diff --git a/doc/src/sgml/ref/pgbench.sgml b/doc/src/sgml/ref/pgbench.sgml
index 15ee7c0f2b..10285d655b 100644
--- a/doc/src/sgml/ref/pgbench.sgml
+++ b/doc/src/sgml/ref/pgbench.sgml
@@ -1613,6 +1613,14 @@ f(x) = PHI(2.0 * parameter * (x - mu) / (max - min + 1)) /
       frequently values to the beginning of the interval are drawn.
       The closer to 0 <replaceable>parameter</replaceable> is,
       the flatter (more uniform) the access distribution.
+      The distribution is such that, assuming the range starts from 1,
+      the ratio of probability of drawing <replaceable>k</replaceable> versus
+      drawing <replaceable>k+1</replaceable> is
+      <literal>((<replaceable>k</replaceable>+1)/<replaceable>k</replaceable>)**<replaceable>parameter</replaceable></literal>.
+      For instance <literal>random_zipfian(1, ..., 2.5)</literal> draws
+      value <literal>1</literal> about <literal>(2/1)**2.5 = 5.66</literal> times more frequently
+      than <literal>2</literal>, which itself is drawn <literal>(3/2)*2.5 = 2.76</literal> times more
+      frequently than <literal>3</literal>, and so on.
      </para>
     </listitem>
    </itemizedlist>