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Welcome to the home page for the ICU4X project.

ICU4X provides components enabling wide range of software internationalization. It draws deeply from the experience of ICU4C, ICU4J and ECMA-402 and relies on data from the CLDR project.

The design goals of ICU4X are:

  • Small and modular code
  • Pluggable locale data
  • Availability and ease of use in multiple programming languages
  • Written by internationalization experts to encourage best practices

Stay informed! Join our public, low-traffic mailing list: Note: After subscribing, check your spam folder for a confirmation.


For an introduction to the project, please visit the "Introduction to ICU4X for Rust" tutorial. Further tutorials can be found in the tutorial index.

For technical information on how to use ICU4X, visit our API docs.

More information about the project can be found in the docs subdirectory.

Quick Start

An example ICU4X powered application in Rust may look like below...


icu = "1.0.0"
icu_testdata = "1.0.0"


use icu::calendar::DateTime;
use icu::datetime::{options::length, DateTimeFormatter};
use icu::locid::locale;

let options =
    length::Bag::from_date_time_style(length::Date::Long, length::Time::Medium).into();

let dtf = DateTimeFormatter::try_new_unstable(&icu_testdata::unstable(), &locale!("es").into(), options)
    .expect("Failed to create DateTimeFormatter instance.");

let date = DateTime::try_new_iso_datetime(2020, 9, 12, 12, 35, 0).expect("Failed to parse date.");
let date = date.to_any();

let formatted_date = dtf.format(&date).expect("Formatting failed");
    "12 de septiembre de 2020, 12:35:00"


ICU4X is developed by the ICU4X-SC. We are a subcommittee of ICU-TC in the Unicode Consortium focused on providing solutions for client-side internationalization. See for more information on our governance.

Please subscribe to this repository to participate in discussions. If you want to contribute, see our


For the full charter, including answers to frequently asked questions, see

ICU4X is a new project whose objective is to solve the needs of clients who wish to provide client-side internationalization for their products in resource-constrained environments.

ICU4X, or "ICU for X", will be built from the start with several key design constraints:

  1. Small and modular code.
  2. Pluggable locale data.
  3. Availability and ease of use in multiple programming languages.
  4. Written by internationalization experts to encourage best practices.

ICU4X will provide an ECMA-402-compatible API surface in the target client-side platforms, including the web platform, iOS, Android, WearOS, WatchOS, Flutter, and Fuchsia, supported in programming languages including Rust, JavaScript, Objective-C, Java, Dart, and C++.

Benchmark dashboards

The performance benchmarks are all run on Ubuntu, and are broken out by component.

The memory benchmarks are run separately on each platform, and examples are individually instrumented.

The binary size benchmarks run on Ubuntu, and are broken out by file type.

The data size benchmark tracks size of provider/testdata/data/testdata.postcard and runs on Linux.