CSS - PX to EM converter

Ethan D. - May 31, 2024

Understanding PX and EM Units

When working with CSS, you need to understand the different units of measurement for sizing elements. Two commonly used units are PX (pixels) and EM (relative unit). Let's look at each of these units and their characteristics.

PX (Pixels)

PX, short for pixels, is an absolute unit of measurement in CSS. It represents a fixed size and does not change based on the size of the parent element or the screen. One pixel is equal to one dot on the screen.

PX is used for specifying sizes of elements, such as widths, heights, padding, and margins. For example, setting font-size: 16px; means the font size will always be 16 pixels, regardless of the parent element's font size.

Example of PX usage

font-size: 16px;

However, using PX has some limitations in responsive web design. Since PX units are fixed, they do not adapt well to different screen sizes and devices. An element sized with PX may appear too small on larger screens or too large on smaller screens, leading to readability and usability issues.

EM (Relative Unit)

EM is a relative unit of measurement in CSS. It is based on the font size of the parent element. One EM is equal to the computed font size of the parent element.

For example, if the parent element has a font size of 16 pixels and you set the child element's font size to 1.5em, the child element's font size will be calculated as 16 pixels * 1.5 = 24 pixels.

Example of EM usage

font-size: 1.5em;

Using EM has several advantages for responsive web design:

Advantage	Description
Adaptability	EM units automatically adjust based on the parent element's font size. This means that when the parent font size changes, all the child elements sized with EM will proportionally change as well.
Scalability	EM allows easy scaling of elements by changing the font size of the parent element. By modifying the root element's font size (usually the `<html>` tag), you can proportionally scale the entire design.
Maintainability	EM promotes a more maintainable and flexible codebase. Instead of specifying fixed sizes for each element, you can define relative sizes based on the parent element, making it easier to make global changes.

It's important to note that EM units are relative to the font size of the immediate parent element. If an element's font size is not explicitly defined, it inherits the font size from its parent. This cascading nature of EM allows for creating consistent and hierarchical designs.

When using EM, you need to consider the font size inheritance and plan your CSS accordingly. Setting a base font size on the root element (usually <html>) and using EM for other elements can help maintain a consistent design throughout your website.

Converting PX to EM

When changing from using PX to EM units in your CSS, you'll need to know how to convert between the two units. This section explains the conversion formula and gives a step-by-step guide on how to apply it. It also looks at some useful conversion tools that can make the process easier.

Conversion Formula

To convert PX to EM, you can use the following formula:

EM = PX / Parent Font Size (in PX)

Step	Description
1	Find the PX value you want to convert (e.g., 16px).
2	Identify the font size of the parent element in pixels (e.g., 16px).
3	Divide the PX value by the parent font size to get the EM value.

For example, if you have an element with a font size of 16px and you want to convert a padding value of 24px to EM:

EM = 24px / 16px = 1.5em

In this case, 24px equals 1.5em.

Here's a step-by-step guide on how to apply the conversion formula:

Find the PX value you want to convert.
Find the font size of the parent element. If not set, it defaults to the browser's default font size (usually 16px).
Divide the PX value by the parent font size to calculate the EM value.
Replace the PX value with the calculated EM value in your CSS.

Conversion Tools

While you can manually calculate the EM values using the conversion formula, there are several tools available that can make the process faster and easier. Here are a few options:

Online PX to EM Converters:
- PX to EM Converter by W3Schools
- PX to EM Converter by Nekocalc
- PX to EM Calculator by PxtoEm.com
These online converters allow you to enter PX values and instantly get the EM values. They are convenient for quick conversions without the need for manual calculations.
CSS Preprocessors:
CSS preprocessors like Sass and Less have built-in functions that make PX to EM conversions easier. In Sass, you can use the em() function:

Example: Sass EM Conversion

font-size: em(24px, 16px); // Converts 24px to 1.5em, assuming a parent font size of 16px

Similarly, in Less, you can use the unit() function:

Example: Less EM Conversion

font-size: unit(24px / 16px, em); // Converts 24px to 1.5em, assuming a parent font size of 16px

Using CSS preprocessors can save you time and make your code more readable by handling the conversions behind the scenes.

Code Snippets for Manual Conversion:
If you prefer to perform the conversions manually in your CSS, you can use code snippets like the following:

Example: Manual PX to EM Conversion

/* Convert PX to EM */
font-size: 24px; /* Original PX value */
font-size: 1.5em; /* EM value, assuming a parent font size of 16px */

By keeping the original PX value as a comment, you can easily refer back to it if needed.

Remember, when converting PX to EM, it's important to consider the font size of the parent element. If the parent element's font size changes, the EM values will adjust, keeping the relative sizing of the elements.

Implementing EM Units in CSS

When using EM units in your CSS, you should set a base font size on the root element and apply EM consistently throughout your stylesheets. This section covers setting a base font size, using EM for various CSS properties, and tips for maintaining consistency and hierarchy in your design.

Setting a Base Font Size

To have a consistent and scalable design, you should set a base font size on the root element of your HTML document. The root element is usually the <html> tag. By setting a base font size, you create a reference point for all other EM-based sizes in your CSS.

