diff --git a/internal/core/src/index/BitmapIndex.cpp b/internal/core/src/index/BitmapIndex.cpp
index 3ead065cc2..e5dc68cbbc 100644
--- a/internal/core/src/index/BitmapIndex.cpp
+++ b/internal/core/src/index/BitmapIndex.cpp
@@ -112,6 +112,7 @@ BitmapIndex<T>::Build(size_t n, const T* data, const bool* valid_data) {
     }
 
     is_built_ = true;
+    ComputeByteSize();
 }
 
 template <typename T>
@@ -168,6 +169,7 @@ BitmapIndex<T>::BuildWithFieldData(
                             proto::schema::DataType_Name(schema_.data_type())));
     }
     is_built_ = true;
+    ComputeByteSize();
 }
 
 template <typename T>
@@ -568,6 +570,7 @@ BitmapIndex<T>::LoadWithoutAssemble(const BinarySet& binary_set,
         is_mmap_);
 
     is_built_ = true;
+    ComputeByteSize();
 }
 
 template <typename T>
diff --git a/internal/core/src/index/BitmapIndex.h b/internal/core/src/index/BitmapIndex.h
index b391958ec8..a6e2b6415e 100644
--- a/internal/core/src/index/BitmapIndex.h
+++ b/internal/core/src/index/BitmapIndex.h
@@ -114,6 +114,69 @@ class BitmapIndex : public ScalarIndex<T> {
         return Count();
     }
 
+    void
+    ComputeByteSize() override {
+        ScalarIndex<T>::ComputeByteSize();
+        int64_t total = this->cached_byte_size_;
+
+        // valid_bitset_
+        total += valid_bitset_.size_in_bytes();
+
+        if (is_mmap_) {
+            // mmap mode
+            total += mmap_size_;
+            // bitmap_info_map_ overhead (keys and roaring metadata in memory)
+            size_t num_entries = bitmap_info_map_.size();
+            if constexpr (std::is_same_v<T, std::string>) {
+                for (const auto& [key, bitmap] : bitmap_info_map_) {
+                    total += key.capacity();
+                }
+            } else {
+                total += num_entries * sizeof(T);
+            }
+            // roaring metadata + map node overhead per entry
+            total += num_entries * (sizeof(roaring::Roaring) + 40);
+        } else if (build_mode_ == BitmapIndexBuildMode::ROARING) {
+            // data_: map<T, roaring::Roaring>
+            for (const auto& [key, bitmap] : data_) {
+                if constexpr (std::is_same_v<T, std::string>) {
+                    total += key.capacity();
+                } else {
+                    total += sizeof(T);
+                }
+                total += bitmap.getSizeInBytes();
+                // std::map red-black tree node overhead (~40 bytes per node)
+                total += 40;
+            }
+        } else {
+            // bitsets_: map<T, TargetBitmap>
+            size_t num_entries = bitsets_.size();
+            if (num_entries > 0) {
+                size_t bitset_bytes = bitsets_.begin()->second.size_in_bytes();
+                total += num_entries * bitset_bytes;
+                if constexpr (std::is_same_v<T, std::string>) {
+                    for (const auto& [key, bitset] : bitsets_) {
+                        total += key.capacity();
+                    }
+                } else {
+                    total += num_entries * sizeof(T);
+                }
+                // std::map red-black tree node overhead (~40 bytes per node)
+                total += num_entries * 40;
+            }
+        }
+
+        // offset cache
+        total += data_offsets_cache_.capacity() *
+                 sizeof(typename decltype(data_offsets_cache_)::value_type);
+        total += bitsets_offsets_cache_.capacity() *
+                 sizeof(typename decltype(bitsets_offsets_cache_)::value_type);
+        total += mmap_offsets_cache_.capacity() *
+                 sizeof(typename decltype(mmap_offsets_cache_)::value_type);
+
+        this->cached_byte_size_ = total;
+    }
+
     IndexStatsPtr
     Upload(const Config& config = {}) override;
 
diff --git a/internal/core/src/index/HybridScalarIndex.cpp b/internal/core/src/index/HybridScalarIndex.cpp
index e541c6c08b..3defa2dc3d 100644
--- a/internal/core/src/index/HybridScalarIndex.cpp
+++ b/internal/core/src/index/HybridScalarIndex.cpp
@@ -351,6 +351,7 @@ HybridScalarIndex<T>::Load(const BinarySet& binary_set, const Config& config) {
     index->Load(binary_set, config);
 
