Reference Guide

ICU MessageFormat Syntax

The complete guide to ICU MessageFormat for internationalization. Learn pluralization, gender select, number formatting, and more.

Simple Arguments Plural Format Select Format Select Ordinal Format Number Format Date and Time Format Rich Text / Tags Escaping Special Characters Nested Messages Plural Rules Libraries FAQ

What is ICU MessageFormat?

ICU MessageFormat is the industry-standard syntax for internationalized messages that adapt to language-specific grammar — pluralization, gender, number, and date formatting — using CLDR rules, so one message string renders correctly in every language.

ICU MessageFormat is a syntax for creating internationalized messages that handle pluralization, gender, and other language-specific variations. Developed by theInternational Components for Unicode (ICU)project, it's the industry standard for complex i18n messages.

Unlike simple string interpolation, ICU MessageFormat handles the grammatical complexities of different languages. For example, Russian has different plural forms for 1, 2-4, 5-20, and 21+ items. ICU MessageFormat handles all of this automatically based on CLDR (Common Locale Data Repository) rules.

Simple Arguments

Insert dynamic values into your messages using curly braces.

Syntax

{argumentName}

Examples

Basic variable interpolation

Code

Hello, {name}!

Output

Hello, John!

Multiple variables

Code

Welcome back, {firstName} {lastName}!

Output

Welcome back, John Doe!

Plural Format

Handle singular and plural forms based on a numeric value. ICU uses CLDR plural rules which vary by language.

Syntax

{count, plural, =0 {zero} one {singular} other {plural}}

Examples

Basic plural with exact match (=0) and categories

Code

{count, plural,
  =0 {No messages}
  one {1 message}
  other {# messages}
}

Output

5 messages

Plural embedded in a sentence

Code

You have {count, plural,
  =0 {no items}
  one {one item}
  other {# items}
} in your cart.

Output

You have 3 items in your cart.

Plural with offset for "You and X others" pattern

Code

{count, plural, offset:1
  =0 {Nobody liked this}
  =1 {You liked this}
  one {You and 1 other liked this}
  other {You and # others liked this}
}

Output

You and 4 others liked this

Notes

•The # symbol is replaced with the formatted number
•CLDR plural categories: zero, one, two, few, many, other
•English only uses "one" and "other", but Arabic has all 6 categories
•Exact matches (=0, =1, =2) take precedence over categories
•The "other" category is required and acts as the fallback

Select Format

Choose different text based on a string value. Commonly used for gender-aware messages.

Syntax

{value, select, option1 {text1} option2 {text2} other {default}}

Examples

Gender-aware pronouns

Code

{gender, select,
  male {He}
  female {She}
  other {They}
} liked your photo.

Output

She liked your photo.

Role-based messaging

Code

{role, select,
  admin {Full access granted}
  editor {Edit access granted}
  viewer {Read-only access}
  other {Limited access}
}

Output

Full access granted

Status messages

Code

{status, select,
  active {Your account is active}
  pending {Verification pending}
  suspended {Account suspended - contact support}
  other {Unknown status}
}

Output

Your account is active

Notes

•The "other" option is required as a fallback
•Option names must be valid identifiers (no spaces or special characters)
•Select is case-sensitive - "Male" and "male" are different options

Select Ordinal Format

Format ordinal numbers (1st, 2nd, 3rd, 4th, etc.) with proper suffixes.

Syntax

{count, selectordinal, one {#st} two {#nd} few {#rd} other {#th}}

Examples

Ranking/position display

Code

You finished in {place, selectordinal,
  one {#st}
  two {#nd}
  few {#rd}
  other {#th}
} place!

Output

You finished in 1st place!

Attempt counter

Code

This is your {attempt, selectordinal,
  one {#st}
  two {#nd}
  few {#rd}
  other {#th}
} attempt.

Output

This is your 3rd attempt.

Notes

•Uses CLDR ordinal plural rules, not cardinal rules
•In English: 1st, 2nd, 3rd, then 4th-20th, then 21st, 22nd, etc.
•Other languages have different ordinal patterns

Number Format

Format numbers with locale-aware formatting, including currencies, percentages, and compact notation.

