diff --git a/core-modules/000QubesVm.py b/core-modules/000QubesVm.py
index b9b32bdd..1e087196 100644
--- a/core-modules/000QubesVm.py
+++ b/core-modules/000QubesVm.py
@@ -1738,6 +1738,26 @@ class QubesVm(object):
             self.force_shutdown()
             raise OSError("ERROR: Cannot execute qubesdb-daemon!")
 
+    def request_memory(self, mem_required = None):
+        # Overhead of per-VM/per-vcpu Xen structures, taken from OpenStack nova/virt/xenapi/driver.py
+        # see https://wiki.openstack.org/wiki/XenServer/Overhead
+        # add an extra MB because Nova rounds up to MBs
+        MEM_OVERHEAD_BASE = (3 + 1) * 1024 * 1024
+        MEM_OVERHEAD_PER_VCPU = 3 * 1024 * 1024 / 2
+        if mem_required is None:
+            mem_required = int(self.memory) * 1024 * 1024
+        if qmemman_present:
+            qmemman_client = QMemmanClient()
+            try:
+                mem_required_with_overhead = mem_required + MEM_OVERHEAD_BASE + self.vcpus * MEM_OVERHEAD_PER_VCPU
+                got_memory = qmemman_client.request_memory(mem_required_with_overhead)
+            except IOError as e:
+                raise IOError("ERROR: Failed to connect to qmemman: %s" % str(e))
+            if not got_memory:
+                qmemman_client.close()
+                raise MemoryError ("ERROR: insufficient memory to start VM '%s'" % self.name)
+            return qmemman_client
+
     def start(self, verbose = False, preparing_dvm = False, start_guid = True,
             notify_function = None, mem_required = None):
         self.log.debug('start('
@@ -1765,17 +1785,7 @@ class QubesVm(object):
 
         self._update_libvirt_domain()
 
-        if mem_required is None:
-            mem_required = int(self.memory) * 1024 * 1024
-        if qmemman_present:
-            qmemman_client = QMemmanClient()
-            try:
-                got_memory = qmemman_client.request_memory(mem_required)
-            except IOError as e:
-                raise IOError("ERROR: Failed to connect to qmemman: %s" % str(e))
-            if not got_memory:
-                qmemman_client.close()
-                raise MemoryError ("ERROR: insufficient memory to start VM '%s'" % self.name)
+        qmemman_client = self.request_memory(mem_required)
 
         # Bind pci devices to pciback driver
         for pci in self.pcidevs:
diff --git a/core-modules/01QubesDisposableVm.py b/core-modules/01QubesDisposableVm.py
index 68b4319f..a758dd74 100644
--- a/core-modules/01QubesDisposableVm.py
+++ b/core-modules/01QubesDisposableVm.py
@@ -173,17 +173,7 @@ class QubesDisposableVm(QubesVm):
         # refresh config file
         domain_config = self.create_config_file()
 
-        if qmemman_present:
-            mem_required = int(self.memory) * 1024 * 1024
-            print >>sys.stderr, "time=%s, getting %d memory" % (str(time.time()), mem_required)
-            qmemman_client = QMemmanClient()
-            try:
-                got_memory = qmemman_client.request_memory(mem_required)
-            except IOError as e:
-                raise IOError("ERROR: Failed to connect to qmemman: %s" % str(e))
-            if not got_memory:
-                qmemman_client.close()
-                raise MemoryError ("ERROR: insufficient memory to start VM '%s'" % self.name)
+        qmemman_client = self.request_memory()
 
         # dispvm cannot have PCI devices
         assert (len(self.pcidevs) == 0), "DispVM cannot have PCI devices"
diff --git a/qmemman/qmemman.py b/qmemman/qmemman.py
index 3de81d75..d40c0b0a 100755
--- a/qmemman/qmemman.py
+++ b/qmemman/qmemman.py
@@ -56,7 +56,11 @@ class SystemState(object):
         self.BALOON_DELAY = 0.1
         self.XEN_FREE_MEM_LEFT = 50*1024*1024
         self.XEN_FREE_MEM_MIN = 25*1024*1024
-        self.ALL_PHYS_MEM = self.xc.physinfo()['total_memory']*1024
+        # Overhead of per-page Xen structures, taken from OpenStack nova/virt/xenapi/driver.py
+        # see https://wiki.openstack.org/wiki/XenServer/Overhead
+        # we divide total and free physical memory by this to get "assignable" memory
+        self.MEM_OVERHEAD_FACTOR = 1.0 / 1.00781
+        self.ALL_PHYS_MEM = int(self.xc.physinfo()['total_memory']*1024 * self.MEM_OVERHEAD_FACTOR)
 
     def add_domain(self, id):
         self.log.debug('add_domain(id={!r})'.format(id))
@@ -67,7 +71,7 @@ class SystemState(object):
         self.domdict.pop(id)
 
     def get_free_xen_memory(self):
-        return self.xc.physinfo()['free_memory']*1024
+        return int(self.xc.physinfo()['free_memory']*1024 * self.MEM_OVERHEAD_FACTOR)
 #        hosts = self.xend_session.session.xenapi.host.get_all()
 #        host_record = self.xend_session.session.xenapi.host.get_record(hosts[0])
 #        host_metrics_record = self.xend_session.session.xenapi.host_metrics.get_record(host_record["metrics"])