     is_built_ = true;
+    ComputeByteSize();
 }
 
 template <typename T>
@@ -382,6 +383,7 @@ HybridScalarIndex<T>::Load(milvus::tracer::TraceContext ctx,
     index->Load(ctx, config);
 
     is_built_ = true;
+    ComputeByteSize();
 }
 
 template class HybridScalarIndex<bool>;
diff --git a/internal/core/src/index/HybridScalarIndex.h b/internal/core/src/index/HybridScalarIndex.h
index 79a7abce90..7b83c4a6e5 100644
--- a/internal/core/src/index/HybridScalarIndex.h
+++ b/internal/core/src/index/HybridScalarIndex.h
@@ -153,6 +153,16 @@ class HybridScalarIndex : public ScalarIndex<T> {
         return internal_index_->Size();
     }
 
+    void
+    ComputeByteSize() override {
+        ScalarIndex<T>::ComputeByteSize();
+        int64_t total = this->cached_byte_size_;
+        if (internal_index_) {
+            total += internal_index_->ByteSize();
+        }
+        this->cached_byte_size_ = total;
+    }
+
     const bool
     HasRawData() const override {
         if (field_type_ == proto::schema::DataType::Array) {
diff --git a/internal/core/src/index/Index.h b/internal/core/src/index/Index.h
index b045de7e1e..7e5c2fe876 100644
--- a/internal/core/src/index/Index.h
+++ b/internal/core/src/index/Index.h
@@ -91,6 +91,29 @@ class IndexBase {
         cell_size_ = cell_size;
     }
 
+    // Returns the memory usage in bytes that scales with data size (O(n)).
+    // Fixed overhead are minimal and thus not included.
+    //
+    // NOTE: This method returns a cached value computed by ComputeByteSize().
+    // It is designed for SEALED SEGMENTS only, where the index is fully built
+    // or loaded and no more data will be added. For GROWING SEGMENTS with
+    // ongoing inserts, the cached value will NOT be updated automatically.
+    // ComputeByteSize() should only be called after Build() or Load() completes
+    // on sealed segments.
+    int64_t
+    ByteSize() const {
+        return cached_byte_size_;
+    }
+
+    // Computes and caches the memory usage in bytes.
+    // Subclasses should override this method to calculate their specific memory usage
+    // and store the result in cached_byte_size_.
+    // This method should be called at the end of Build() or Load() for sealed segments.
+    virtual void
+    ComputeByteSize() {
+        cached_byte_size_ = 0;
+    }
+
  protected:
     explicit IndexBase(IndexType index_type)
         : index_type_(std::move(index_type)) {
@@ -98,6 +121,7 @@ class IndexBase {
 
     IndexType index_type_ = "";
     cachinglayer::ResourceUsage cell_size_ = {0, 0};
+    mutable int64_t cached_byte_size_ = 0;
 
     std::unique_ptr<MmapFileRAII> mmap_file_raii_;
 };
diff --git a/internal/core/src/index/InvertedIndexTantivy.cpp b/internal/core/src/index/InvertedIndexTantivy.cpp
index 46d50cdb10..c1e99cf545 100644
--- a/internal/core/src/index/InvertedIndexTantivy.cpp
+++ b/internal/core/src/index/InvertedIndexTantivy.cpp
@@ -218,6 +218,7 @@ InvertedIndexTantivy<T>::Load(milvus::tracer::TraceContext ctx,
         // the index is loaded in ram, so we can remove files in advance
         disk_file_manager_->RemoveIndexFiles();
     }
+    ComputeByteSize();
 }
 
 template <typename T>
@@ -564,6 +565,7 @@ InvertedIndexTantivy<T>::BuildWithRawDataForUT(size_t n,
     wrapper_->create_reader(milvus::index::SetBitsetSealed);
     finish();
     wrapper_->reload();
+    ComputeByteSize();
 }
 
 template <typename T>
diff --git a/internal/core/src/index/InvertedIndexTantivy.h b/internal/core/src/index/InvertedIndexTantivy.h
index 750ac767e8..bdc0271b9d 100644
--- a/internal/core/src/index/InvertedIndexTantivy.h
+++ b/internal/core/src/index/InvertedIndexTantivy.h
@@ -190,9 +190,18 @@ class InvertedIndexTantivy : public ScalarIndex<T> {
         return Count();
     }
 
