape / older laptop
1280 px 338.67 13.33 Common laptop
1440 px 381.00 15.00 Larger laptop
1920 px 508.00 20.00 Full HD desktop
2560 px 677.33 26.67 QHD monitor
3840 px 1016.00 40.00 4K monitor (at 100% scaling)

Display scaling caveat: a 4K monitor at 200% OS scaling shows 3840 px as visually equivalent to 1920 px on a 1080p display — so the physical width to a user is closer to 50 cm, not 100. CSS pixels and the 96 DPI assumption already handle this; the table is what the bare math says.

6. Device PPI quick reference (real-world physical density)

When you measure a real device with a physical ruler, the effective PPI is much higher than 96. This is what device makers actually pack onto the screen — useful when calibrating physical 1:1 measurement on a specific screen.

Device Resolution Physical size Real PPI
iPhone 16 Pro 2622 × 1206 6.3 inch diag 460
iPhone 15 / 14 2556 × 1179 6.1 inch diag 460
iPhone SE (3rd gen) 1334 × 750 4.7 inch diag 326
Samsung Galaxy S24 Ultra 3120 × 1440 6.8 inch diag 505
iPad Pro 12.9 (M4) 2752 × 2064 13 inch diag 264
iPad Air 2360 × 1640 11 inch diag 264
MacBook Pro 14 (M4) 3024 × 1964 14.2 inch diag 254
MacBook Air 13 (M3) 2560 × 1664 13.6 inch diag 224
Dell UltraSharp 27" 4K 3840 × 2160 27 inch diag 163
24" 1080p monitor 1920 × 1080 24 inch diag 92
27" 1440p monitor 2560 × 1440 27 inch diag 109

Why this matters: when CSS asks for 1in of width, the browser draws 96 CSS pixels regardless of the real PPI. On a 460-PPI iPhone, that's roughly 460/96 ≈ 4.8 physical pixels per CSS pixel. The HiDPI scaling handles itself; you don't need to do that math by hand. But if you're calibrating a screen to display physical 1:1 measurements, you do need the real PPI — that's where the Device Specs Database and Screen Ruler come in.

7. CSS units cheat sheet (web only)

For web work, several CSS length units are equivalent at 96 DPI:

CSS unit Pixels (96 DPI) Notes
1px 1 The CSS reference pixel
1pt 1.333 Typographic point (used in print CSS)
1pc 16 12 points
1in 96 Inch
1cm 37.795 Centimeter
1mm 3.7795 Millimeter
1em varies Equal to current font-size — defaults to 16 px
1rem varies Equal to root font-size — defaults to 16 px

Why this rarely comes up: most CSS in production is in px, rem, or %. The physical units (mm, cm, in) are mostly for print stylesheets (@media print). When you do see them, the conversion is purely the table above.

8. DPI for common workflows

A flat list of "what DPI should I use" for the most common output mediums.

Workflow DPI Justification
Web design (CSS, HTML, app UI) 96 Browser/OS standard. Always.
Mobile app design 96 Same as web — Retina is handled separately as @2x/@3x assets
Print proofs, A2/A1 large-format 150 Detail is fine at viewing distance
Magazines, books, brochures 300 Industry standard for press output
Photo prints (4×6 to 16×20) 300 Standard photo lab default
Photo prints (gallery, fine art) 300–600 Higher for close inspection
Line art, technical drawings 600 Crisp edges at any zoom
Screen-printed apparel 300 DPI doesn't matter as much; vector preferred
Billboard / outdoor 30–50 Viewing distance is far; pixels can be large
Architectural drawings 600 Fine line work
LCD signage 96 or device-specific Treat as a screen, not print

9. Quick mental shortcuts

For when you don't have the converter open:

  • 96 px ≈ 1 inch. Half it for half-inch (48 px), double for 2 inches (192 px).
  • 1 mm ≈ 4 px at 96 DPI (technically 3.78). Useful for sanity-checking margins.
  • 1 cm = 37 or 38 px at 96 DPI. Same trick scaled up.
  • At 300 DPI, 1 mm ≈ 12 px. So a 50 mm logo is ~600 px wide for high-quality print.
  • A4 paper at 300 DPI is roughly 2500 × 3500 px — anchor that and scale up/down from there.
  • 6000 px is "20 inches at 300 DPI" — the ceiling of a 24 MP photo's native print size.

10. When the cheat sheet isn't enough

These tables cover the 95% case. For the other 5% — non-standard DPI, exotic paper sizes, kiosk screens at unusual physical sizes — open the Pixel Converter and use the Custom DPI option to enter the value you need. The math is the same; only the constants change.

And when you need to verify against a physical object on your specific screen — bookmark the Screen Ruler so you can hold a credit card up to your monitor and confirm the calibration.

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