There Is No Such Factor As A CSS Absolute Unit

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After we begin studying CSS, we discover that CSS models of measurement are categorized as relative or absolute. Absolute models are rooted in bodily models, comparable to pixels, centimeters, and inches. However through the years, all absolute models in CSS have misplaced their connection to the bodily world and have turn out to be completely different sorts of relative models, a minimum of from the attitude of the online.

It’s essential to notice that there are nonetheless important variations between relative and absolute models. CSS relative models are sized in response to different fashion definitions outlined by mother or father parts or are affected by the dimensions of a mother or father container. As for absolute models, we are going to dive in and see how they’re affected by different issues, such because the display screen and the system’s working system.

Relative models embody models comparable to %, em, rem, viewport models (vw and vh), and extra. The commonest absolute unit is the pixel (px). Apart from that, we now have the centimeter unit (cm) and the inches unit (in).

Now, let’s discover why CSS absolute models aren’t so absolute.

CSS Pixels

Pixels have been the commonest unit of CSS, courting to the start of the online. Within the outdated world of desktop screens, earlier than we had any smartphones, the display screen’s pixels have been all the time equal to CSS pixels.

In 2007, for instance, the commonest desktop decision was 1024×768 pixels. Again then, we might usually give our net pages a set width of 1000 pixels to suit all the web page, and the leftover pixels could be saved for the browser’s scrollbar.

Smartphone Screens

Smartphones introduced one other quiet evolution, beginning the period of high-density screens. If we take into account an iPhone 12 Professional, whose display screen is 1170 pixels vast, we might depend each 3 pixels on the system as 1 pixel within the CSS.

After we measurement in cell, we measure in response to CSS pixels, not in response to system pixels. To sum up:

CSS pixel are logical pixels.
Machine pixels are actual bodily pixels.

Okay, however what about desktop units? Do they nonetheless work with the identical outdated pixel calculation? Let’s discuss that.

Desktop Screens In 2021

Excessive-density screens got here to laptops a number of years later. The 2014 MacBooks obtained the primary “retina” screens (retina being synonymous with excessive density).

As of late, most laptops have a high-density display screen.

Let’s take into account MacBooks:

The 13.3-inch MacBook Professional has a display screen that’s 2560 pixels vast however that behaves like 1440 pixels. Because of this each 1.778 bodily pixels act like 1 logical pixel.
The 16-inch MacBook Professional has a display screen that’s 3072 pixels vast however that behaves like 1792 pixels. Because of this each 1.714 bodily pixels act like 1 logical pixel.

Amongst PC laptops, I examined two 15.6-inch screens, one with full HD decision and the opposite with 4K decision. The outcomes have been attention-grabbing:

The 15.6-inch full-HD display screen is 1920 pixels vast however behaves like 1536 pixels. Because of this each 1.25 bodily pixels act like 1 logical pixel.
The 15.6-inch 4K display screen is 3840 pixels vast however behaves, once more, like 1536 pixels. Because of this each 2.5 bodily pixels act like 1 logical pixel.

As you’ll be able to see, the connection between the actual bodily (i.e. system) pixels and the CSS (i.e. logical) pixels has virtually vanished.

Screens Have Develop into Denser Over The Years

Up to now, in the event you appeared carefully at a display screen, you can really see its pixels. When the expertise of screens improved, producers began to create higher-density screens.

Really helpful studying: What Does A Foldable Internet Truly Imply?

Why Do We Calculate Logical Pixels Otherwise?

Through the years, as screens turned denser, we couldn’t match extra content material in the identical display screen measurement merely as a result of the display screen has extra pixels.

Give it some thought for a second. Think about the Samsung Galaxy S21 Extremely. Its narrower dimension is 1440 bodily pixels. We may simply match it in a daily desktop display screen. But when we did, the textual content could be small to the purpose of being unreadable. Due to this, we separate bodily pixels from logical pixels.

