diff --git a/lib/compat/wordpress-6.1/class-wp-theme-json-6-1.php b/lib/compat/wordpress-6.1/class-wp-theme-json-6-1.php
index 28a16190d00a1..fbc6a2cdd3094 100644
--- a/lib/compat/wordpress-6.1/class-wp-theme-json-6-1.php
+++ b/lib/compat/wordpress-6.1/class-wp-theme-json-6-1.php
@@ -1212,7 +1212,7 @@ public function set_spacing_sizes() {
 			$below_sizes[] = array(
 				/* translators: %s: Multiple of t-shirt sizing, eg. 2X-Small */
 				'name' => $x === $steps_mid_point - 1 ? __( 'Small', 'gutenberg' ) : sprintf( __( '%sX-Small', 'gutenberg' ), strval( $x_small_count ) ),
-				'slug' => $slug,
+				'slug' => (string) $slug,
 				'size' => round( $current_step, 2 ) . $unit,
 			);
 
@@ -1231,7 +1231,7 @@ public function set_spacing_sizes() {
 
 		$below_sizes[] = array(
 			'name' => __( 'Medium', 'gutenberg' ),
-			'slug' => 50,
+			'slug' => '50',
 			'size' => $spacing_scale['mediumStep'] . $unit,
 		);
 
@@ -1249,7 +1249,7 @@ public function set_spacing_sizes() {
 			$above_sizes[] = array(
 				/* translators: %s: Multiple of t-shirt sizing, eg. 2X-Large */
 				'name' => 0 === $x ? __( 'Large', 'gutenberg' ) : sprintf( __( '%sX-Large', 'gutenberg' ), strval( $x_large_count ) ),
-				'slug' => $slug,
+				'slug' => (string) $slug,
 				'size' => round( $current_step, 2 ) . $unit,
 			);
 
diff --git a/packages/block-editor/src/components/spacing-sizes-control/test/utils.js b/packages/block-editor/src/components/spacing-sizes-control/test/utils.js
index a820927a2a073..f893ce021e3cf 100644
--- a/packages/block-editor/src/components/spacing-sizes-control/test/utils.js
+++ b/packages/block-editor/src/components/spacing-sizes-control/test/utils.js
@@ -57,7 +57,7 @@ describe( 'getSpacingPresetSlug', () => {
 		expect( getSpacingPresetSlug( 'default' ) ).toBe( 'default' );
 	} );
 	it( 'should return the int value of the slug portion of a valid preset var', () => {
-		expect( getSpacingPresetSlug( 'var:preset|spacing|20' ) ).toBe( 20 );
+		expect( getSpacingPresetSlug( 'var:preset|spacing|20' ) ).toBe( '20' );
 	} );
 } );
 
diff --git a/packages/block-editor/src/components/spacing-sizes-control/utils.js b/packages/block-editor/src/components/spacing-sizes-control/utils.js
index 12071e3c63dc4..27e6f2e7609ef 100644
--- a/packages/block-editor/src/components/spacing-sizes-control/utils.js
+++ b/packages/block-editor/src/components/spacing-sizes-control/utils.js
@@ -36,7 +36,9 @@ export function getCustomValueFromPreset( value, spacingSizes ) {
 	}
 
 	const slug = getSpacingPresetSlug( value );
-	const spacingSize = spacingSizes.find( ( size ) => size.slug === slug );
+	const spacingSize = spacingSizes.find(
+		( size ) => String( size.slug ) === slug
+	);
 
 	return spacingSize?.size;
 }
@@ -80,7 +82,7 @@ export function getSpacingPresetSlug( value ) {
 
 	const slug = value.match( /var:preset\|spacing\|(.+)/ );
 
-	return slug ? parseInt( slug[ 1 ], 10 ) : undefined;
+	return slug ? slug[ 1 ] : undefined;
 }
 
 /**
@@ -100,7 +102,7 @@ export function getSliderValueFromPreset( presetValue, spacingSizes ) {
 			? '0'
 			: getSpacingPresetSlug( presetValue );
 	const sliderValue = spacingSizes.findIndex( ( spacingSize ) => {
-		return spacingSize.slug === slug;
+		return String( spacingSize.slug ) === slug;
 	} );
 
