How to Calculate PPI

PPI — pixels per inch — measures how densely pixels are packed on a display. It is the single number that best predicts whether a screen looks sharp or whether you can see individual pixels, and it is easy to calculate from two specifications every display lists: its resolution and its diagonal size.

The formula

PPI uses the diagonal, because that is how screen sizes are sold and because it captures both dimensions in one number:

1. Diagonal pixels = sqrt(width² + height²)
2. PPI = Diagonal pixels / Diagonal size in inches

That is all there is to it. No other specification is involved — not the panel type, not the refresh rate, not the brand.

Worked examples, step by step

Example 1: a 24-inch Full HD monitor

Resolution 1920 × 1080, diagonal 24 inches:

• Diagonal pixels = sqrt(1920² + 1080²) = sqrt(3,686,400 + 1,166,400) = sqrt(4,852,800) ≈ 2203 pixels
• PPI = 2203 / 24 ≈ 91.8 PPI

Example 2: the same resolution on a larger screen

The same 1920 × 1080 resolution on a 27-inch monitor:

• Diagonal pixels are still ≈ 2203 (the resolution did not change)
• PPI = 2203 / 27 ≈ 81.6 PPI

Same pixels, larger area, lower density. This is why a 27-inch Full HD monitor looks noticeably less sharp than a 24-inch one at the same desk distance, and why buying a larger monitor without increasing resolution is a trade-off, not a pure upgrade.

Example 3: a 15.6-inch laptop

The same 1920 × 1080 resolution on a 15.6-inch laptop:

• PPI = 2203 / 15.6 ≈ 141.2 PPI

This is why laptop text looks sharper than the same resolution on a large monitor: the pixels are packed much more densely, because the screen is smaller.

Example 4: a 4K monitor

A 3840 × 2160 monitor at 27 inches:

• Diagonal pixels = sqrt(3840² + 2160²) = sqrt(14,745,600 + 4,665,600) = sqrt(19,411,200) ≈ 4406 pixels
• PPI = 4406 / 27 ≈ 163.2 PPI

And the same 4K resolution at 32 inches: 4406 / 32 ≈ 137.7 PPI. Even at 32 inches, 4K is far denser than Full HD at 24 inches.

A reference table

The Monitor PPI Calculator and the comparison table on the PPI page cover the most common displays, so you usually do not need to do this arithmetic by hand at all.

What PPI means in practice

Higher PPI means smaller, sharper-looking pixels at the same viewing distance. Text edges look cleaner, fine lines hold together, and individual pixels disappear. But viewing distance matters just as much as PPI itself:

• A phone held close to your face needs very high PPI to look smooth — which is why phones reach 400+ PPI.
• A monitor at arm’s length looks sharp at much lower PPI — 100–160 PPI is typical for a good desk setup.
• A TV viewed from across a room can look perfectly sharp at 40–60 PPI, because the distance hides the pixels.

This is the same principle as print DPI and viewing distance: density only matters relative to how far away the viewer is. Beyond a certain point, extra PPI at a given distance produces no visible improvement — it just costs more in hardware and, for rendering, in graphics performance.

PPI vs resolution: which one should you compare?

Both, but for different questions:

• Resolution (1920 × 1080) tells you how much content fits on screen — how many windows, how much detail in a photo, how much workspace.
• PPI tells you how fine that content looks — whether text is crisp or pixelated.

A 27-inch 4K monitor and a 32-inch 4K monitor have the same resolution and the same workspace, but different PPI (163.2 vs 137.7), so text looks finer on the smaller one. A 24-inch Full HD monitor and a 27-inch Full HD monitor also share a resolution, but the larger one looks coarser (91.8 vs 81.6 PPI). When you shop, check both numbers: resolution for space, PPI for sharpness.

Common mistakes

• Using width instead of the diagonal. PPI is defined on the diagonal. Dividing pixel width by diagonal inches gives a wrong, inflated number.
• Confusing PPI with DPI. PPI is for screens; DPI is for print. They are related concepts but different measurements — see DPI vs PPI.
• Assuming a higher resolution setting changes PPI. Changing your computer’s resolution setting does not change the physical panel. It scales the image, usually making it blurrier, while the panel’s PPI stays exactly the same.
• Comparing PPI across very different viewing distances. A phone at 457 PPI and a monitor at 92 PPI are not in a contest — they are viewed from completely different distances, and both can look sharp in their own use.

Frequently asked questions

What is a good PPI for a monitor?

For desk use at arm’s length, most people find 90–110 PPI acceptable and 130–165 PPI comfortably sharp. Below about 80 PPI, individual pixels start to become noticeable in text. These are comfort ranges, not specifications — eyes and preferences vary.

Does PPI affect gaming performance?

Indirectly. Higher resolution at the same screen size means more pixels for the graphics card to render, which can lower frame rates. PPI itself does not cost performance; pixel count does. A 27-inch 4K monitor (163 PPI) renders over four times the pixels of a 27-inch Full HD monitor (82 PPI), and that is what taxes the GPU.

How do I find my display’s PPI?

Look up its resolution and diagonal size (the manufacturer’s specifications list both), then enter them in the PPI Calculator. If you only know the model name, the Monitor PPI Calculator covers common sizes.

Is there a PPI where more is pointless?

At any fixed viewing distance, yes — once pixels are smaller than the eye can resolve at that distance, further increases are invisible. That threshold is much higher for a phone held close than for a TV across a room, which is why “enough PPI” is always relative to distance.

Try it: PPI Calculator · Monitor PPI Calculator · Screen Size Calculator