Dynamic Font Scaling
Dynamic Font Scaling is a feature that allows users to choose the size of the text displayed on the screen. This helps users who need larger text for better readability, and it also accommodates users who can read smaller text.
Dynamic Font Scaling is supported on Android, iOS, and iPadOS starting in Ionic v7.5.
Try It Out
Be sure to try this on an Android, iOS, or iPadOS device.
If you are testing on Chrome for Android, make sure "Accessibility Page Zoom" is enabled.
Follow the Changing the Font Size on a Device guide to set your preferred font size, and watch the text in the demo below grow or shrink according to your preferences.
Enabling Dynamic Font Scaling in Ionic
This feature is currently opt-in on iOS. However, it will be enabled by default starting in Ionic 8 at which point the following CSS will no longer be necessary.
Dynamic Font Scaling is already enabled by default on Android. Developers can enable it on iOS using the following steps:
- Ensure that the typography.css file is imported.
- Add the following CSS to a global stylesheet:
html {
--ion-dynamic-font: var(--ion-default-dynamic-font);
}
Under the hood, Ionic sets the following CSS on iOS devices to enable Dynamic Font Scaling:
html {
font: var(--ion-dynamic-font);
}
Using Dynamic Font Scaling
Integrating Custom Components
Developers can configure their custom components to take advantage of Dynamic Font Scaling by converting any font-size declarations that use px units to use rem units instead. An easy way to convert from px to rem is to divide the pixel font size by the default browser font size, which is typically 16px. For example, if a component has a font size of 14px, then this could be converted to rem by doing 14px / 16px = 0.875rem. Also note that any Ionic components that have had their font sizes overridden should also be updated to use rem units.
One thing to keep in mind is that the dimensions of your components may need to change to accommodate the larger font sizes. For example, width and height properties may need to change to min-width and min-height, respectively. Developers should audit their applications for any CSS properties that use length values and make any applicable conversions from px to rem. We also recommend having long text wrap to the next line instead of truncating to keep large text readable.
Custom Font Family
We recommend using the default fonts in Ionic as they are designed to look good at any size and ensure consistency with other mobile apps. However, developers can use a custom font family with Dynamic Font Scaling via CSS:
html {
--ion-dynamic-font: var(--ion-default-dynamic-font);
--ion-font-family: 'Comic Sans MS';
}
em units versus rem units
Developers have two options for relative font sizes: em and rem.
em units set the font size of an element relative to the font size of its parent.
In the following example, the computed font size of .child is 40px because it is a child of .parent (2em * 20px = 40px).
.parent {
font-size: 20px;
}
.child {
font-size: 2em;
}
However, the em unit has a compounding effect which can cause issues. In the following example, the second .child element has a computed font size of 80px since the font sizes compound.
<div class="parent">
Parent element with 20px
<div class="child">
Child element with 40px
<div class="child">Child element with 80px</div>
</div>
</div>
Parent element with 20px
Child element with 40px
Due to this compounding effect, we strongly recommend using rem units instead of em units when working with Dynamic Font Scaling. rem units set the font size of an element relative to the font size of the root element, which is typically <html>. The default font size of the root element is typically 16px.
In the following example, the computed font size of .child is 32px because the font size is being computed relative to html, not .parent.
.parent {
font-size: 20px;
}
.child {
font-size: 2rem;
}
How Dynamic Font Scaling works in Ionic
Ionic components that define font sizes and participate in Dynamic Font Scaling typically use rem units. This sizes the text in each component relative to the font size of the root element, which is usually the html element. This means that as the root element's font size changes, the text in all Ionic components scale in a consistent manner. This avoids the need to manually override each component's font size. Some elements inside of these components, such as icons, use em units instead so the elements are sized relative to the text, though the text itself is sized using rem units.
iOS
Dynamic Font Scaling in Ionic builds on top of an iOS feature called Dynamic Type. To do this, Ionic sets the font of the root element to an Apple-defined text style. For consistency, Ionic uses the body text style.
Using the Apple-defined text style enables Dynamic Type, allowing all text in Ionic components to scale according to the system-level preference. Note that these Apple-defined fonts only work on Apple devices. As a result, these fonts will not work on Android devices even if your app is using ios mode.
Ionic follows Apple's Human Interface Guidelines for Typography when an app is in ios mode. As a result, important content is prioritized when the text size changes. This means a few things:
- Content in an
ion-headeror anion-footerwill have maximum font sizes to prioritize content inion-contentwhich is deemed more important than content in theion-headerandion-footer. - Components such as
ion-badgeandion-back-buttonwill have minimum font sizes so they remain readable. - Text in components such as
ion-tab-barandion-pickerdo not participate in Dynamic Font Scaling according to Apple's Human Interface Guidelines.
