diff --git a/src/Pure/System/java_statistics.scala b/src/Pure/System/java_statistics.scala
--- a/src/Pure/System/java_statistics.scala
+++ b/src/Pure/System/java_statistics.scala
@@ -1,45 +1,45 @@
 /*  Title:      Pure/System/java_statistics.scala
     Author:     Makarius
 
 Java runtime statistics.
 */
 
 package isabelle
 
 
 object Java_Statistics
 {
   /* memory status */
 
   sealed case class Memory_Status(heap_size: Long, heap_free: Long)
   {
     def heap_used: Long = (heap_size - heap_free) max 0
     def heap_used_fraction: Double =
       if (heap_size == 0) 0.0 else heap_used.toDouble / heap_size
   }
 
   def memory_status(): Memory_Status =
   {
     val heap_size = Runtime.getRuntime.totalMemory()
-    val heap_used = heap_size - Runtime.getRuntime.freeMemory()
-    Memory_Status(heap_size, heap_used)
+    val heap_free = Runtime.getRuntime.freeMemory()
+    Memory_Status(heap_size, heap_free)
   }
 
 
   /* JVM statistics */
 
   def jvm_statistics(): Properties.T =
   {
     val status = memory_status()
     val threads = Thread.activeCount()
     val workers = Isabelle_Thread.pool.getPoolSize
     val workers_active = Isabelle_Thread.pool.getActiveCount
 
     List(
       "java_heap_size" -> status.heap_size.toString,
       "java_heap_used" -> status.heap_used.toString,
       "java_threads_total" -> threads.toString,
       "java_workers_total" -> workers.toString,
       "java_workers_active" -> workers_active.toString)
   }
 }