Retina Display Calculator
Type a resolution, diagonal and viewing distance to get PPI and pixels per degree. About 60 PPD, one arcminute per pixel, is the usual Retina rule of thumb. It is not an eye test and it does not prove an Apple Retina panel.
Retina calculator
Formulas: PPI = √(width² + height²) ÷ diagonal in inches. Centimetre lengths are divided by 2.54 first. PPD = 2 × distance in inches × tan(0.5°) × PPI. The 60 PPD distance is 1 ÷ (PPI × tan(1 arcminute)). The 120 PPD distance is 1 ÷ (PPI × tan(0.5 arcminute)). Retina here means the PPD is at or above the line you picked, about 1 arcminute at 60 PPD. That is a rule of thumb, not an eye test and not proof of an Apple Retina panel. Square pixels, at the centre of the screen.
What Retina really means
Retina is a distance, not a PPI number
On this page, Retina means about 60 pixels per degree, about 1 arcminute per pixel. That is a rule of thumb. It is not a medical eye test, and it is not proof that the panel is an Apple Retina display. Apple used the word for the iPhone 4 in 2010, a 326 PPI phone held close to the face. There is no fixed PPI cut-off. At 60 PPD, a phone at 12 inches needs about 286 PPI, a laptop at 20 inches about 172, and a TV at 8 feet about 36. Pixels per degree joins the PPI from the typed diagonal with the typed viewing distance.
The 1 arcminute rule of thumb
The 60 PPD line treats one pixel as 1 arcminute, the angle behind the old 20/20 eye chart. This page does not test your eyes, and it does not score 20/20 or 20/10 vision. Some people resolve finer detail, and a jagged edge can still show up below 1 arcminute, so 60 PPD text can look a little soft. The 120 PPD switch uses 0.5 arcminute per pixel. It is a stricter rule of thumb, not a vision result.
Why a 27 inch 4K and a 32 inch 4K differ
Both have 3840 × 2160 pixels, but the 32 inch spreads them over more glass. The 27 inch is about 163 PPI and passes 60 PPD from about 21 inches. The 32 inch is about 138 PPI and passes 60 PPD from about 25 inches. At a typed distance of 24 inches the 27 inch passes and the 32 inch does not. Compare mode runs the same formulas on any pair you enter.
Check the claim on the box
A Retina line on a spec sheet depends on viewing distance, and the box rarely names that distance. Type the distance from your eyes to the screen, plus the resolution and the diagonal, and read the rule-of-thumb result. A pass only means the typed numbers reach the PPD you picked. It is not an Apple Retina badge, and a miss does not mean the panel is broken.
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Frequently asked questions
How is PPI calculated?
PPI is the diagonal pixel count divided by the diagonal you type, after that diagonal is converted to inches. Diagonal pixels are the square root of width squared plus height squared. A diagonal in centimetres is divided by 2.54 before the division. The page does not measure the panel.
How are pixels per degree calculated?
Pixels per degree use that PPI and the viewing distance you type, in inches: PPD = 2 × distance × tan(0.5°) × PPI. That is how many pixels of this pitch fit in one degree at the centre of the screen. It assumes square pixels. It is not a reading of your eyes.
What does the Retina result mean?
Retina on this page is the common rule of thumb of about 60 pixels per degree, about 1 arcminute per pixel. The other switch uses 120 pixels per degree, about 0.5 arcminute. The distance shown is 1 ÷ (PPI × tan of that angle). It is not a medical eye test, and a pass is not proof that the panel is an Apple Retina display.
Why do the results show a dash?
A blank, zero, negative, or non-numeric entry does not get a number. The same is true when a result is not a finite number. The page shows a dash instead of Infinity or NaN. Width and height must be whole numbers from 1 to 30,000. The diagonal must be from 0.5 to 1,000 inches, and the distance from 1 to 10,000 inches.
What does Fill device-pixel estimate do?
It copies screen.width and screen.height, which are CSS pixels, multiplied by devicePixelRatio. The note lists those CSS pixels and the device-pixel estimate separately. The button copies only the device-pixel estimate into the width and height fields (screen A in compare mode), not the CSS size. OS scaling and browser zoom can change it, so it may not match the panel. The diagonal is never copied. Nothing is copied until you press the button.
Why do a 27 inch 4K and a 32 inch 4K differ?
Both are 3840 × 2160 pixels. The 27 inch diagonal is about 163 PPI and reaches 60 PPD at about 21 inches. The 32 inch diagonal is about 138 PPI and reaches 60 PPD at about 25 inches. At a typed distance of 24 inches, only the 27 inch passes the 60 PPD rule. Compare mode runs the same math on the two screens you type.
Are the phone and laptop presets this device?
No. The named examples are approximate published resolutions and diagonals, not a reading of this computer or phone. Picking one fills the fields so you can edit them. Check the resolution and the size printed for that panel before you trust the result.