diff --git a/doc/thermo_style.html b/doc/thermo_style.html
index fe94a4d8d5..9e91728793 100644
--- a/doc/thermo_style.html
+++ b/doc/thermo_style.html
@@ -69,6 +69,7 @@
       fnorm = length of force vector for all atoms
       cella,cellb,cellc = periodic cell lattice constants a,b,c 
       cellalpha, cellbeta, cellgamma = periodic cell angles alpha,beta,gamma
+      density = mass density of system
       c_ID = global scalar value calculated by a compute with ID
       c_ID[I] = Ith component of global vector calculated by a compute with ID
       c_ID[I][J] = I,J component of global array calculated by a compute with ID
@@ -249,7 +250,10 @@ define the periodic unit cell of a crystal.  See <A HREF = "Section_howto.html#h
 section</A> of the doc pages for a geometric
 description of triclinic periodic cells, including a precise defintion
 of these quantities in terms of the internal LAMMPS cell dimensions
-<I>lx</I>, <I>ly</I>, <I>lz</I>, <I>yz</I>, <I>xz</I>, <I>xy</I>,
+<I>lx</I>, <I>ly</I>, <I>lz</I>, <I>yz</I>, <I>xz</I>, <I>xy</I>.
+</P>
+<P>The keyword <I>density</I> gives the mass density of the system in density
+<A HREF = "units.html">units</A>.
 </P>
 <HR>
 
diff --git a/doc/thermo_style.txt b/doc/thermo_style.txt
index a4f06ed840..16682ea6a0 100644
--- a/doc/thermo_style.txt
+++ b/doc/thermo_style.txt
@@ -64,6 +64,7 @@ args = list of arguments for a particular style :l
       fnorm = length of force vector for all atoms
       cella,cellb,cellc = periodic cell lattice constants a,b,c 
       cellalpha, cellbeta, cellgamma = periodic cell angles alpha,beta,gamma
+      density = mass density of system
       c_ID = global scalar value calculated by a compute with ID
       c_ID\[I\] = Ith component of global vector calculated by a compute with ID
       c_ID\[I\]\[J\] = I,J component of global array calculated by a compute with ID
@@ -243,7 +244,10 @@ define the periodic unit cell of a crystal.  See "this
 section"_Section_howto.html#howto_12 of the doc pages for a geometric
 description of triclinic periodic cells, including a precise defintion
 of these quantities in terms of the internal LAMMPS cell dimensions
-{lx}, {ly}, {lz}, {yz}, {xz}, {xy},
+{lx}, {ly}, {lz}, {yz}, {xz}, {xy}.
+
+The keyword {density} gives the mass density of the system in density
+"units"_units.html.
 
 :line