diff --git a/SimPEG/PropMaps.py b/SimPEG/PropMaps.py
index 7c22e149..2a8e7ffc 100644
--- a/SimPEG/PropMaps.py
+++ b/SimPEG/PropMaps.py
@@ -73,8 +73,22 @@ class PropModel(object):
     def __init__(self, propMap, vector):
         self.propMap = propMap
         self.vector  = vector
+        assert len(self.vector) == self.nP
+
+    @property
+    def nP(self):
+        inds = []
+        if getattr(self, '_nP', None) is None:
+            for name in self.propMap._properties:
+                index = getattr(self.propMap, '%sIndex'%name, None)
+                if index is not None:
+                    if type(index) is slice:
+                        inds += range(*index.indices(len(self.vector)))
+                    else:
+                        inds += list(index)
+            self._nP = len(set(inds))
+        return self._nP
 
-    # TODO: nP
 
 
 _PROPMAPCLASSREGISTRY = {}
diff --git a/SimPEG/Tests/test_PropMaps.py b/SimPEG/Tests/test_PropMaps.py
index 4e7c8bb7..b869502b 100644
--- a/SimPEG/Tests/test_PropMaps.py
+++ b/SimPEG/Tests/test_PropMaps.py
@@ -1,11 +1,11 @@
 import unittest
 from SimPEG import *
-
+from scipy.constants import mu_0
 
 
 class MyPropMap(Maps.PropMap):
     sigma = Maps.Property("Electrical Conductivity", defaultInvProp=True)
-    mu    = Maps.Property("Electrical Conductivity", defaultVal=4e-10)
+    mu    = Maps.Property("Electrical Conductivity", defaultVal=mu_0)
 
 
 class TestPropMaps(unittest.TestCase):
@@ -31,7 +31,7 @@ class TestPropMaps(unittest.TestCase):
             assert PM.muIndex is None
 
             m = PM(np.r_[1,2,3])
-            assert m.mu == 4e-10
+            assert m.mu == mu_0
             assert m.muModel is None
             assert m.muMap is None
             assert m.muDeriv is None
@@ -41,6 +41,8 @@ class TestPropMaps(unittest.TestCase):
             assert np.all(m.sigma == np.exp(np.r_[1,2,3]))
             assert m.sigmaDeriv is not None
 
+            assert m.nP == 3
+
     def test_slices(self):
         expMap = Maps.ExpMap(Mesh.TensorMesh((3,)))
         PM = MyPropMap({'maps':[('sigma', expMap)], 'slices':{'sigma':[2,1,0]}})
@@ -49,7 +51,36 @@ class TestPropMaps(unittest.TestCase):
         assert np.all(m.sigmaModel == np.r_[3,2,1])
         assert np.all(m.sigma == np.exp(np.r_[3,2,1]))
 
+    def test_multiMap(self):
+        m = Mesh.TensorMesh((3,))
+        expMap = Maps.ExpMap(m)
+        iMap = Maps.IdentityMap(m)
+        PM = MyPropMap([('sigma', expMap), ('mu', iMap)])
 
+        pm = PM(np.r_[1,2,3,4,5,6])
+
+        assert pm.nP == 6
+
+        assert np.all(pm.sigmaModel == [1,2,3])
+        assert np.all(pm.sigma == np.exp([1,2,3]))
+        assert np.all(pm.muModel == [4,5,6])
+        assert np.all(pm.mu == [4,5,6])
+
+
+    def test_multiMapCompressed(self):
+        m = Mesh.TensorMesh((3,))
+        expMap = Maps.ExpMap(m)
+        iMap = Maps.IdentityMap(m)
+        PM = MyPropMap({'maps':[('sigma', expMap), ('mu', iMap)],'slices':{'mu':[0,1,2]}})
+
+        pm = PM(np.r_[1,2,3])
+
+        assert pm.nP == 3
+
+        assert np.all(pm.sigmaModel == [1,2,3])
+        assert np.all(pm.sigma == np.exp([1,2,3]))
+        assert np.all(pm.muModel == [1,2,3])
+        assert np.all(pm.mu == [1,2,3])
 
 if __name__ == '__main__':
     unittest.main()