diff --git a/src/wp-includes/js/media-views.js b/src/wp-includes/js/media-views.js
index 7be686827a..0e1ea805ec 100644
--- a/src/wp-includes/js/media-views.js
+++ b/src/wp-includes/js/media-views.js
@@ -400,13 +400,18 @@ var Controller = wp.media.controller,
 CustomizeImageCropper = Controller.Cropper.extend({
 	doCrop: function( attachment ) {
 		var cropDetails = attachment.get( 'cropDetails' ),
-			control = this.get( 'control' );
+			control = this.get( 'control' ),
+			ratio = cropDetails.width / cropDetails.height;
 
-		if ( ! control.params.flex_width ) {
-			cropDetails.dst_width = control.params.width;
-		}
-		if ( ! control.params.flex_height ) {
-			cropDetails.dst_height = control.params.height;
+		// Use crop measurements when flexible in both directions.
+		if ( control.params.flex_width && control.params.flex_height ) {
+			cropDetails.dst_width  = cropDetails.width;
+			cropDetails.dst_height = cropDetails.height;
+
+		// Constrain flexible side based on image ratio and size of the fixed side.
+		} else {
+			cropDetails.dst_width  = control.params.flex_width  ? control.params.height * ratio : control.params.width;
+			cropDetails.dst_height = control.params.flex_height ? control.params.width  / ratio : control.params.height;
 		}
 
 		return wp.ajax.post( 'crop-image', {
diff --git a/src/wp-includes/js/media/controllers/customize-image-cropper.js b/src/wp-includes/js/media/controllers/customize-image-cropper.js
index df2d77baca..3456ba36ff 100644
--- a/src/wp-includes/js/media/controllers/customize-image-cropper.js
+++ b/src/wp-includes/js/media/controllers/customize-image-cropper.js
@@ -14,13 +14,18 @@ var Controller = wp.media.controller,
 CustomizeImageCropper = Controller.Cropper.extend({
 	doCrop: function( attachment ) {
 		var cropDetails = attachment.get( 'cropDetails' ),
-			control = this.get( 'control' );
+			control = this.get( 'control' ),
+			ratio = cropDetails.width / cropDetails.height;
 
-		if ( ! control.params.flex_width ) {
-			cropDetails.dst_width = control.params.width;
-		}
-		if ( ! control.params.flex_height ) {
-			cropDetails.dst_height = control.params.height;
+		// Use crop measurements when flexible in both directions.
+		if ( control.params.flex_width && control.params.flex_height ) {
+			cropDetails.dst_width  = cropDetails.width;
+			cropDetails.dst_height = cropDetails.height;
+
+		// Constrain flexible side based on image ratio and size of the fixed side.
+		} else {
+			cropDetails.dst_width  = control.params.flex_width  ? control.params.height * ratio : control.params.width;
+			cropDetails.dst_height = control.params.flex_height ? control.params.width  / ratio : control.params.height;
 		}
 
 		return wp.ajax.post( 'crop-image', {