diff --git a/builtin/providers/azurerm/resource_arm_managed_disk.go b/builtin/providers/azurerm/resource_arm_managed_disk.go
index 3cd677252..8aa64bcb5 100644
--- a/builtin/providers/azurerm/resource_arm_managed_disk.go
+++ b/builtin/providers/azurerm/resource_arm_managed_disk.go
@@ -79,7 +79,7 @@ func resourceArmManagedDisk() *schema.Resource {
 
 			"disk_size_gb": {
 				Type:         schema.TypeInt,
-				Optional:     true,
+				Required:     true,
 				ValidateFunc: validateDiskSizeGB,
 			},