diff --git a/rosbag2_cpp/include/rosbag2_cpp/writers/sequential_writer.hpp b/rosbag2_cpp/include/rosbag2_cpp/writers/sequential_writer.hpp
index 1bece470bd..8617e71385 100644
--- a/rosbag2_cpp/include/rosbag2_cpp/writers/sequential_writer.hpp
+++ b/rosbag2_cpp/include/rosbag2_cpp/writers/sequential_writer.hpp
@@ -118,8 +118,10 @@ class ROSBAG2_CPP_PUBLIC SequentialWriter
   std::chrono::seconds max_bagfile_duration;
 
   // Intermediate cache to write multiple messages into the storage.
-  // `max_cache_size` is the amount of messages to hold in storage before writing to disk.
+  // `max_cache_size` is the number of bytes of messages to hold in storage
+  // before writing to disk.
   uint64_t max_cache_size_;
+  uint64_t current_cache_size_;
   std::vector<std::shared_ptr<const rosbag2_storage::SerializedBagMessage>> cache_;
 
   // Used to track topic -> message count
diff --git a/rosbag2_cpp/src/rosbag2_cpp/writers/sequential_writer.cpp b/rosbag2_cpp/src/rosbag2_cpp/writers/sequential_writer.cpp
index d8b17cb7cc..15d5e9a41e 100644
--- a/rosbag2_cpp/src/rosbag2_cpp/writers/sequential_writer.cpp
+++ b/rosbag2_cpp/src/rosbag2_cpp/writers/sequential_writer.cpp
@@ -89,8 +89,8 @@ void SequentialWriter::open(
   max_bagfile_size_ = storage_options.max_bagfile_size;
   max_bagfile_duration = std::chrono::seconds(storage_options.max_bagfile_duration);
   max_cache_size_ = storage_options.max_cache_size;
-
   cache_.reserve(max_cache_size_);
+  current_cache_size_ = 0u;
 
   if (converter_options.output_serialization_format !=
     converter_options.input_serialization_format)
@@ -249,11 +249,13 @@ void SequentialWriter::write(std::shared_ptr<rosbag2_storage::SerializedBagMessa
     storage_->write(converted_msg);
   } else {
     cache_.push_back(converted_msg);
-    if (cache_.size() >= max_cache_size_) {
+    current_cache_size_ += converted_msg->serialized_data->buffer_length;
+    if (current_cache_size_ >= max_cache_size_) {
       storage_->write(cache_);
       // reset cache
       cache_.clear();
       cache_.reserve(max_cache_size_);
+      current_cache_size_ = 0u;
     }
   }
 }
diff --git a/rosbag2_cpp/test/rosbag2_cpp/test_sequential_writer.cpp b/rosbag2_cpp/test/rosbag2_cpp/test_sequential_writer.cpp
index 2553b4b135..fce161781d 100644
--- a/rosbag2_cpp/test/rosbag2_cpp/test_sequential_writer.cpp
+++ b/rosbag2_cpp/test/rosbag2_cpp/test_sequential_writer.cpp
@@ -25,6 +25,7 @@
 #include "rosbag2_cpp/writer.hpp"
 
 #include "rosbag2_storage/bag_metadata.hpp"
+#include "rosbag2_storage/ros_helper.hpp"
 #include "rosbag2_storage/topic_metadata.hpp"
 
 #include "mock_converter.hpp"
@@ -265,13 +266,14 @@ TEST_F(SequentialWriterTest, writer_splits_when_storage_bagfile_size_gt_max_bagf
 }
 
 TEST_F(SequentialWriterTest, only_write_after_cache_is_full) {
-  const size_t counter = 1000;
+  const uint64_t counter = 1000;
   const uint64_t max_cache_size = 100;
-
+  std::string msg_content = "Hello";
+  const auto msg_length = msg_content.length();
   EXPECT_CALL(
     *storage_,
     write(An<const std::vector<std::shared_ptr<const rosbag2_storage::SerializedBagMessage>> &>())).
-  Times(counter / max_cache_size);
+  Times(static_cast<int>(counter * msg_length / max_cache_size));
   EXPECT_CALL(
     *storage_,
     write(An<std::shared_ptr<const rosbag2_storage::SerializedBagMessage>>())).Times(0);
@@ -284,6 +286,8 @@ TEST_F(SequentialWriterTest, only_write_after_cache_is_full) {
 
   auto message = std::make_shared<rosbag2_storage::SerializedBagMessage>();
   message->topic_name = "test_topic";
+  message->serialized_data = rosbag2_storage::make_serialized_message(
+    msg_content.c_str(), msg_length);
 
   storage_options_.max_bagfile_size = 0;
   storage_options_.max_cache_size = max_cache_size;
@@ -314,8 +318,13 @@ TEST_F(SequentialWriterTest, do_not_use_cache_if_cache_size_is_zero) {
 
   std::string rmw_format = "rmw_format";
 
+  std::string msg_content = "Hello";
+  auto msg_length = msg_content.length();
   auto message = std::make_shared<rosbag2_storage::SerializedBagMessage>();
   message->topic_name = "test_topic";
+  message->serialized_data = rosbag2_storage::make_serialized_message(
+    msg_content.c_str(), msg_length);
+
 
   storage_options_.max_bagfile_size = 0;
   storage_options_.max_cache_size = max_cache_size;