diff --git a/changelog.d/6025.bugfix b/changelog.d/6025.bugfix
new file mode 100644
index 0000000000..50d7f9aab5
--- /dev/null
+++ b/changelog.d/6025.bugfix
@@ -0,0 +1 @@
+Fix bug in calculating the federation retry backoff period.
\ No newline at end of file
diff --git a/synapse/util/retryutils.py b/synapse/util/retryutils.py
index 5b16a81617..33263fe20f 100644
--- a/synapse/util/retryutils.py
+++ b/synapse/util/retryutils.py
@@ -193,8 +193,9 @@ class RetryDestinationLimiter(object):
         else:
             # We couldn't connect.
             if self.retry_interval:
-                self.retry_interval *= RETRY_MULTIPLIER
-                self.retry_interval *= int(random.uniform(0.8, 1.4))
+                self.retry_interval = int(
+                    self.retry_interval * RETRY_MULTIPLIER * random.uniform(0.8, 1.4)
+                )
 
                 if self.retry_interval >= MAX_RETRY_INTERVAL:
                     self.retry_interval = MAX_RETRY_INTERVAL