How do you calculate PPI from resolution and screen size?

Take the diagonal pixel count with Pythagoras — √(width² + height²) — then divide by the diagonal size in inches. For a 1920×1080 screen at 24 inches: √(1920² + 1080²) = 2202.9 pixels, ÷ 24 = 91.8 PPI. That single number is the density every other metric derives from.

What is a good PPI for a monitor?

For a desk monitor viewed at roughly 60 cm, anything from about 90 PPI (a 24-inch 1080p panel) is usable, 109 PPI (27-inch 1440p) is comfortable, and 160+ PPI (27-inch 4K) looks Retina-sharp with no visible pixels. Higher PPI mainly helps text crispness; past ~220 PPI at desk distance the gains are hard to see.

What PPI counts as a Retina display?

There is no fixed PPI — Retina depends on viewing distance. Apple's rule is that pixels stop being individually resolvable once the display shows at least 60 pixels per degree of your vision (the 20/20 acuity limit). A phone at 460 PPI held 30 cm away easily clears it; a 4K TV at 80 PPI clears it too because you sit metres back.

What is the difference between PPI and DPI?

PPI (pixels per inch) describes a screen's pixel density. DPI (dots per inch) describes a printer's ink dots. They're often used interchangeably online, and this tool computes screen PPI, but strictly DPI is a printing term. If someone types "DPI" for a monitor, they almost always mean PPI.

How many megapixels is a 4K screen?

A 3840×2160 "4K UHD" screen is 3840 × 2160 = 8,294,400 pixels, i.e. 8.29 megapixels. True DCI 4K (4096×2160) is 8.85 MP. Note this is the panel's pixel count, not a camera sensor — it's the same number regardless of screen size, only the density (PPI) changes.

What is dot pitch and why does it matter?

Dot pitch is the physical width of a single pixel, in millimetres: 25.4 ÷ PPI. A 24-inch 1080p monitor has a 0.277 mm dot pitch; a 27-inch 4K panel drops to 0.156 mm. Smaller dot pitch means finer detail and smoother text edges at the same viewing distance.

Does higher PPI always look better?

Only up to the point your eyes can resolve. Once a screen exceeds 60 pixels per degree at your normal viewing distance, extra density is invisible — it just costs more GPU power and battery. This tool shows your pixels-per-degree so you can tell whether more resolution would actually be noticeable.

Can I enter the diagonal in centimetres?

Yes. Toggle the diagonal (and viewing distance) unit to cm and enter the value in centimetres — the tool converts internally at 2.54 cm per inch. A 68.58 cm diagonal is the same as 27 inches and returns identical PPI, so use whichever unit your spec sheet quotes.

Utility · Displays

PPI & Pixel Density Calculator

Enter a resolution and screen size to get exact pixel density (PPI), dot pitch, physical dimensions, megapixels, and whether the display is Retina-sharp at your viewing distance. Compare any phone, laptop, monitor or TV before you buy. No signup, sources cited below.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 11, 2026

Pixel density & PPI

Screen resolution (pixels)

Horizontal (width)

Vertical (height)

Diagonal size

The screen's corner-to-corner size, as advertised.

Viewing distance

How far your eyes sit from the screen.

Presets

Pixel density

91.8 PPI

36.1 pixels per cm

Dot pitch

0.2767 mm

Physical size of one pixel

Screen size

20.9″ × 11.8″

53.1 × 29.9 cm

Megapixels

2.07 MP

2,073,600 px · 16:9

Pixels may be visible here

You'd need to sit at least 37.5 in away for this screen to look Retina-sharp.

38.4

pixels / degree

Compared with common screens

Device	PPI	vs yours
iPhone 15Phone	461	Sharper
iPhone 15 Pro MaxPhone	460	Sharper
Galaxy S24Phone	416	Sharper
iPad 10.9"Tablet	264	Sharper
MacBook Air 13"Laptop	225	Sharper
15.6" laptop (FHD)Laptop	141	Sharper
24" monitor (1080p)Monitor	92	About equal
27" monitor (1440p)Monitor	109	Sharper
27" monitor (4K)Monitor	163	Sharper
32" monitor (4K)Monitor	138	Sharper
55" TV (4K)TV	80	Softer

Preset PPIs are derived from each device's published resolution and diagonal (Apple, Samsung, Dell, LG spec sheets). PPI is quoted on the standard whole-pixel basis, as manufacturers do.

