diff --git a/io/blob_store.cc b/io/blob_store.cc
index 9ce7b098..176f46e6 100644
--- a/io/blob_store.cc
+++ b/io/blob_store.cc
@@ -476,6 +476,7 @@ BlobWriter::BlobWriter(const Path& filename, hwy::ThreadPool& pool)
   // we will later write a footer, else we will update the header.
   std::vector<uint8_t> bytes_before_blobs = BlobStore::BytesBeforeBlobsV2();
   file_->Write(bytes_before_blobs.data(), bytes_before_blobs.size(), 0);
+  curr_offset_ = bytes_before_blobs.size();
 }
 
 void BlobWriter::Add(const std::string& key, const void* data, size_t bytes) {
@@ -485,7 +486,7 @@ void BlobWriter::Add(const std::string& key, const void* data, size_t bytes) {
   blob_sizes_.push_back(bytes);
 
   std::vector<BlobIO> writes;
-  EnqueueChunks(keys_.size() - 1, file_->FileSize(), bytes,
+  EnqueueChunks(keys_.size() - 1, curr_offset_, bytes,
                 static_cast<const uint8_t*>(data), writes);
 
   hwy::ThreadPool null_pool(0);
@@ -493,7 +494,6 @@ void BlobWriter::Add(const std::string& key, const void* data, size_t bytes) {
   pool_or_serial.Run(
       0, writes.size(), [this, &writes](uint64_t i, size_t /*thread*/) {
         const BlobRange& range = writes[i].range;
-
         if (!file_->Write(writes[i].data, range.bytes, range.offset)) {
           const std::string& key = StringFromKey(keys_[range.key_idx]);
           HWY_ABORT("Write failed for %s from %zu, %zu bytes to %p.",
@@ -501,16 +501,20 @@ void BlobWriter::Add(const std::string& key, const void* data, size_t bytes) {
                     writes[i].data);
         }
       });
+  curr_offset_ = writes.back().range.End();
 }
 
 void BlobWriter::Finalize() {
+  if (!file_->IsAppendOnly() && curr_offset_ != file_->FileSize()) {
+    HWY_WARN("Computed offset %zu does not match file size %zu.",
+             curr_offset_, file_->FileSize());
+  }
   const BlobStore bs = BlobStore(keys_, blob_sizes_);
 
   // Write the rest of the bytes, which contains: paddings + directory + header.
   const auto bytes_after_blobs = bs.BytesAfterBlobs();
   file_->Write(bytes_after_blobs.data(), bytes_after_blobs.size(),
-               file_->FileSize());
-
+               curr_offset_);
   file_.reset();  // closes the file
 }
 
diff --git a/io/blob_store.h b/io/blob_store.h
index 916091c3..7ee57d21 100644
--- a/io/blob_store.h
+++ b/io/blob_store.h
@@ -130,6 +130,8 @@ class BlobWriter {
   std::vector<hwy::uint128_t> keys_;
   std::vector<size_t> blob_sizes_;
   hwy::ThreadPool& pool_;
+  // Current offset in the file used for writing.
+  int64_t curr_offset_ = 0;
 };
 
 }  // namespace gcpp