There are two common approaches to setting a base font size:

Using pixels (PX):

Example: Set base font size using pixels

html {
  font-size: 16px;
}

Using a percentage:

Example: Set base font size using percentage

html {
  font-size: 100%;
}

By setting a base font size, you create a foundation for using EM units consistently throughout your CSS.

Applying EM Units to CSS Properties

Once you have set a base font size, you can start using EM units for various CSS properties. EM can be used for font sizes, padding, margins, and other properties that need sizing.

Example: Using EM units in CSS

h1 {
  font-size: 2.5em; /* Equivalent to 40px if base font size is 16px */
}

p {
  font-size: 1em; /* Equivalent to 16px if base font size is 16px */
}

Another example of using EM units in CSS

.button {
  padding: 0.75em 1.5em; /* Equivalent to 12px 24px if base font size is 16px */
  margin-bottom: 1em; /* Equivalent to 16px if base font size is 16px */
}

By using EM units, you make sure that the sizes of elements adapt proportionally based on the base font size and the font size of their parent elements.

Maintaining Consistency and Hierarchy

When implementing EM units in your CSS, you should maintain consistency and establish a clear hierarchy of elements. Here are some tips to help you do that:

Use a modular scale: A modular scale is a sequence of sizes that follow a consistent ratio. By using a modular scale for your font sizes and spacing, you can create a harmonious and visually pleasing design. Tools like Modular Scale can help you generate a scale based on your base font size and desired ratio.
Keep it simple: Avoid using too many different font sizes or spacing values. Stick to a limited set of sizes to maintain consistency throughout your design. Use a typography style guide to define the font sizes and hierarchy for different elements like headings, paragraphs, and lists.
Use relative units for media queries: When defining breakpoints for responsive design, use EM units instead of fixed pixel values. This makes sure that your breakpoints adapt to the base font size and maintain their relative positions.
Use EM for padding and margins: By using EM units for padding and margins, you create consistent spacing that scales proportionally with the font size. This helps maintain the visual balance and rhythm of your design across different screen sizes.
Test and iterate: As you implement EM units in your CSS, test your design on various devices and screen sizes. Make adjustments as needed to make sure readability, usability, and visual appeal. Iterate on your design until you achieve a consistent and hierarchical layout.

Remember, the key to success with EM units is to use them consistently, establish a clear hierarchy, and maintain a balanced and scalable design.

Responsive Design with EM Units

Using EM units in your CSS offers benefits when creating responsive web designs that adapt to different screen sizes and devices. EM units allow your design elements to scale based on the font size, providing a flexible layout.

Benefits of EM in Responsive Web Design

One of the main advantages of using EM units for responsive design is their ability to adapt to different screen sizes and devices. When you set sizes using EM, they are relative to the font size of the parent element. This means that if the font size changes, all the elements sized with EM will adjust.

For example, consider a layout where you have set the base font size to 16px on the root element. If you have a heading with a font size of 2em, it will be calculated as 32px (16px 2). Now, if you change the base font size to 20px, the heading will automatically adjust to 40px (20px 2) without needing to modify the EM value.

This adaptability of EM units makes them suited for responsive design. As the screen size changes, you can modify the base font size using media queries, and all the EM-based sizes will adapt. This helps maintain the relative proportions and visual hierarchy of your design across different devices.

Other relative units in CSS include:

Unit	Description
REM	Relative to the root element's font size
VW	Relative to 1% of the viewport's width
VH	Relative to 1% of the viewport's height

Each of these relative units has its own use cases and benefits. REM units offer more predictable sizing as they are always relative to the root font size. VW and VH units are useful for creating full-screen layouts or elements that scale based on the viewport size.

However, EM units have the advantage of being more flexible and allowing for easier scaling of individual components based on their parent element's font size. They provide a level of relative sizing that is close to the design's typography, making it intuitive to work with.

Media Queries and EM

Media queries play a role in responsive web design, allowing you to apply different styles based on the characteristics of the device or viewport. When using EM units, you can also use them in your media queries to create responsive breakpoints.

Instead of using fixed pixel values for breakpoints, you can define them using EM units. This approach ensures that your breakpoints adapt to the base font size and maintain their relative positions.

Example: Using EM units in media queries

/* Base styles */
html {
  font-size: 16px;
}

/* Media queries */
@media (min-width: 48em) { /* Equivalent to 768px if base font size is 16px */
  html {
    font-size: 18px;
  }
}

@media (min-width: 64em) { /* Equivalent to 1024px if base font size is 16px */
  html {
    font-size: 20px;
  }
}

By using EM units in media queries, you ensure that the breakpoints adapt to the base font size. If you change the base font size, the breakpoints will adjust, maintaining the intended responsive behavior.

Remember to test your responsive design across different devices and screen sizes to ensure that your EM-based layout adapts well and provides a good user experience.

Using EM units in combination with responsive design techniques, such as media queries and flexible layouts, allows you to create websites that scale and adapt to different devices and screen sizes. By using the power of EM units, you can build responsive designs that are both visually appealing and user-friendly.