diff --git a/pym/calculate/lib/utils/partition.py b/pym/calculate/lib/utils/partition.py
index 6fdf725..f5c9281 100644
--- a/pym/calculate/lib/utils/partition.py
+++ b/pym/calculate/lib/utils/partition.py
@@ -230,7 +230,7 @@ class VirtualDisk(SizeableDisk, DosDisk, GptDisk):
             self.mount = mount
             self.part = part
             self.partid = partid
-            self.size = round_disk_size(size)
+            self.size = size
             self.format = ""
 
     def __init__(self, parent, dev, size):
@@ -330,6 +330,7 @@ class VirtualDisk(SizeableDisk, DosDisk, GptDisk):
 
 class VirtualDosDisk(VirtualDisk, DosDisk):
     parttype = "dos"
+    header_size = Sizes().M
 
     def write(self):
         res = super(VirtualDosDisk, self).write()
@@ -338,6 +339,7 @@ class VirtualDosDisk(VirtualDisk, DosDisk):
 
 class VirtualGptDisk(VirtualDisk, GptDisk):
     parttype = "gpt"
+    header_size = Sizes().M + Sizes().Sector * 34
 
 class VirtualLvm(SizeableDisk, LvmDisk):