diff --git a/lib/ShortDev.Microsoft.ConnectedDevices/Messages/CommonHeaders.cs b/lib/ShortDev.Microsoft.ConnectedDevices/Messages/CommonHeaders.cs
index d16259bd..763d9c4c 100644
--- a/lib/ShortDev.Microsoft.ConnectedDevices/Messages/CommonHeaders.cs
+++ b/lib/ShortDev.Microsoft.ConnectedDevices/Messages/CommonHeaders.cs
@@ -1,12 +1,13 @@
 using System.Buffers.Binary;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 
 namespace ShortDev.Microsoft.ConnectedDevices.Messages;
 
 /// <summary>
 /// The <see cref="CommonHeader"/> is common for all Messages.
 /// </summary>
-public sealed class CommonHeader : ICdpHeader<CommonHeader>
+public sealed class CommonHeader : ICdpSerializable<CommonHeader>
 {
     public static CommonHeader Parse(ref EndianReader reader)
     {
@@ -93,6 +94,35 @@ public void Write(EndianWriter writer)
         writer.Write((byte)0);
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveOptimization | MethodImplOptions.AggressiveInlining)]
+    internal int CalcSize()
+    {
+        int size =
+            sizeof(ushort) + // Constants.Signature
+            sizeof(ushort) + // MessageLength
+            sizeof(byte) + // Constants.ProtocolVersion
+            sizeof(byte) + // Type
+            sizeof(short) + // Flags
+            sizeof(uint) + // SequenceNumber
+            sizeof(ulong) + // RequestID
+            sizeof(ushort) + // FragmentIndex
+            sizeof(ushort) + // FragmentCount
+            sizeof(ulong) + // SessionId
+            sizeof(ulong); // ChannelId
+
+        foreach (var header in AdditionalHeaders)
+        {
+            size +=
+                sizeof(byte) + // Type
+                sizeof(byte) + // Value.Length
+                header.Value.Length; // Value
+        }
+
+        return size +
+            sizeof(byte) + // AdditionalHeaderType.None
+            sizeof(byte); // 0
+    }
+
     /// <summary>
     /// Entire message length in bytes including signature.
     /// </summary>
@@ -150,7 +180,7 @@ public void Write(EndianWriter writer)
     /// Returns size of the whole rest of the message (excluding headers) (including hmac)
     /// </summary>
     public int PayloadSize
-        => MessageLength - (int)((ICdpSerializable<CommonHeader>)this).CalcSize();
+        => MessageLength - (int)CalcSize();
 
 
     #region Flags
@@ -171,7 +201,7 @@ public void CorrectClientSessionBit()
     public const int MessageLengthOffset = 2;
     public void SetPayloadLength(int payloadSize)
     {
-        MessageLength = (ushort)(payloadSize + ((ICdpSerializable<CommonHeader>)this).CalcSize());
+        MessageLength = (ushort)(payloadSize + CalcSize());
     }
 
     public static void ModifyMessageLength(Span<byte> msgBuffer, short delta)
diff --git a/lib/ShortDev.Microsoft.ConnectedDevices/Messages/ICdpSerializable.cs b/lib/ShortDev.Microsoft.ConnectedDevices/Messages/ICdpSerializable.cs
index af683176..9642636f 100644
--- a/lib/ShortDev.Microsoft.ConnectedDevices/Messages/ICdpSerializable.cs
+++ b/lib/ShortDev.Microsoft.ConnectedDevices/Messages/ICdpSerializable.cs
@@ -5,13 +5,6 @@ public interface ICdpSerializable<T> : ICdpWriteable where T : ICdpSerializable<
     static abstract T Parse(ref EndianReader reader);
     public static bool TryParse(ref EndianReader reader, out T? result, out Exception? error)
         => throw new NotImplementedException();
-
-    public long CalcSize()
-    {
-        EndianWriter writer = new(Endianness.BigEndian);
-        Write(writer);
-        return writer.Buffer.Size;
-    }
 }
 
 public interface ICdpArraySerializable<T> where T : ICdpArraySerializable<T>
diff --git a/tests/ShortDev.Microsoft.ConnectedDevices.Test/Messages/CommonHeaderTest.cs b/tests/ShortDev.Microsoft.ConnectedDevices.Test/Messages/CommonHeaderTest.cs
new file mode 100644
index 00000000..7b48ee12
--- /dev/null
+++ b/tests/ShortDev.Microsoft.ConnectedDevices.Test/Messages/CommonHeaderTest.cs
@@ -0,0 +1,43 @@
+using ShortDev.Microsoft.ConnectedDevices.Messages;
+using System.Buffers;
+
+namespace ShortDev.Microsoft.ConnectedDevices.Test.Messages;
+
+public class CommonHeaderTest
+{
+    [Fact]
+    public void CalcSize_YieldsCorrectResult_WhenNoHeaders()
+    {
+        using EndianWriter writer = EndianWriter.Create(Endianness.BigEndian, ArrayPool<byte>.Shared);
+
+        CommonHeader header = new();
+        header.Write(writer);
+        var expected = writer.Buffer.WrittenSpan.Length;
+
+        var actual = header.CalcSize();
+
+        Assert.Equal(expected, actual);
+    }
+
+    [Fact]
+    public void CalcSize_YieldsCorrectResult_WhenWithHeaders()
+    {
+        using EndianWriter writer = EndianWriter.Create(Endianness.BigEndian, ArrayPool<byte>.Shared);
+
+        CommonHeader header = new()
+        {
+            Type = MessageType.Connect,
+            AdditionalHeaders = {
+                AdditionalHeader.FromUInt32(AdditionalHeaderType.Header129, 0x70_00_00_03),
+                AdditionalHeader.FromUInt64(AdditionalHeaderType.PeerCapabilities, (ulong)PeerCapabilities.All),
+                AdditionalHeader.FromUInt64(AdditionalHeaderType.Header131, 6u)
+            }
+        };
+        header.Write(writer);
+        var expected = writer.Buffer.WrittenSpan.Length;
+
+        var actual = header.CalcSize();
+
+        Assert.Equal(expected, actual);
+    }
+}