Why pixel-to-inch converters disagree, and which answer is yours

Type 1920 pixels into three converters and you can get three answers: 20 inches, 26.67 inches and 6.4 inches. None of them has made an arithmetic mistake. Each one has decided what a pixel is, and most of them do not tell you which decision they made.

One number, four answers

Every pixel-to-inch conversion is the same division: inches = pixels ÷ pixels per inch. The whole disagreement lives in the divisor.

What the converter assumes	Pixels per inch	1920 px in inches	1920 px in centimetres
A CSS pixel	96	20	50.8
The old Macintosh figure	72	26.6667	67.7333
An image printed at 150 ppi	150	12.8	32.512
An image printed at 300 ppi	300	6.4	16.256
A pixel on your own monitor	Depends on the screen	Unknown until measured	Unknown until measured

The top answer and the bottom known answer differ by more than four times (26.6667 ÷ 6.4 = 4.17). That is not rounding. It is three different questions sharing one word.

A pixel is three different things

A CSS pixel is 1/96 of a CSS inch, by definition. The CSS Values and Units Level 4 specification (§6.2, Absolute lengths) fixes 1in = 2.54cm = 96px, so 96 is neither an average nor a setting. It is the right divisor for anything written in a stylesheet, set in a design tool that outputs CSS, or measured in a browser. The same section defines the reference pixel as the visual angle of one pixel at 96 dpi viewed from arm’s length, about 0.0213 degrees at a nominal 28 inches.

An image pixel has no physical size until it is printed. Its inch is whatever resolution you print at. Adobe’s documentation of Image Size with resampling switched off makes the point exactly: changing the resolution changes the printed size and nothing else, not the pixel dimensions, not the file size and not how the image looks on screen.

A hardware pixel is a physical dot on one particular display. A CSS pixel spans devicePixelRatio of them, which on a high-density phone or laptop is several. The specification recommends that screens anchor their lengths to the pixel and print anchors to physical units. The consequence: in print, a CSS inch is a real inch; on screen, it is 96 CSS pixels, however long that happens to measure on the glass.

Where 72 and 96 came from

The 1984 Macintosh had a 72 ppi screen, so one typographic point (1/72 of an inch) was exactly one pixel and what you saw matched the paper. Windows reported screens of roughly the same density as 96 “logical” dpi, a third larger (96 ÷ 72 = 4/3), so that 10-point text got enough pixels to be legible. Microsoft’s typography team told that story in “Where does 96 DPI come from in Windows?”, and CSS later adopted the Windows figure as its ratio of pixels to inches.

72 survived in two places: as the default resolution field in image software, and as the belief that 72 dpi is “web resolution”. That belief was never true. A browser ignores the resolution stored in an image and draws its pixels as CSS pixels, scaled by the device pixel ratio. The field only matters once the file is printed or placed into a print layout. A converter that divides by 72 for the web gives an answer a third larger than the CSS one: 26.6667 ÷ 20 = 1.333.

What the converters in the search results assume

When we checked the top ten results for “px to inches” in October 2026, two patterns covered almost all of them:

• One formula for everything. The most common page stated that 1 inch is 96 pixels “for web”, then applied the same inches = px ÷ PPI to web, print and devices alike, with no line between CSS, image and hardware pixels and no mention of devicePixelRatio.
• Presets with no stated default. Another offered 72, 96 and 300 as presets and called 72 or 96 “historically standard” for the web, without saying which one its result was using.

The px-to-rem converters checked at the same time showed the same habit: almost all assumed a 16px root with an editable base, and none showed the arithmetic behind each result.

None of this is a calculation error. It is an unstated premise, and the visitor whose premise differs gets a confident wrong number. Someone sending a 1920-pixel image to a 300 ppi printer who reads “20 inches” has been told the print will be more than three times wider than it will be (20 ÷ 6.4 = 3.125).

Which answer is yours

Writing CSS or measuring in a browser: 96

Divide by 96, every time. 1920 CSS pixels is 20 CSS inches. In a print stylesheet those are real inches; on a monitor they are not, and nothing in CSS can make them so.

Sending an image to paper: your print resolution

Inches = pixels ÷ ppi, and pixels needed = inches × ppi. 300 ppi is the trade convention for prints looked at up close; it is a convention, not a standard. Printers usually say “dpi” when they mean ppi. Strictly, PPI is pixels per inch in an image and DPI is the dots of ink a printer lays down, but a request for a “300 dpi image” means 300 pixels per inch.

Knowing how big something looks on your screen: measure it

No web interface reports a screen’s physical size or true density. devicePixelRatio gives hardware pixels per CSS pixel and says nothing about length. An object of known size is the only honest reference, and the obvious one is in your wallet: bank cards and most ID cards are 85.60 × 53.98 mm under ISO/IEC 7810.

Suppose a rectangle matches a card’s long edge at 400 CSS pixels. 85.60 mm is 3.3701 inches, so 400 ÷ 3.3701 = 118.7 CSS pixels per real inch, and a CSS inch measures 96 ÷ 118.7 = 0.81 real inches, or 20.5 mm. Browser zoom changes the size of a CSS pixel, so that figure holds only at the zoom it was measured at.

Doing it in the pixel converter

1. Open the pixel converter and answer its first question, what the pixels are for: “For CSS”, “For print” or “On this screen”.
2. Type the size into “Value”. A bare number uses the unit in the “From” menu; typing the unit as well, such as 1920px or 8.5in, sets the menu for you.
3. Set the one thing the answer depends on. For print, pick 72, 96, 150, 300 or 600, or type your own resolution beside them, and a table of paper sizes in whole pixels appears under the results. For CSS, the “Root font size”, “Parent font size” and “Viewport width” fields set the basis for rem, em and vw.
4. For “On this screen”, hold a card flat against the rectangle under “Measure this screen with a bank card”, its corner on the rectangle’s top-left corner, and adjust the slider or the − and + buttons (half a pixel per press) until the right-hand edges line up. On a narrow phone, press “Use the short edge” and match the 53.98 mm side. Until you measure, this mode shows CSS inches, and it tells you to measure again if the zoom changes.
5. Read the result cards. Each one prints its working; at 300 ppi, 1920 pixels reads “1920px ÷ 300 = 6.4in (300 px per inch)”. The copy button takes the value with its unit attached.

Other ways to do it

There is no built-in converter on macOS or Windows to point you to, and the first two meanings do not need one: it is a single division on any calculator, by 96 for CSS or by your print resolution for paper. The third meaning has no formula at all, because nothing on a web page knows how big the screen is. That gap is what the card fills.

Checking that nothing is sent

The arithmetic runs in the tab. The screen measurement is saved in this browser’s own storage so you do not have to repeat it, and it does not leave your device. To confirm, open your browser’s developer tools on the Network tab, then type values and switch modes: no new requests should appear.

Start with the pixel converter and the question it asks first. For reference tables in both directions and paper sizes in whole pixels, see pixels to inches and pixels to centimetres; for the root-size question behind rem, see px to rem.