Sources: geometric PPI definition (VESA display metrology), Apple Retina / 20-20 acuity (60 px per degree). Full citations below.

How it works

Pixel density is a fixed geometric identity, not a matter of opinion. A screen's sharpness comes from how many pixels are packed into each physical inch, and that follows directly from Pythagoras.

Diagonal pixel count. The diagonal spans dp = √(width² + height²) pixels. For 1920×1080 that is √(1920² + 1080²) = 2202.9 pixels.
PPI. Divide the diagonal pixels by the diagonal size in inches: PPI = dp ÷ diagonal_inches. Pixels per centimetre is simply PPI ÷ 2.54. If you enter the diagonal in centimetres, it is converted at 2.54 cm per inch first.
Dot pitch.One pixel's physical width in millimetres is 25.4 ÷ PPI (there are 25.4 mm in an inch).
Physical size & megapixels. Width is width_px ÷ PPI inches (height likewise), and megapixels is width × height ÷ 1,000,000.
Pixels per degree (PPD). At a viewing distance d inches, one degree of vision subtends d × (π ÷ 180) inches of screen, so PPD = PPI × d × (π ÷ 180).
Retina verdict. Human 20/20 acuity resolves about one arc-minute, i.e. 60pixels per degree — the basis Apple uses for “Retina”. When PPD ≥ 60 the screen is Retina-sharp where you sit; otherwise the tool reports the minimum distance 60 ÷ (PPI × π ÷ 180) at which it becomes sharp.

Each PPI figure is cross-checked by a second, independent route: the physical width equals diagonal × cos θ, where θ = atan2(height, width), so PPI = width_px ÷ physical_width. That trigonometric path is algebraically identical to the diagonal formula — when both agree to the pixel, the result is verified. Every preset in the comparison table computes its own PPI the same way, reconciled to the manufacturer's published density.

Worked examples

24″ Full-HD monitor — 1920×1080 at 24 in

Diagonal pixels: √(1920² + 1080²) = √4,852,800 = 2202.9 px
PPI: 2202.9 ÷ 24 = 91.8 PPI
Dot pitch: 25.4 ÷ 91.8 = 0.277 mm
Megapixels: 1920 × 1080 ÷ 1,000,000 = 2.07 MP
PPD at 24 in: 91.8 × 24 × (π/180) = 38.4 → below 60
Verdict: not Retina at 24 in; sharp beyond 60 ÷ (91.8 × π/180) = 37.4 in

iPhone 15 — 2556×1179 at 6.1 in (fractional diagonal)

Diagonal pixels: √(2556² + 1179²) = √7,923,177 = 2814.8 px
PPI: 2814.8 ÷ 6.1 = 461.4 PPI (Apple publishes 460 ppi — matches)
Dot pitch: 25.4 ÷ 461.4 = 0.055 mm
PPD at 12 in: 461.4 × 12 × (π/180) = 96.6 → above 60
Verdict: Retina-sharp at normal phone distance

27″ 4K monitor — 3840×2160 at 27 in

Diagonal pixels: √(3840² + 2160²) = √19,411,200 = 4405.8 px
PPI: 4405.8 ÷ 27 = 163.2 PPI
Dot pitch: 25.4 ÷ 163.2 = 0.156 mm
Megapixels: 3840 × 2160 ÷ 1,000,000 = 8.29 MP
PPD at 24 in: 163.2 × 24 × (π/180) = 68.3 → above 60
Verdict: Retina-sharp at a 24 in (≈60 cm) desk distance

Frequently asked questions

Sources & references

The formulas and the Retina acuity constant on this page were last reconciled against these sources on 2026-07-11. Preset densities are checked against current Apple, Samsung, Dell and LG spec sheets.

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