Syntax

{value, number} or {value, number, style}

Examples

Basic number formatting with thousands separator

Code

Total: {amount, number}

Output

Total: 1,234,567

Currency formatting

Code

Price: {price, number, ::currency/USD}

Output

Price: $99.99

Percentage formatting

Code

Discount: {discount, number, ::percent}

Output

Discount: 25%

Compact notation for large numbers

Code

{views, number, ::compact-short} views

Output

1.2M views

Fixed decimal places

Code

Rating: {rating, number, ::.0}

Output

Rating: 4.5

Notes

•Number skeleton syntax starts with ::
•Common currency codes: USD, EUR, GBP, JPY, CNY
•Compact notations: compact-short (1.2M), compact-long (1.2 million)
•Formatting is locale-aware (1,234.56 in en-US vs 1.234,56 in de-DE)

Date and Time Format

Format dates and times with locale-aware patterns.

Syntax

{value, date, style} or {value, time, style}

Examples

Short date format

Code

Date: {date, date, short}

Output

Date: 1/15/24

Medium date format

Code

Date: {date, date, medium}

Output

Date: Jan 15, 2024

Long date format

Code

Date: {date, date, long}

Output

Date: January 15, 2024

Full date format with weekday

Code

Date: {date, date, full}

Output

Date: Monday, January 15, 2024

Short time format

Code

Time: {time, time, short}

Output

Time: 3:30 PM

Combined date and time

Code

Posted on {date, date, medium} at {date, time, short}

Output

Posted on Jan 15, 2024 at 3:30 PM

Notes

•Available styles: short, medium, long, full
•Date/time formatting is fully locale-aware
•The same date value can be used for both date and time formatting

Rich Text / Tags

Use XML-like tags for rich text formatting. Tags are passed as functions in your code.

Syntax

<tagName>content</tagName>

Examples

Inline links

Code

Read our <link>Terms of Service</link>.

Output

Read our Terms of Service (as a link).

Bold text

Code

This is <b>very important</b> information.

Output

This is very important information (bold).

Call-to-action link

Code

Don't have an account? <signup>Sign up free</signup>

Output

Don't have an account? Sign up free (as a link)

Keyboard shortcuts

Code

Press <kbd>Ctrl</kbd> + <kbd>S</kbd> to save.

Output

Press Ctrl + S to save (keyboard keys styled).

Notes

•Tags are passed as render functions in your code
•Tags can wrap variables: <b>{name}</b>
•Tags can be nested: <b><link>text</link></b>
•Self-closing tags are not supported

Escaping Special Characters

Escape special characters when you need them as literal text.

Syntax

Use '' to escape or wrap in quotes

Examples

Escape single quote with double quote

Code

It''s a beautiful day!

Output

It's a beautiful day!

Escape curly braces

Code

Use ''{'' and ''}}'' for literal braces.

Output

Use { and } for literal braces.

Show literal variable syntax

Code

The variable is called '{name}'.

Output

The variable is called {name}.

Notes

•Single quotes are used for escaping in ICU
•Double the single quote ('') to get a literal single quote
•Text inside single quotes is treated as literal

Nested Messages

Combine multiple ICU constructs for complex messages.

Syntax

Nest plural, select, and other formats inside each other

Examples

Gender + Plural combination

Code

{gender, select,
  male {{count, plural,
    =0 {He has no messages}
    one {He has 1 message}
    other {He has # messages}
  }}
  female {{count, plural,
    =0 {She has no messages}
    one {She has 1 message}
    other {She has # messages}
  }}
  other {{count, plural,
    =0 {They have no messages}
    one {They have 1 message}
    other {They have # messages}
  }}
}

Output

She has 5 messages

Conditional with formatted numbers

Code

{taxable, select,
  yes {Total: {total, number, ::currency/USD} (incl. {tax, number, ::currency/USD} tax)}
  other {Total: {total, number, ::currency/USD}}
}

Output

Total: $99.99 (incl. $8.00 tax)

Notes

•Nesting can make messages complex - consider breaking into separate keys
•Each nested level must have its own opening and closing braces
•The "other" case is always required in nested constructs

CLDR Plural Rules by Language

Different languages have different plural categories. ICU uses CLDR (Common Locale Data Repository) rules to determine which category applies for any given number.

