diff --git a/google-cloud-clients/google-cloud-firestore/src/main/java/com/google/cloud/firestore/UpdateBuilder.java b/google-cloud-clients/google-cloud-firestore/src/main/java/com/google/cloud/firestore/UpdateBuilder.java
index b5d0c0a7a6ce..e211be9ed79e 100644
--- a/google-cloud-clients/google-cloud-firestore/src/main/java/com/google/cloud/firestore/UpdateBuilder.java
+++ b/google-cloud-clients/google-cloud-firestore/src/main/java/com/google/cloud/firestore/UpdateBuilder.java
@@ -664,4 +664,9 @@ public List<WriteResult> apply(CommitResponse commitResponse) {
   boolean isEmpty() {
     return mutations.isEmpty();
   }
+
+  /** Get the number of mutations. */
+  public int getMutationsSize() {
+    return mutations.size();
+  }
 }
diff --git a/google-cloud-clients/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WriteBatchTest.java b/google-cloud-clients/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WriteBatchTest.java
index f29d87ca0948..5af3ae8b6d9f 100644
--- a/google-cloud-clients/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WriteBatchTest.java
+++ b/google-cloud-clients/google-cloud-firestore/src/test/java/com/google/cloud/firestore/WriteBatchTest.java
@@ -91,6 +91,8 @@ public void updateDocument() throws Exception {
     batch.update(documentReference, updateTime, "foo", "bar");
     batch.update(documentReference, LocalFirestoreHelper.SINGLE_FIELD_MAP, updateTime);
 
+    assertEquals(4, batch.getMutationsSize());
+
     List<WriteResult> writeResults = batch.commit().get();
     List<Write> writes = new ArrayList<>();
 
@@ -126,6 +128,8 @@ public void setDocument() throws Exception {
     writes.add(set(LocalFirestoreHelper.SINGLE_FIELD_PROTO, Arrays.asList("foo")));
     writes.add(set(LocalFirestoreHelper.SINGLE_FIELD_PROTO, Arrays.asList("foo")));
 
+    assertEquals(4, batch.getMutationsSize());
+
     List<WriteResult> writeResults = batch.commit().get();
     for (int i = 0; i < writeResults.size(); ++i) {
       assertEquals(Timestamp.ofTimeSecondsAndNanos(i, i), writeResults.get(i).getUpdateTime());
@@ -144,6 +148,8 @@ public void omitWriteResultForDocumentTransforms() throws Exception {
 
     batch.set(documentReference, map("time", FieldValue.serverTimestamp()));
 
+    assertEquals(1, batch.getMutationsSize());
+
     List<WriteResult> writeResults = batch.commit().get();
     assertEquals(1, writeResults.size());
   }
@@ -159,6 +165,8 @@ public void createDocument() throws Exception {
         .create(documentReference, LocalFirestoreHelper.SINGLE_FIELD_MAP)
         .create(documentReference, LocalFirestoreHelper.SINGLE_FIELD_OBJECT);
 
+    assertEquals(2, batch.getMutationsSize());
+
     List<WriteResult> writeResults = batch.commit().get();
     List<Write> writes = new ArrayList<>();
 
@@ -188,6 +196,8 @@ public void deleteDocument() throws Exception {
     precondition.getUpdateTimeBuilder().setSeconds(1).setNanos(2);
     writes.add(delete(precondition.build()));
 
+    assertEquals(2, batch.getMutationsSize());
+
     List<WriteResult> writeResults = batch.commit().get();
 
     for (int i = 0; i < writeResults.size(); ++i) {