diff --git a/README.md b/README.md
index ace749b..baa5b51 100644
--- a/README.md
+++ b/README.md
@@ -228,6 +228,11 @@ Twig filters.
 {{ '<b>bold</b> <strong>strong</strong>'|check_markup('restricted_html') }}
 ```
 
+### Format size
+```twig
+{{ 12345|format_size() }}
+```
+
 ### Truncate
 ```twig
 {# Truncates a UTF-8-encoded string safely to 10 characters. #}
diff --git a/src/TwigTweakExtension.php b/src/TwigTweakExtension.php
index 404a3b0..c298446 100644
--- a/src/TwigTweakExtension.php
+++ b/src/TwigTweakExtension.php
@@ -80,6 +80,7 @@ class TwigTweakExtension extends AbstractExtension {
       new TwigFilter('image_style', [self::class, 'imageStyleFilter']),
       new TwigFilter('transliterate', [self::class, 'transliterateFilter']),
       new TwigFilter('check_markup', 'check_markup'),
+      new TwigFilter('format_size', 'format_size'),
       new TwigFilter('truncate', [Unicode::class, 'truncate']),
       new TwigFilter('view', [self::class, 'viewFilter']),
       new TwigFilter('with', [self::class, 'withFilter']),
diff --git a/tests/src/Functional/TwigTweakTest.php b/tests/src/Functional/TwigTweakTest.php
index e85ea68..a23fe6e 100644
--- a/tests/src/Functional/TwigTweakTest.php
+++ b/tests/src/Functional/TwigTweakTest.php
@@ -310,6 +310,10 @@ final class TwigTweakTest extends BrowserTestBase {
     $xpath = '//div[@class = "tt-check-markup"]';
     self::assertEquals('<b>bold</b> strong', trim($this->xpath($xpath)[0]->getHtml()));
 
+    // -- Format size.
+    $xpath = '//div[@class = "tt-format-size"]';
+    self::assertSame('12.06 KB', $this->xpath($xpath)[0]->getHtml());
+
     // -- Truncate.
     $xpath = '//div[@class = "tt-truncate" and text() = "Hello…"]';
     $this->assertXpath($xpath);
diff --git a/tests/twig_tweak_test/templates/twig-tweak-test.html.twig b/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
index 99201a1..dcc9f28 100644
--- a/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
+++ b/tests/twig_tweak_test/templates/twig-tweak-test.html.twig
@@ -64,6 +64,7 @@
   <div class="tt-image-style">{{ 'public://images/ocean.jpg'|image_style('thumbnail') }}</div>
   <div class="tt-transliterate">{{ 'Привет!'|transliterate('ru') }}</div>
   <div class="tt-check-markup">{{ '<b>bold</b> <strong>strong</strong>'|check_markup('twig_tweak_test') }}</div>
+  <div class="tt-format-size">{{ 12345|format_size() }}</div>
   <div class="tt-truncate">{{ 'Hello world!'|truncate(10, true, true) }}</div>
   <div class="tt-with">{{ {'#markup':'Example'}|with('#prefix', '<b>')|with('#suffix', '</b>') }}</div>
   <div class="tt-with-nested">{{ {alpha: {beta: {gamma: 123}}}|with(['alpha', 'beta', 'gamma'], 456)|json_encode|replace({'"':''}) }}</div>