diff --git a/macros/src/main/scala/MacroCompiler.scala b/macros/src/main/scala/MacroCompiler.scala
index d26f5912..642b85ef 100644
--- a/macros/src/main/scala/MacroCompiler.scala
+++ b/macros/src/main/scala/MacroCompiler.scala
@@ -32,7 +32,9 @@ trait CostMetric {
   def cost(mem: Macro, lib: Macro): Option[BigInt]
 }
 
-/** Some default cost functions. */
+// Some default cost functions.
+
+/** Palmer's old metric. */
 object PalmerMetric extends CostMetric {
   override def cost(mem: Macro, lib: Macro): Option[BigInt] = {
     /* Palmer: A quick cost function (that must be kept in sync with
@@ -45,7 +47,43 @@ object PalmerMetric extends CostMetric {
   }
 }
 
-// The current default metric in barstools, re-defined by Donggyu.
+/**
+ * An external cost function.
+ * Calls the specified path with paths to the JSON MDF representation of the mem
+ * and lib macros. The external executable should return a BigInt.
+ * None will be returned if the external executable does not return a valid
+ * BigInt.
+ */
+class ExternalMetric(path: String) extends CostMetric {
+  import mdf.macrolib.Utils.writeMacroToPath
+  import java.io._
+  import scala.language.postfixOps // for !! postfix op
+  import sys.process._
+
+  override def cost(mem: Macro, lib: Macro): Option[BigInt] = {
+    // Create temporary files.
+    val memFile = File.createTempFile("_macrocompiler_mem_", ".json")
+    val libFile = File.createTempFile("_macrocompiler_lib_", ".json")
+
+    writeMacroToPath(Some(memFile.getAbsolutePath), mem.src)
+    writeMacroToPath(Some(libFile.getAbsolutePath), lib.src)
+
+    // !! executes the given command
+    val result: String = (s"${path} ${memFile.getAbsolutePath} ${libFile.getAbsolutePath}" !!).trim
+
+    // Remove temporary files.
+    memFile.delete()
+    libFile.delete()
+
+    try {
+      Some(BigInt(result))
+    } catch {
+      case e: NumberFormatException => None
+    }
+  }
+}
+
+/** The current default metric in barstools, re-defined by Donggyu. */
 object NewDefaultMetric extends CostMetric {
   override def cost(mem: Macro, lib: Macro): Option[BigInt] = {
     val memMask = mem.src.ports map (_.maskGran) find (_.isDefined) map (_.get)