diff --git a/matplotlib_scalebar/dimension.py b/matplotlib_scalebar/dimension.py
index 4239b36..245a27e 100644
--- a/matplotlib_scalebar/dimension.py
+++ b/matplotlib_scalebar/dimension.py
@@ -121,7 +121,7 @@ def __init__(self):
             latexrepr = "{0}m$^{{-1}}$".format(prefix)
             if prefix == "\u00b5" or prefix == "u":
                 latexrepr = _LATEX_MU + "m$^{-1}$"
-            self.add_units("1/{0}m".format(prefix), factor, latexrepr)
+            self.add_units("1/{0}m".format(prefix), 1/factor, latexrepr)
 
 
 class ImperialLengthDimension(_Dimension):
diff --git a/matplotlib_scalebar/test_dimension.py b/matplotlib_scalebar/test_dimension.py
index 6af6606..d2caa88 100644
--- a/matplotlib_scalebar/test_dimension.py
+++ b/matplotlib_scalebar/test_dimension.py
@@ -118,13 +118,13 @@ def tearDown(self):
 
     def testcalculate_preferred_cm(self):
         value, units = self.dim.calculate_preferred(0.02, "1/m")
-        self.assertAlmostEqual(2.0, value, 2)
-        self.assertEqual("1/cm", units)
+        self.assertAlmostEqual(20.0, value, 2)
+        self.assertEqual("1/km", units)
 
     def testcalculate_preferred_mm1(self):
         value, units = self.dim.calculate_preferred(0.002, "1/m")
         self.assertAlmostEqual(2.0, value, 2)
-        self.assertEqual("1/mm", units)
+        self.assertEqual("1/km", units)
 
     def testto_latex_cm(self):
         self.assertEqual("cm$^{-1}$", self.dim.to_latex("1/cm"))