diff --git a/xray/utils.py b/xray/utils.py
index 24d6b72..066435b 100644
--- a/xray/utils.py
+++ b/xray/utils.py
@@ -134,6 +134,14 @@ def plotdata(*args,x=None,plot=True,showTrend=True,title=None,clim='auto',fig=No
   if (plot or showTrend) and title is not None:
     plt.title(title)
 
+def volumeFraction(concentration=1,molWeight=17,density=1.347):
+  """ molWeight is kDa 
+      concentration in mM
+      density g/ml
+  """
+  concentration_mg_ml = concentration*molWeight
+  volume_fraction    = concentration_mg_ml/density/1e3
+  return volume_fraction
 
 def plotdiffs(*args,select=None,err=None,absSignal=None,absSignalScale=10,
               showErr=False,cmap=plt.cm.jet,fig=None,title=None):