Sizes in CSS (i.e. width and peak), then, are calculated in response to CSS logical pixels. In fact, we are able to use bodily pixels to load high-density content material, comparable to pictures and movies, like so:

<img src=”image-size-1200px.jpg” width=”300″ >

OK, CSS pixels aren’t equal to a tool’s bodily pixels. However we now have centimeters and inches. These are bodily models linked to the bodily world, proper? Let’s test them out.

CSS Inches And CSS Centimeters

Wherever we use bodily models like inches and centimeters, we all know these are absolute models.

I had a thought that if CSS pixels aren’t equal to system pixels, then perhaps it might be a good suggestion to make use of bodily models comparable to inches and centimeters on the internet. They’re absolute models, proper?

To make certain, I examined it. I created a field with a width and peak of 1 centimeter and gave it a background shade of crimson. I grabbed an actual tape measure and obtained a shock:

A CSS centimeter isn’t equal to a bodily centimeter.

Right here I’m testing a CSS centimeter unit with a tape measure on a mid-2019 13-inch MacBook:

The outcome is identical for CSS inches:

A CSS inch isn’t equal to a bodily inch.

The identical holds true for pica (laptop) and millimeter (mm) models. These correspond to part of both a CSS inch or a CSS centimeter, neither of which is linked to an actual inch or an actual centimeter.

Why CSS Inches And Centimeters Aren’t Actual Inches And Centimeters

Because the Nineteen Eighties, the PC market has decided a CSS inch to be equal to 96 pixels. This calculation of pixels was straight tied to the DPI/PPI (pixels per inch) commonplace of Microsoft’s Home windows working system for displays on the time, the commonest of which had a regular of 96 DPI.

This meant that 1 CSS inch would all the time be equal to 96 CSS pixels.

As for CSS centimeters, each centimeter is straight calculated from inches, which signifies that 1 inch is equal to 2.54 centimeters. Because of this each 1 CSS centimeter will all the time be equal to 37.7952756 CSS pixels.

In different phrases: 1cm = 37.7952756px (96px / 2.54).

As a result of a CSS inch and a CSS centimeter are straight transformed from CSS pixels, and since screens have gotten extra DPIs through the years, we’ve gotten to the purpose the place these models don’t characterize what they’re speculated to characterize on screens.

CSS Level Unit

The purpose (pt) unit is without doubt one of the less-recognized models of CSS. As Wikipedia states:

“In typography, the purpose is the smallest unit of measure. It’s used for measuring font measurement, main, and different gadgets on a printed web page.”

The Wikipedia web page reveals a ruler with the purpose scale on the underside and the inch scale on the highest:

Earlier than we get into why this unit isn’t actually an absolute unit for the online, let’s go over the fundamental models of screens and printers.


We’ve already talked about DPI, and also you may need heard these phrases prior to now, however in the event you’ve by no means understood what precisely they’re all about, right here is fast primer:

Screens are constructed from plenty of small gentle dots, known as pixels. To measure the density of pixels, we depend the variety of pixels that match 1 inch, known as pixels per inch (PPI).
Printers print shade dots. To characterize the density of printer dots, we depend the variety of dots that match 1 inch of paper, known as dots per inch (DPI).

Briefly, these are two methods to measure the density of visible info that we are able to slot in 1 inch.

PPI: pixels per inch (for screens)
DPI: dots per inch (for printers)

It is very important point out that the depend of CSS pixels and dots in 1 inch are for each the width and the peak. Because of this on a display screen of 96 PPI, a field with a peak and width of 1 inch could have a complete measurement of 9216 pixels (96×96 px = 9216 px).

Here’s a visible demonstration of 1 inch with a display screen of 10 PPI:

Listed below are some examples of actual calculations of CSS PPI:

CSS Decision
CSS Inches
(width and peak)



“DPI” For Screens

Producers of cell and desktop units favor to specific their display screen measurements in DPI, not PPI. However don’t let that confuse you: It’s all the time PPI for screens and DPI for printers.

DPI/PPI Requirements

To characterize all these dots and pixels, we now have the purpose (pt) unit.

