diff --git a/hrv/classical.py b/hrv/classical.py
index 36b1477..99b8687 100644
--- a/hrv/classical.py
+++ b/hrv/classical.py
@@ -27,7 +27,7 @@ def _nn50(rri):
 
 
 def _pnn50(rri):
-    return _nn50(rri) / len(rri) * 100
+    return float(_nn50(rri)) / (len(rri) - 1) * 100
 
 
 @validate_frequency_domain_arguments