diff --git a/pymc3/diagnostics.py b/pymc3/diagnostics.py
index dfdaa3a8b9..838b29bcd0 100644
--- a/pymc3/diagnostics.py
+++ b/pymc3/diagnostics.py
@@ -259,7 +259,7 @@ def calc_n_eff(x):
 
             t += 1
 
-        return int(m * n / (1. + 2 * rho[1:t].sum()))
+        return min(m * n, int(m * n / (1. + 2 * rho[1:t].sum())))
 
     n_eff = {}
     for var in mtrace.varnames: