diff --git a/src/main/java/us/ihmc/fastddsjava/cdr/idl/IDLObjectSequence.java b/src/main/java/us/ihmc/fastddsjava/cdr/idl/IDLObjectSequence.java
index 42dc4485..4cdf4301 100644
--- a/src/main/java/us/ihmc/fastddsjava/cdr/idl/IDLObjectSequence.java
+++ b/src/main/java/us/ihmc/fastddsjava/cdr/idl/IDLObjectSequence.java
@@ -231,6 +231,25 @@ public int elementSizeBytes(int currentAlignment, int i)
       return elements[i].calculateSizeBytes(currentAlignment);
    }
 
+   /**
+    * Object elements already include their own alignment padding in {@link #elementSizeBytes(int, int)},
+    * so the base {@link IDLSequence#calculateSizeBytes(int)} loop must not add a second alignment pass.
+    */
+   @Override
+   public int calculateSizeBytes(int currentAlignment)
+   {
+      int initialAlignment = currentAlignment;
+
+      currentAlignment += 4 + CDRBuffer.alignment(currentAlignment, 4); // Length header
+
+      for (int i = 0; i < size(); i++)
+      {
+         currentAlignment += elementSizeBytes(currentAlignment, i);
+      }
+
+      return currentAlignment - initialAlignment;
+   }
+
    @Override
    public void readElement(CDRBuffer buffer)
    {
diff --git a/src/main/java/us/ihmc/jros2/ROS2Publisher.java b/src/main/java/us/ihmc/jros2/ROS2Publisher.java
index 63cc80e0..314559fa 100644
--- a/src/main/java/us/ihmc/jros2/ROS2Publisher.java
+++ b/src/main/java/us/ihmc/jros2/ROS2Publisher.java
@@ -205,11 +205,11 @@ private void writeAndPublish(T message, boolean recordStatistics)
       {
          writeBuffer.rewind();
 
-         // For fixed-size messages the payload length is stable after construction preallocation.
-         payloadSizeBytes = CDRBuffer.PAYLOAD_HEADER.length + message.calculateSizeBytes(0);
-         if (payloadSizeBytes > writeBuffer.getBufferUnsafe().capacity())
+         // Presize from calculateSizeBytes; actual payload length is the buffer position after serialize.
+         int estimatedSizeBytes = CDRBuffer.PAYLOAD_HEADER.length + message.calculateSizeBytes(0);
+         if (estimatedSizeBytes > writeBuffer.getBufferUnsafe().capacity())
          {
-            writeBuffer.ensureRemainingCapacity(payloadSizeBytes);
+            writeBuffer.ensureRemainingCapacity(estimatedSizeBytes);
          }
 
          writeBuffer.writePayloadHeader();
diff --git a/src/test/java/us/ihmc/fastddsjava/cdr/idl/IDLObjectSequenceVariableSizeElementTest.java b/src/test/java/us/ihmc/fastddsjava/cdr/idl/IDLObjectSequenceVariableSizeElementTest.java
new file mode 100644
index 00000000..3cf97185
--- /dev/null
+++ b/src/test/java/us/ihmc/fastddsjava/cdr/idl/IDLObjectSequenceVariableSizeElementTest.java
@@ -0,0 +1,94 @@
+package us.ihmc.fastddsjava.cdr.idl;
+
+import org.junit.jupiter.api.Test;
+import us.ihmc.fastddsjava.cdr.CDRBuffer;
+import us.ihmc.fastddsjava.cdr.CDRSerializable;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Checks {@link IDLObjectSequence#calculateSizeBytes(int)} against serialize() for variable-size elements.
+ */
+public class IDLObjectSequenceVariableSizeElementTest
+{
+   /** Element with a string field so encoded size is not a fixed CDR alignment boundary. */
+   static class VariableSizeMsg implements CDRSerializable
+   {
+      private int id;
+      private final StringBuilder name = new StringBuilder();
+
+      @Override
+      public int calculateSizeBytes(int currentAlignment)
+      {
+         int initialAlignment = currentAlignment;
+         currentAlignment += 4 + CDRBuffer.alignment(currentAlignment, 4); // id
+         currentAlignment += 4 + CDRBuffer.alignment(currentAlignment, 4) + name.length() + 1; // name
+         return currentAlignment - initialAlignment;
+      }
+
+      @Override
+      public void serialize(CDRBuffer buffer)
+      {
+         buffer.writeInt(id);
+         buffer.writeString(name);
+      }
+
+      @Override
+      public void deserialize(CDRBuffer buffer)
+      {
+         id = buffer.readInt();
+         buffer.readString(name);
+      }
+   }
+
+   @Test
+   public void testCalculateSizeBytesMatchesActualSerializedSize()
+   {
+      IDLObjectSequence<VariableSizeMsg> sequence = new IDLObjectSequence<>(VariableSizeMsg.class);
+
+      VariableSizeMsg first = sequence.add();
+      first.id = 1;
+      first.name.append("pelvis"); // 6 chars -> element size is not a power of two
+
+      VariableSizeMsg second = sequence.add();
+      second.id = 2;
+      second.name.append("thigh"); // 5 chars
+
+      int calculatedSizeBytes = sequence.calculateSizeBytes(0);
+
+      CDRBuffer buffer = new CDRBuffer();
+      buffer.ensureRemainingCapacity(CDRBuffer.PAYLOAD_HEADER.length + calculatedSizeBytes);
+      buffer.writePayloadHeader();
+      sequence.serialize(buffer);
+
+      int actualSizeBytes = buffer.getBufferUnsafe().position() - CDRBuffer.PAYLOAD_HEADER.length;
+
+      assertEquals(actualSizeBytes, calculatedSizeBytes, "calculateSizeBytes must match serialize byte count");
+   }
+
+   @Test
+   public void testDeserializeRoundTrip()
+   {
+      IDLObjectSequence<VariableSizeMsg> sequence = new IDLObjectSequence<>(VariableSizeMsg.class);
+      sequence.add().id = 42;
+      sequence.get(0).name.append("longer_frame_name_example");
+      sequence.add().id = 7;
+      sequence.get(1).name.append("x");
+
+      CDRBuffer buffer = new CDRBuffer();
+      buffer.ensureRemainingCapacity(CDRBuffer.PAYLOAD_HEADER.length + sequence.calculateSizeBytes(0));
+      buffer.writePayloadHeader();
+      sequence.serialize(buffer);
+      buffer.rewind();
+      buffer.readPayloadHeader();
+
+      IDLObjectSequence<VariableSizeMsg> deserialized = new IDLObjectSequence<>(VariableSizeMsg.class);
+      deserialized.deserialize(buffer);
+
+      assertEquals(2, deserialized.size());
+      assertEquals(42, deserialized.get(0).id);
+      assertEquals("longer_frame_name_example", deserialized.get(0).name.toString());
+      assertEquals(7, deserialized.get(1).id);
+      assertEquals("x", deserialized.get(1).name.toString());
+   }
+}