diff --git a/src/mash/CommandScreen.cpp b/src/mash/CommandScreen.cpp
index 9d6d7a1..3030dda 100644
--- a/src/mash/CommandScreen.cpp
+++ b/src/mash/CommandScreen.cpp
@@ -606,7 +606,7 @@
         return 1.;
     }
     
-    double r = 1. / (1. + kmerSpace / setSize);
+    double r = double(setSize) / kmerSpace;
     
 #ifdef USE_BOOST
     return cdf(complement(binomial(sketchSize, r), x - 1));