diff --git a/pymongo/asynchronous/client_session.py b/pymongo/asynchronous/client_session.py
index 27f26c35b2..545c8b247b 100644
--- a/pymongo/asynchronous/client_session.py
+++ b/pymongo/asynchronous/client_session.py
@@ -717,7 +717,7 @@ async def callback(session, custom_arg, custom_kwarg=None):
         while True:
             if retry:  # Implement exponential backoff on retry.
                 jitter = random.random()  # noqa: S311
-                backoff = jitter * min(_BACKOFF_INITIAL * (1.5**retry), _BACKOFF_MAX)
+                backoff = jitter * min(_BACKOFF_INITIAL * (1.5 ** (retry - 1)), _BACKOFF_MAX)
                 if _would_exceed_time_limit(start_time, backoff):
                     assert last_error is not None
                     raise last_error
diff --git a/pymongo/synchronous/client_session.py b/pymongo/synchronous/client_session.py
index 28999bcd62..c4d02ded11 100644
--- a/pymongo/synchronous/client_session.py
+++ b/pymongo/synchronous/client_session.py
@@ -715,7 +715,7 @@ def callback(session, custom_arg, custom_kwarg=None):
         while True:
             if retry:  # Implement exponential backoff on retry.
                 jitter = random.random()  # noqa: S311
-                backoff = jitter * min(_BACKOFF_INITIAL * (1.5**retry), _BACKOFF_MAX)
+                backoff = jitter * min(_BACKOFF_INITIAL * (1.5 ** (retry - 1)), _BACKOFF_MAX)
                 if _would_exceed_time_limit(start_time, backoff):
                     assert last_error is not None
                     raise last_error
diff --git a/test/asynchronous/test_transactions.py b/test/asynchronous/test_transactions.py
index 18f9778463..968594883e 100644
--- a/test/asynchronous/test_transactions.py
+++ b/test/asynchronous/test_transactions.py
@@ -677,7 +677,9 @@ async def callback(session):
         async with self.client.start_session() as s:
             await s.with_transaction(callback)
         end = time.monotonic()
-        self.assertLess(abs(end - start - (no_backoff_time + 2.2)), 1)  # sum of 13 backoffs is 2.2
+        self.assertLess(
+            abs(end - start - (no_backoff_time + 1.8)), 0.5
+        )  # sum of 13 backoffs is 1.8
 
         random.random = _original_random_random
 
diff --git a/test/test_transactions.py b/test/test_transactions.py
index 94d70396fc..0ceb456e58 100644
--- a/test/test_transactions.py
+++ b/test/test_transactions.py
@@ -661,7 +661,9 @@ def callback(session):
         with self.client.start_session() as s:
             s.with_transaction(callback)
         end = time.monotonic()
-        self.assertLess(abs(end - start - (no_backoff_time + 2.2)), 1)  # sum of 13 backoffs is 2.2
+        self.assertLess(
+            abs(end - start - (no_backoff_time + 1.8)), 0.5
+        )  # sum of 13 backoffs is 1.8
 
         random.random = _original_random_random