diff --git a/src/objective-c/tests/InteropTestsLocalCleartext.m b/src/objective-c/tests/InteropTestsLocalCleartext.m
index b41210f50f8..49876608087 100644
--- a/src/objective-c/tests/InteropTestsLocalCleartext.m
+++ b/src/objective-c/tests/InteropTestsLocalCleartext.m
@@ -37,6 +37,10 @@
 
 static NSString * const kLocalCleartextHost = @"localhost:5050";
 
+// The Protocol Buffers encoding overhead of local interop server. Acquired
+// by experiment. Adjust this when server's proto file changes.
+static int32_t kLocalInteropServerOverhead = 10;
+
 /** Tests in InteropTests.m, sending the RPCs to a local cleartext server. */
 @interface InteropTestsLocalCleartext : InteropTests
 @end
@@ -48,7 +52,7 @@ static NSString * const kLocalCleartextHost = @"localhost:5050";
 }
 
 - (int32_t)encodingOverhead {
-  return 10; // bytes
+  return kLocalInteropServerOverhead; // bytes
 }
 
 - (void)setUp {
diff --git a/src/objective-c/tests/InteropTestsLocalSSL.m b/src/objective-c/tests/InteropTestsLocalSSL.m
index 1479c5896c3..934d500abc0 100644
--- a/src/objective-c/tests/InteropTestsLocalSSL.m
+++ b/src/objective-c/tests/InteropTestsLocalSSL.m
@@ -37,6 +37,10 @@
 
 static NSString * const kLocalSSLHost = @"localhost:5051";
 
+// The Protocol Buffers encoding overhead of local interop server. Acquired
+// by experiment. Adjust this when server's proto file changes.
+static int32_t kLocalInteropServerOverhead = 10;
+
 /** Tests in InteropTests.m, sending the RPCs to a local SSL server. */
 @interface InteropTestsLocalSSL : InteropTests
 @end
@@ -48,7 +52,7 @@ static NSString * const kLocalSSLHost = @"localhost:5051";
 }
 
 - (int32_t)encodingOverhead {
-  return 10; // bytes
+  return kLocalInteropServerOverhead; // bytes
 }
 
 - (void)setUp {
diff --git a/src/objective-c/tests/InteropTestsRemote.m b/src/objective-c/tests/InteropTestsRemote.m
index 70f84753bb6..9fb30aa43d0 100644
--- a/src/objective-c/tests/InteropTestsRemote.m
+++ b/src/objective-c/tests/InteropTestsRemote.m
@@ -37,6 +37,10 @@
 
 static NSString * const kRemoteSSLHost = @"grpc-test.sandbox.googleapis.com";
 
+// The Protocol Buffers encoding overhead of remote interop server. Acquired
+// by experiment. Adjust this when server's proto file changes.
+static int32_t kRemoteInteropServerOverhead = 12;
+
 /** Tests in InteropTests.m, sending the RPCs to a remote SSL server. */
 @interface InteropTestsRemote : InteropTests
 @end
@@ -48,7 +52,7 @@ static NSString * const kRemoteSSLHost = @"grpc-test.sandbox.googleapis.com";
 }
 
 - (int32_t)encodingOverhead {
-  return 12; // bytes
+  return kRemoteInteropServerOverhead; // bytes
 }
 
 @end
diff --git a/src/objective-c/tests/InteropTestsRemoteWithCronet/InteropTestsRemoteWithCronet.m b/src/objective-c/tests/InteropTestsRemoteWithCronet/InteropTestsRemoteWithCronet.m
index 7bc303ac10d..005bac0a0d4 100644
--- a/src/objective-c/tests/InteropTestsRemoteWithCronet/InteropTestsRemoteWithCronet.m
+++ b/src/objective-c/tests/InteropTestsRemoteWithCronet/InteropTestsRemoteWithCronet.m
@@ -37,6 +37,10 @@
 
 static NSString * const kRemoteSSLHost = @"grpc-test.sandbox.googleapis.com";
 
+// The Protocol Buffers encoding overhead of remote interop server. Acquired
+// by experiment. Adjust this when server's proto file changes.
+static int32_t kRemoteInteropServerOverhead = 12;
+
 /** Tests in InteropTests.m, sending the RPCs to a remote SSL server. */
 @interface InteropTestsRemoteWithCronet : InteropTests
 @end
@@ -48,7 +52,7 @@ static NSString * const kRemoteSSLHost = @"grpc-test.sandbox.googleapis.com";
 }
 
 - (int32_t)encodingOverhead {
-  return 12; // bytes
+  return kRemoteInteropServerOverhead; // bytes
 }
 
 @end