diff --git a/docs/source/usersguide/depletion.rst b/docs/source/usersguide/depletion.rst
index 6c31170951..ee0d09b8d1 100644
--- a/docs/source/usersguide/depletion.rst
+++ b/docs/source/usersguide/depletion.rst
@@ -319,7 +319,7 @@ normalizing reaction rates:
 
    .. math::
 
-      \phi = \frac{P}{V \cdot \sum_i (Q_i \cdot \Sigma^f_i \cdot \rho_i)}
+      \phi = \frac{P}{V \cdot \sum_i (Q_i \cdot \sigma^f_i \cdot \rho_i)}
 
    where :math:`\sum_i` is the sum over all nuclides :math:`i`. This equation
    makes the same assumptions and issues as discussed in