diff --git a/packages/block-library/src/calendar/block.json b/packages/block-library/src/calendar/block.json
index 6ba8f7d725e6e5..0735148fdc8973 100644
--- a/packages/block-library/src/calendar/block.json
+++ b/packages/block-library/src/calendar/block.json
@@ -40,6 +40,18 @@
 		},
 		"interactivity": {
 			"clientNavigation": true
+		},
+		"__experimentalBorder": {
+			"radius": true,
+			"color": true,
+			"width": true,
+			"style": true,
+			"__experimentalDefaultControls": {
+				"radius": false,
+				"color": false,
+				"width": false,
+				"style": false
+			}
 		}
 	},
 	"style": "wp-block-calendar"
diff --git a/packages/block-library/src/calendar/style.scss b/packages/block-library/src/calendar/style.scss
index 969d1aafec99ef..d68d47a5fa4836 100644
--- a/packages/block-library/src/calendar/style.scss
+++ b/packages/block-library/src/calendar/style.scss
@@ -1,6 +1,7 @@
 .wp-block-calendar {
 	text-align: center;
-
+	// This block has customizable borders. Border-box sizing makes that more predictable.
+	box-sizing: border-box;
 	th,
 	td {
 		padding: 0.25em;