diff --git a/Code/Mantid/Framework/DataHandling/src/SetSampleMaterial.cpp b/Code/Mantid/Framework/DataHandling/src/SetSampleMaterial.cpp
index 613d7da35b4ed842f7ace6b284b95af6ff7cdf64..31802eb33d182e4c63266e68ec134a4d8fe7f46e 100644
--- a/Code/Mantid/Framework/DataHandling/src/SetSampleMaterial.cpp
+++ b/Code/Mantid/Framework/DataHandling/src/SetSampleMaterial.cpp
@@ -60,9 +60,9 @@ namespace Mantid
       declareProperty("SampleNumberDensity", EMPTY_DBL(), mustBePositive,
         "Optional:  This number density of the sample in number of formulas per cubic angstrom will be used instead of calculated");
       declareProperty("ZParameter", EMPTY_DBL(), mustBePositive,
-        "Number of formulas in the unit cell needed for chemical formulas with more than 1 atom");
+        "Number of atoms in the unit cell");
       declareProperty("UnitCellVolume", EMPTY_DBL(), mustBePositive,
-        "Unit cell volume in Angstoms^3 needed for chemical formulas with more than 1 atom");
+        "Unit cell volume in Angstoms^3. Will be calculated from the OrientedLattice if not supplied.");
       declareProperty("CoherentXSection", EMPTY_DBL(), mustBePositive,
         "Optional:  This coherent cross-section for the sample material in barns will be used instead of tabulated");
       declareProperty("IncoherentXSection", EMPTY_DBL(), mustBePositive,
diff --git a/Code/Mantid/docs/source/algorithms/SetSampleMaterial-v1.rst b/Code/Mantid/docs/source/algorithms/SetSampleMaterial-v1.rst
index 0d7ca7c07f8eacad310163e64972134da50e574b..4f4d227297d632c06f8053bce94fb363c09cce4c 100644
--- a/Code/Mantid/docs/source/algorithms/SetSampleMaterial-v1.rst
+++ b/Code/Mantid/docs/source/algorithms/SetSampleMaterial-v1.rst
@@ -44,4 +44,30 @@ Also Note that for isotopes specified in a molecular expression, the
 isotope must be enclosed by parenthesis, except for two special cases, 
 ``D`` and ``T``, which stand for ``H2`` and ``H3``, respectively.
 
+Cross Section Calculations
+##########################
+
+Each of the cross sections (:math:`\sigma`) are calculated according to
+
+.. math:: \sigma = \frac{1}{N_{atoms}}\sum_{i}\sigma_{i}n_{i}
+
+where :math:`N_{atoms} = \sum_{i}n_{i}`. A concrete example for the total
+cross section of ``D2 O``
+
+.. math:: \sigma = \frac{1}{2+1}\left( 7.64*2 + 4.232*1\right) = 6.504\ barns
+
+Number Density
+##############
+
+The number density is defined as
+
+.. math:: \rho_n = \frac{ZParameter}{UnitCellVolume}
+
+It can can be generated in one of two ways:
+
+1. Specifying it directly with ``SampleNumberDensity``
+2. Specifying the ``ZParameter`` and the ``UnitCellVolume`` (or letting
+   the algorithm calculate it from the OrientedLattice on the 
+   ``InputWorkspace``).
+
 .. categories::