diff --git a/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp b/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp
index aceed24..27accdc 100644
--- a/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp
+++ b/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp
@@ -15,17 +15,15 @@
 #include <sys/stat.h>
 
 #include <cstdio>
-#include <iostream>
 #include <memory>
-#include <string_view>
 
 #include <android-base/file.h>
 #include <android-base/logging.h>
+#include <android-base/unique_fd.h>
 #include <gtest/gtest.h>
+#include <libsnapshot/cow_format.h>
 #include <libsnapshot/cow_reader.h>
 #include <libsnapshot/cow_writer.h>
-#include "cow_decompress.h"
-#include "libsnapshot/cow_format.h"
 #include "writer_v2.h"
 #include "writer_v3.h"
 
@@ -658,5 +656,20 @@
     ASSERT_EQ(op.type(), kCowReplaceOp);
 }
 
+TEST_F(CowTestV3, CowSizeEstimate) {
+    CowOptions options{};
+    options.compression = "none";
+    auto estimator = android::snapshot::CreateCowEstimator(3, options);
+    ASSERT_TRUE(estimator->AddZeroBlocks(0, 1024 * 1024));
+    const auto cow_size = estimator->GetCowSize();
+    options.op_count_max = 1024 * 1024;
+    options.max_blocks = 1024 * 1024;
+    CowWriterV3 writer(options, GetCowFd());
+    ASSERT_TRUE(writer.Initialize());
+    ASSERT_TRUE(writer.AddZeroBlocks(0, 1024 * 1024));
+
+    ASSERT_LE(writer.GetCowSize(), cow_size);
+}
+
 }  // namespace snapshot
 }  // namespace android
diff --git a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
index 5bdc5fd..07f6f00 100644
--- a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
+++ b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
@@ -327,7 +327,12 @@
 bool CowWriterV3::WriteOperation(const CowOperationV3& op, const void* data, size_t size) {
     if (IsEstimating()) {
         header_.op_count++;
-        header_.op_count_max++;
+        if (header_.op_count > header_.op_count_max) {
+            // If we increment op_count_max, the offset of data section would
+            // change. So need to update |next_data_pos_|
+            next_data_pos_ += (header_.op_count - header_.op_count_max) * sizeof(CowOperationV3);
+            header_.op_count_max = header_.op_count;
+        }
         next_data_pos_ += op.data_length;
         return true;
     }