diff --git a/sys/amd64/include/vmparam.h b/sys/amd64/include/vmparam.h
index 755a7d9824e0..b2092589cce6 100644
--- a/sys/amd64/include/vmparam.h
+++ b/sys/amd64/include/vmparam.h
@@ -202,7 +202,8 @@
  * Ceiling on amount of kmem_map kva space.
  */
 #ifndef VM_KMEM_SIZE_MAX
-#define	VM_KMEM_SIZE_MAX	(400 * 1024 * 1024)
+#define	VM_KMEM_SIZE_MAX	((VM_MAX_KERNEL_ADDRESS - \
+    VM_MIN_KERNEL_ADDRESS + 1) * 3 / 5)
 #endif
 
 /* initial pagein size of beginning of executable file */