Android Web View
The Android Web View's font scaling mechanism is always enabled in web content and will automatically scale font sizes defined using the px unit. This means that any maximum or minimum font sizes specified using px will still be scaled even if the final font size does not align with the maximum or minimum font sizes specified.
In the following example we are using the min() function to indicate that the font size of .foo should be no larger than 14px.
.foo {
font-size: min(1rem, 14px);
}
If the root element's default font size is 16px, and the system-level font scale is 1.5 (i.e text sizes should be increased by 50%), then 1rem will evaluate to 24px because 16 * 1.5 = 24.
This is larger than our defined maximum of 14px, so one might assume that the evaluated font size of .foo is 14px. However, since the Android Web View scales any font sizes defined using the px unit, this means the 14px used in our min() function will also be scaled by 1.5.
As a result, this means that the maximum computed font size is actually 21px since 14 * 1.5 = 21 and therefore the overall computed font size of .foo is 21px.
Chrome for Android
The Chrome Web Browser on Android behaves differently than the Android Web View. By default, Chrome for Android does not respect the system-level font scale setting. However, the Chromium team is working on a new feature to allow for this. When enabled, this feature will change the zoom level of the html element which will cause the layout to increase in size in addition to the text.
Developers can test this behavior by enabling the experimental "Accessibility Page Zoom" feature in chrome://flags.
See https://bugs.chromium.org/p/chromium/issues/detail?id=645717 for more information.
Using Modes on Different Platforms
Each platform has slightly different font scaling behaviors, and the ios and md modes have been implemented to take advantage of the scaling behaviors on their respective platforms.
For example, ios mode makes use of maximum and minimum font sizes to follow Apple's Human Interface Guidelines for Typography. md mode does not implement this same behavior because Material Design does not have that same guidance. This means that using md mode on an iOS device may allow for very large font sizes in headers and footers.
As a result, we strongly recommend using ios mode on iOS devices and md mode on Android devices when using Dynamic Font Scaling.
Changing the Font Size on a Device
Font scaling preferences are configured on a per-device basis by the user. This allows the user to scale the font for all applications that support this behavior. This guide shows how to enable font scaling for each platform.
iOS
Font scaling on iOS can be configured in the Settings app.
See Apple Support for more information.
Android
Where users access the font scaling configuration varies across devices, but it is typically found in the "Accessibility" page in the Settings app.
The Chrome Web Browser on Android has some limitations with respecting system-level font scales. See Chrome for Android for more information.
Troubleshooting
Dynamic Font Scaling is not working
There are a number of reasons why Dynamic Font Scaling may not have any effect on an app. The following list, while not exhaustive, provides some things to check to debug why Dynamic Font Scaling is not working.
- Verify that your version of Ionic supports Dynamic Font Scaling. Dynamic Font Scaling was added starting in Ionic v7.5.
- Dynamic Font Scaling is opt-in on iOS in Ionic 7. Verify that the proper CSS has been set. See Enabling Dynamic Font Scaling in Ionic for more information.
- Verify that your code does not override the root element's default font size. Manually setting a font size on the root element will prevent Dynamic Font Scaling from working as intended.
- Verify that your code does not override font sizes on Ionic components. Ionic components that set
font-sizerules will useremunits. However, if your app overrides that to usepx, then that custom rule will need to be converted to userem. See Integrating Custom Components for more information. - Verify "Accessibility Page Zoom" is enabled if using Chrome for Android. See Chrome for Android for more information.
Maximum and minimum font sizes are not being respected on Android
The Android Web View scales any font sizes defined using the px unit by the system-level font scale preference. This means that actual font sizes may be larger or smaller than the font sizes defined in min(), max(), or clamp().
See how font scaling works on Android for more information.
Font sizes are larger/smaller even with Dynamic Font Scaling disabled
Ionic components define font sizes using rem units even when Dynamic Font Scaling is disabled. This sizes the text in each component relative to the font size of the root element, which is usually the html element. As a result, if the font size of html changes, the computed font size of all Ionic components will change too.
Scaled Ionic iOS component font sizes do not exactly match native iOS equivalents
Certain native iOS components such as the Action Sheet make use of private font scales that Ionic does not have access to. While we try to stay as close as possible to the native behavior, text in some components may render slightly larger or smaller than their native counterparts.
The text size in my Ionic app on iOS changed when enabling Dynamic Font Scaling
The root element's default font size is typically 16px. However, Dynamic Font Scaling on iOS devices make use of the "Body" text style which has a default font size of 17px. Since the text in Ionic components is scaled relative to the root element's font size, some text may get larger or smaller when Dynamic Font Scaling is enabled, even if the system-level text scale did not change.
iOS provides a "Callout" text style which has a default font size of 16px. However, this font style is currently not exposed to web content. See the supported text styles in WebKit for more information.