diff --git a/pycoloquinte/module.cpp b/pycoloquinte/module.cpp
index e4529e6..a0acaab 100644
--- a/pycoloquinte/module.cpp
+++ b/pycoloquinte/module.cpp
@@ -317,7 +317,8 @@ Construct a circuit.
       .def("expand_cells_by_factor", &Circuit::expandCellsByFactor,
            py::arg("expansion_factor"), py::arg("max_density") = 1.0,
            py::arg("row_side_margin") = 0.0,
-           "Expand the standard cells by an individual factor")
+           "Expand the standard cells by an individual factor, up to a maximum "
+           "density")
       .def("compute_cell_expansion", &Circuit::computeCellExpansion,
            py::arg("congestion_map"), py::arg("fixed_penalty") = 0.0,
            py::arg("penalty_factor") = 1.0,
@@ -325,7 +326,7 @@ Construct a circuit.
       .def("check", &Circuit::check, "Check the datastructure")
       .def("report", &Circuit::report)
       .def("export_ispd", &Circuit::exportIspd,
-           "Export the circuit to ISPD benchmark files")
+           py::arg("filename") "Export the circuit to ISPD benchmark files")
       .def("__str__", &Circuit::toString)
       .def("__repr__", &Circuit::toString);
 }
diff --git a/src/coloquinte.cpp b/src/coloquinte.cpp
index 8e5302c..22fa00e 100644
--- a/src/coloquinte.cpp
+++ b/src/coloquinte.cpp
@@ -669,8 +669,8 @@ void Circuit::expandCellsToDensity(double targetDensity, double rowSideMargin) {
   }
 }
 
-void Circuit::expandCellsByFactor(const std::vector<float> &expansionFactor,
-                                  double maxDensity, double rowSideMargin) {
+double Circuit::expandCellsByFactor(const std::vector<float> &expansionFactor,
+                                    double maxDensity, double rowSideMargin) {
   // Setup the target expansion if not done explicitly
   if ((int)expansionFactor.size() != nbCells()) {
     throw std::runtime_error(
@@ -697,14 +697,14 @@ void Circuit::expandCellsByFactor(const std::vector<float> &expansionFactor,
 
   // Compute the density and return if no further work is needed
   if (cellArea == 0LL || rowArea == 0LL) {
-    return;
+    return 1.0;
   }
 
   std::vector<float> expansion = expansionFactor;
   double density = (double)cellArea / (double)rowArea;
   if (density >= maxDensity) {
     // No point in applying any expansion
-    return;
+    return 1.0;
   }
 
   double expandedDensity = (double)expandedArea / (double)rowArea;
@@ -724,6 +724,8 @@ void Circuit::expandCellsByFactor(const std::vector<float> &expansionFactor,
       cellWidth_[i] *= expansion[i];
     }
   }
+
+  return expandedDensity / density;
 }
 
 std::vector<float> Circuit::computeCellExpansion(
diff --git a/src/coloquinte.hpp b/src/coloquinte.hpp
index 1eb9305..76742ff 100644
--- a/src/coloquinte.hpp
+++ b/src/coloquinte.hpp
@@ -1014,9 +1014,12 @@ class Circuit {
    * will be adjusted to not go over this.
    * @param rowSideMargin Margin applied to each row before computing available
    * area, in standard cell heights.
+   *
+   * @return Average expansion
    */
-  void expandCellsByFactor(const std::vector<float> &expansionFactor,
-                           double maxDensity = 1.0, double rowSideMargin = 0.0);
+  double expandCellsByFactor(const std::vector<float> &expansionFactor,
+                             double maxDensity = 1.0,
+                             double rowSideMargin = 0.0);
 
   /*
    * TODO: find a way that the expansion could happen on both sides (cleaner),