Language	Plural Categories	Example
English	`one, other`	1 item, 2 items
French	`one, other`	0 élément, 1 élément, 2 éléments
Russian	`one, few, many, other`	1 сообщение, 2 сообщения, 5 сообщений
Arabic	`zero, one, two, few, many, other`	All 6 forms used
Japanese	`other`	No plural forms
Polish	`one, few, many, other`	1 wiadomość, 2 wiadomości, 5 wiadomości

Important: Always include the "other" category as a fallback. Some languages like Japanese have only "other", while Arabic uses all 6 categories.View all CLDR plural rules →

Using ICU in Popular Libraries

ICU MessageFormat is supported by most major i18n libraries. Here's how to use it:

react-intl / FormatJS

import { FormattedMessage } from 'react-intl';

<FormattedMessage
  id="cart.items"
  defaultMessage="{count, plural, one {# item} other {# items}}"
  values={{ count: 5 }}
/>

next-intl

import { useTranslation } from '@/hooks/useTranslation';

const { t } = useTranslation('cart');
t('items', { count: 5 });
// messages: { "cart": { "items": "{count, plural, one {# item} other {# items}}" } }

i18next (with ICU plugin)

import i18next from 'i18next';
import ICU from 'i18next-icu';

i18next.use(ICU).init({ ... });
i18next.t('cart.items', { count: 5 });

@formatjs/intl

import { createIntl } from '@formatjs/intl';

const intl = createIntl({ locale: 'en', messages });
intl.formatMessage(
  { id: 'cart.items' },
  { count: 5 }
);

Best Practices

Do

✓Always include the "other" fallback case
✓Use # for the formatted number in plurals
✓Test with various numbers (0, 1, 2, 5, 11, 21)
✓Keep messages readable - break complex ones into parts
✓Use descriptive variable names

Don't

✗Forget "other" - it causes runtime errors
✗Assume all languages have the same plural forms
✗Nest more than 2-3 levels deep
✗Use string concatenation for i18n messages
✗Hardcode number/date formats

Frequently Asked Questions

What is ICU MessageFormat?

ICU MessageFormat is a standardized syntax for writing internationalized messages that adapt to language-specific grammar — pluralization, gender, number, and date formatting. Developed by the Unicode ICU project, it uses CLDR plural rules so a single message string renders correctly across languages, from English (one, other) to Arabic (all six plural categories).

What is ICU plural syntax and how does it work?

ICU plural syntax is written as {count, plural, one {# item} other {# items}}, where the engine picks a branch based on the CLDR plural category for the number. The # symbol is replaced by the formatted number, exact matches like =0 take precedence over categories, and the "other" branch is required as a fallback. Categories are zero, one, two, few, many, and other.

How is the ICU select format different from plural?

The select format chooses text based on a string value rather than a number, written as {gender, select, male {He} female {She} other {They}}. It is most often used for gender-aware pronouns or role/status messages. Like plural, an "other" branch is required, and option names are case-sensitive identifiers.

How do I escape curly braces and apostrophes in ICU MessageFormat?

Wrap literal text in single quotes to escape special characters: '{' and '}' produce literal braces, and a doubled single quote ('') produces a literal apostrophe — for example "It''s a beautiful day!". Text inside single quotes is treated as literal and is not parsed as ICU syntax.

Which i18n libraries support ICU MessageFormat?

react-intl (FormatJS) and @formatjs/intl support ICU MessageFormat natively. next-intl supports it out of the box, and i18next supports it through the i18next-icu plugin. The same ICU string works across all of them, which makes ICU a portable choice for shared message catalogs.

Is ICU MessageFormat better than simple string interpolation?

Yes, for any user-facing text that varies by count, gender, or locale. Simple interpolation cannot express that Russian has four plural forms or that French and English pluralize differently, so it produces grammatically wrong translations. ICU MessageFormat encodes those rules in one string, which is why it is the industry standard for complex i18n.

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ICU MessageFormat Syntax Guide 2026 (with Examples) | IntlPull