However the level unit of CSS derives from the default printer DPI, which, once more, was determined within the Nineteen Eighties and is the same as 72 DPI. Because of this 1 inch of CSS all the time equals 72 factors.

1 inch = 72 factors
1 level = 1/72nd of 1 inch

Pixels For Internet, Dots For Printers

For the online, the DPI unit has no that means. The online DPI is outlined in response to a special commonplace (96 DPI), which we already talked about after we calculated a CSS inch and CSS centimeter. Due to this, there isn’t any purpose to make use of the purpose unit on the internet.

Notice: 1 level isn’t equal to (CSS) pixels.

1 level = 1.333 pixels
72 factors = 1 inch
72 factors = 96 pixels


On this article, I primarily needed to show why there aren’t any absolute models for the online. However what about utilizing them for printers? Is there a purpose to make use of CSS inches or centimeters or level models for printers?

My Printing Take a look at

I ran a small check to test whether or not the Nineteen Eighties commonplace of DPI works accurately on printers. I created two bins: one with a width and peak of 72 factors, and the second with a width and peak of 1 inch.

I printed these two bins on a laser printer that I’ve in my workplace. Right here is my Codepen for testing factors and inches for printers:

Printers are capable of print extra DPIs, but when we’re working at 100% zoom on the printer, then 72 factors (or 1 inch) of CSS will equal an actual bodily inch.

Reminder: This text is extra concerning the connection of absolute models to the online somewhat than to printers. In fact, the outcomes would possibly change on various kinds of printers.

Really helpful studying: Utilizing HSL Colours In CSS

Attempting To Create Correct Sizes On The Internet

If we have a look at the 16-inch MacBook Professional, which has a ratio of 1.714 bodily pixels to each 1 CSS pixel, we are able to’t precisely predict sizes on the internet.

If we attempt to guess the actual system pixel ratio on the 16-inch MacBook Professional utilizing JavaScript’s window.devicePixelRatio, it would return an incorrect ratio of two, as a substitute of 1.714. (And that is with out making an allowance for the zoom state of the online browser and working system.)

Why We Want Actual Absolute CSS Models

After we need to outline a set measurement for a sidebar ingredient, we might use CSS pixels. But when you concentrate on it, CSS pixels haven’t any that means nowadays. As we noticed above, on most smartphones and desktops, CSS pixels don’t describe system pixels anymore.

Based mostly on this, I consider we want precise bodily models for CSS (like an actual centimeter or inch unit) as a result of CSS pixels not have any true that means on the internet.

It’s price mentioning that Firefox had carried out an precise bodily millimeter unit (mozmm), however eliminated it in model 59. I don’t know why they eliminated it. Maybe it’s as a result of so many issues already rely on CSS pixels, comparable to responsive pictures and the em and rem models. If we tried so as to add a brand new bodily measurement, perhaps it might trigger extra issues than it solves.

Plainly net folks have gotten so used to pondering in pixels that, though the CSS pixel unit has misplaced its connection to system pixels, we’ll hold utilizing the unit.

And in case you continue to assume that CSS pixels are a wonderful unit of measurement, attempt to clarify to a brand new net developer what this unit is definitely measuring.

For now, we don’t have any actual approach to describe bodily sizes in CSS.

So, the CSS pixel is the worst sort of absolute unit — apart from all of the others.

To Summarize

Firstly of this text, I mentioned that absolutely the CSS models have turn out to be like new sorts of relative models. We began with CSS pixels, and we noticed the distinction between CSS pixels and system pixels.

Then, we discovered that CSS inches and CSS centimeters are straight transformed from CSS pixels and aren’t linked to actual inches and centimeters. In the long run, we talked concerning the level unit and, once more, about how this unit has no absolute that means for the online.

Closing Phrases

That’s all. I hope you’ve loved this text and discovered from my expertise. Should you like this put up, I’d respect listening to about it and sharing it.


CSS Size Defined”, Tim Chien, Robert Nyman, Mozilla Hacks
Dots Per Inch”, Wikipedia
Level (Typography)”, Wikipedia
CSS Values and Models”, W3C

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