PPI & Screen Size Calculator

How this is calculated

Pixels per inch is the diagonal pixel count divided by the diagonal in inches. The diagonal pixel count comes from Pythagoras: the square root of the horizontal resolution squared plus the vertical resolution squared. For 2560 by 1440 that is about 2937 pixels across the diagonal, and on a 27 inch panel that gives roughly 108.8 PPI. The physical width and height come from the same triangle, scaled: the diagonal in inches multiplied by each resolution divided by the diagonal pixel count. This works because the pixels are square on every mainstream display, so the pixel grid and the physical panel are the same shape. Dot pitch is the flip side - the distance between the centres of neighbouring pixels, 25.4 millimetres divided by the PPI. Monitor specifications often quote dot pitch instead of PPI, and smaller is sharper, which is the opposite direction from PPI. The point of all this is that RESOLUTION ALONE MEANS NOTHING. A 4K panel is only sharper than a 1440p one if it is not too much bigger: 3840 by 2160 at 32 inches is about 138 PPI, while 2560 by 1440 at 27 inches is about 109 - but the same 4K resolution stretched to 43 inches drops to about 102, which is LESS dense than the 1440p monitor. The number on the box is a pixel count, not a sharpness. Viewing distance matters just as much and is not modelled here, because it depends entirely on you. A phone held at 30 centimetres needs several times the density of a monitor at arm's length, which in turn needs far more than a television across a room. That is why a 55 inch 4K television at about 80 PPI looks perfectly sharp on the sofa and unusable as a desk monitor. This assumes square pixels and a flat rectangular panel, which covers essentially all monitors, phones and televisions. It does not account for subpixel arrangements, which is why two panels at identical PPI can render text with visibly different quality.

Common questions

Is a 4K screen always sharper than a 1440p one?

No - it depends entirely on the size. 3840 by 2160 at 32 inches gives about 138 PPI, comfortably sharper than 2560 by 1440 at 27 inches at about 109. But stretch that same 4K resolution to 43 inches and it falls to about 102 PPI, which is LESS dense than the smaller 1440p monitor. Resolution is a pixel count; sharpness is how thinly those pixels are spread, and only the second one is visible.

What PPI do I actually need?

It depends on viewing distance more than anything else, which is why no single number is the answer. A phone held at 30 centimetres needs several times the density of a monitor at arm's length, which needs far more than a television across a room. This is exactly why a 55 inch 4K television at roughly 80 PPI looks flawless from the sofa and unpleasant as a desk monitor - the panel has not changed, the distance has.

Why does it show the actual width and height?

Because the diagonal is a poor guide to whether something fits, and manufacturers only ever quote the diagonal. Two 34 inch monitors at different aspect ratios have noticeably different widths, and an ultrawide can be a great deal broader than people expect from the diagonal alone. Check the width against your desk before ordering, and remember the figure here is the panel - a stand and bezels add more.

What is dot pitch and why is it backwards?

It is the distance between the centres of neighbouring pixels, in millimetres, and it measures the same thing as PPI from the other end - so SMALLER is sharper, where with PPI bigger is sharper. Monitor specifications often quote one or the other without explaining the direction, which is a reliable way to compare two screens and conclude the wrong thing.

Does higher PPI mean I need scaling?

Usually, yes. Operating systems assume a rough density, so a high-PPI display makes text and interface elements very small unless the system scales them up. That is fine and expected - the extra pixels go into rendering the same size text more smoothly rather than into fitting more on screen. It is worth knowing before buying a very dense monitor expecting more workspace: you may get a sharper picture of the same amount.

Two monitors have the same PPI but text looks different.

Subpixel arrangement, most likely, which this calculator knows nothing about. PPI counts pixels; how the red, green and blue subpixels are laid out within each one affects how text edges are rendered and can be visibly different between panel technologies at identical density. Panel coating, contrast and how your operating system renders text all contribute too.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the PPI & Screen Size Calculator at https://www.bfcbrilliance.com/tools/ppi-calculator.

What I entered:
- Screen diagonal (in): ___
- Horizontal resolution (px): ___
- Vertical resolution (px): ___

What it calculated:
- Pixel density: ___
- Actual width: ___
- Actual height: ___
- Dot pitch: ___
- Total pixels: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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