 	// Returning NaN rather than undefined as undefined makes range control thumb sit in center
diff --git a/phpunit/class-wp-theme-json-test.php b/phpunit/class-wp-theme-json-test.php
index 92ab586dbf7d6..993675fd4d4fd 100644
--- a/phpunit/class-wp-theme-json-test.php
+++ b/phpunit/class-wp-theme-json-test.php
@@ -780,7 +780,7 @@ function data_generate_spacing_scale_fixtures() {
 				'expected_output' => array(
 					array(
 						'name' => 'Medium',
-						'slug' => 50,
+						'slug' => '50',
 						'size' => '4rem',
 					),
 				),
@@ -797,12 +797,12 @@ function data_generate_spacing_scale_fixtures() {
 				'expected_output' => array(
 					array(
 						'name' => 'Medium',
-						'slug' => 50,
+						'slug' => '50',
 						'size' => '4rem',
 					),
 					array(
 						'name' => 'Large',
-						'slug' => 60,
+						'slug' => '60',
 						'size' => '5.5rem',
 					),
 				),
@@ -819,17 +819,17 @@ function data_generate_spacing_scale_fixtures() {
 				'expected_output' => array(
 					array(
 						'name' => 'Small',
-						'slug' => 40,
+						'slug' => '40',
 						'size' => '2.5rem',
 					),
 					array(
 						'name' => 'Medium',
-						'slug' => 50,
+						'slug' => '50',
 						'size' => '4rem',
 					),
 					array(
 						'name' => 'Large',
-						'slug' => 60,
+						'slug' => '60',
 						'size' => '5.5rem',
 					),
 				),
@@ -846,22 +846,22 @@ function data_generate_spacing_scale_fixtures() {
 				'expected_output' => array(
 					array(
 						'name' => 'Small',
-						'slug' => 40,
+						'slug' => '40',
 						'size' => '2.5rem',
 					),
 					array(
 						'name' => 'Medium',
-						'slug' => 50,
+						'slug' => '50',
 						'size' => '4rem',
 					),
 					array(
 						'name' => 'Large',
-						'slug' => 60,
+						'slug' => '60',
 						'size' => '5.5rem',
 					),
 					array(
 						'name' => 'X-Large',
-						'slug' => 70,
+						'slug' => '70',
 						'size' => '7rem',
 					),
 				),
@@ -878,27 +878,27 @@ function data_generate_spacing_scale_fixtures() {
 				'expected_output' => array(
 					array(
 						'name' => 'Small',
-						'slug' => 40,
+						'slug' => '40',
 						'size' => '2.5rem',
 					),
 					array(
 						'name' => 'Medium',
-						'slug' => 50,
+						'slug' => '50',
 						'size' => '5rem',
 					),
 					array(
 						'name' => 'Large',
-						'slug' => 60,
+						'slug' => '60',
 						'size' => '7.5rem',
 					),
 					array(
 						'name' => 'X-Large',
-						'slug' => 70,
+						'slug' => '70',
 						'size' => '10rem',
 					),
 					array(
 						'name' => '2X-Large',
-						'slug' => 80,
+						'slug' => '80',
 						'size' => '12.5rem',
 					),
 				),
@@ -915,27 +915,27 @@ function data_generate_spacing_scale_fixtures() {
 				'expected_output' => array(
 					array(
 						'name' => 'X-Small',
-						'slug' => 30,
+						'slug' => '30',
 						'size' => '0.67rem',
 					),
 					array(
 						'name' => 'Small',
-						'slug' => 40,
+						'slug' => '40',
 						'size' => '1rem',
 					),
 					array(
 						'name' => 'Medium',
-						'slug' => 50,
+						'slug' => '50',
 						'size' => '1.5rem',
 					),
 					array(
 						'name' => 'Large',
-						'slug' => 60,
+						'slug' => '60',
 						'size' => '2.25rem',
 					),
 					array(
 						'name' => 'X-Large',
-						'slug' => 70,
+						'slug' => '70',
 						'size' => '3.38rem',
 					),
 				),
@@ -952,27 +952,27 @@ function data_generate_spacing_scale_fixtures() {
 				'expected_output' => array(
 					array(
 						'name' => 'X-Small',
-						'slug' => 30,
+						'slug' => '30',
 						'size' => '0.09rem',
 					),
 					array(
 						'name' => 'Small',
-						'slug' => 40,
+						'slug' => '40',
 						'size' => '0.38rem',
 					),
 					array(
 						'name' => 'Medium',
-						'slug' => 50,
+						'slug' => '50',
 						'size' => '1.5rem',
 					),
 					array(
 						'name' => 'Large',
-						'slug' => 60,
+						'slug' => '60',
 						'size' => '6rem',
 					),
 					array(
 						'name' => 'X-Large',
-						'slug' => 70,
+						'slug' => '70',
 						'size' => '24rem',
 					),
 				),
diff --git a/schemas/json/theme.json b/schemas/json/theme.json
index 25a973d26ff5a..9fef064661d05 100644
--- a/schemas/json/theme.json
+++ b/schemas/json/theme.json
@@ -243,7 +243,7 @@
 										"type": "string"
 									},
 									"slug": {
-										"description": "Kebab-case unique identifier for the space size preset.",
+										"description": "Unique identifier for the space size preset. For best cross theme compatibility these should be in the form '10','20','30','40','50','60', etc. with '50' representing the 'Medium' size step.",
 										"type": "string"
 									},
 									"size": {