diff --git a/ess/TraceWin.py b/ess/TraceWin.py
index a44a303d5adf124127fab7e4ad70fa02d433ae89..418be1d87eb29d839b9ebee631df5d4fd5f0193a 100644
--- a/ess/TraceWin.py
+++ b/ess/TraceWin.py
@@ -244,6 +244,25 @@ class plt:
             for v in vals:
                 self.avg[v].append(numpy.average(data[v]))
 
+    def calc_std(self):
+        '''
+        Calculates the beam sizes
+
+        '''
+
+        import numpy
+
+        if not hasattr(self,'sigma'):
+               self.calc_sigma()
+
+        vals=self._columns[:-1]
+
+        self.std={}
+
+        for j in xrange(len(vals)):
+            v=vals[j]
+            self.std[v]=numpy.sqrt(self.sigma[:,j,j])
+
     def calc_minmax(self,pmin=5,pmax=95):
         '''
         Calculates min/max values of beam coordinates