-    int64_t
-    ByteSize() const {
-        return wrapper_->index_size_bytes();
+    void
+    ComputeByteSize() override {
+        ScalarIndex<T>::ComputeByteSize();
+        int64_t total = this->cached_byte_size_;
+
+        // Tantivy index size
+        total += wrapper_->index_size_bytes();
+
+        // null_offset_: vector<size_t>
+        total += null_offset_.capacity() * sizeof(size_t);
+
+        this->cached_byte_size_ = total;
     }
 
     virtual const TargetBitmap
diff --git a/internal/core/src/index/RTreeIndex.cpp b/internal/core/src/index/RTreeIndex.cpp
index 159977aec4..10c1a220ae 100644
--- a/internal/core/src/index/RTreeIndex.cpp
+++ b/internal/core/src/index/RTreeIndex.cpp
@@ -221,6 +221,7 @@ RTreeIndex<T>::Load(milvus::tracer::TraceContext ctx, const Config& config) {
     total_num_rows_ =
         wrapper_->count() + static_cast<int64_t>(null_offset_.size());
     is_built_ = true;
+    ComputeByteSize();
 
     LOG_INFO(
         "Loaded R-Tree index from {} with {} rows", path_, total_num_rows_);
@@ -238,6 +239,7 @@ RTreeIndex<T>::Build(const Config& config) {
     total_num_rows_ =
         wrapper_->count() + static_cast<int64_t>(null_offset_.size());
     is_built_ = true;
+    ComputeByteSize();
 }
 
 template <typename T>
@@ -279,6 +281,7 @@ RTreeIndex<T>::BuildWithFieldData(
         wrapper_->bulk_load_from_field_data(field_datas, schema_.nullable());
         total_num_rows_ = total_rows;
         is_built_ = true;
+        ComputeByteSize();
         return;
     }
 }
diff --git a/internal/core/src/index/RTreeIndex.h b/internal/core/src/index/RTreeIndex.h
index 8c88d40475..0773b3dccc 100644
--- a/internal/core/src/index/RTreeIndex.h
+++ b/internal/core/src/index/RTreeIndex.h
@@ -143,6 +143,22 @@ class RTreeIndex : public ScalarIndex<T> {
         return Count();
     }
 
+    void
+    ComputeByteSize() override {
+        ScalarIndex<T>::ComputeByteSize();
+        int64_t total = this->cached_byte_size_;
+
+        // null_offset_ vector
+        total += null_offset_.capacity() * sizeof(size_t);
+
+        // wrapper_ (RTreeIndexWrapper)
+        if (wrapper_) {
+            total += wrapper_->ByteSize();
+        }
+
+        this->cached_byte_size_ = total;
+    }
+
     // GIS-specific query methods
     /**
      * @brief Query candidates based on spatial operation
diff --git a/internal/core/src/index/RTreeIndexWrapper.cpp b/internal/core/src/index/RTreeIndexWrapper.cpp
index e1e0fe337b..9061836225 100644
--- a/internal/core/src/index/RTreeIndexWrapper.cpp
+++ b/internal/core/src/index/RTreeIndexWrapper.cpp
@@ -285,5 +285,22 @@ RTreeIndexWrapper::count() const {
     return static_cast<int64_t>(rtree_.size());
 }
 
+int64_t
+RTreeIndexWrapper::ByteSize() const {
+    int64_t total = 0;
+
+    // values_: vector<Value> where Value = std::pair<Box, int64_t>
+    // Box = bg::model::box<Point> = 2 Points = 2 * 2 * sizeof(double) = 32 bytes
+    // Value = Box + int64_t = 32 + 8 = 40 bytes
+    total += values_.capacity() * sizeof(Value);
+
+    // rtree_ internal structure (nodes, pointers, MBRs)
+    // R*-tree with max 16 entries per node has overhead per entry
+    // Estimated ~18 bytes per entry for internal tree structure
+    total += rtree_.size() * 18;
+
+    return total;
+}
+
 // index/leaf capacity setters removed; not applicable for Boost rtree
 }  // namespace milvus::index
\ No newline at end of file
diff --git a/internal/core/src/index/RTreeIndexWrapper.h b/internal/core/src/index/RTreeIndexWrapper.h
index bac2454676..030e284e1e 100644
--- a/internal/core/src/index/RTreeIndexWrapper.h
+++ b/internal/core/src/index/RTreeIndexWrapper.h
@@ -97,6 +97,13 @@ class RTreeIndexWrapper {
     int64_t
     count() const;
 
+    /**
+     * @brief Get the estimated memory usage of the R-tree index
+     * @return Memory usage in bytes
+     */
+    int64_t
+    ByteSize() const;
+
     // Boost rtree does not use index/leaf capacities; keep only fill factor for
     // compatibility (no-op currently)
 
diff --git a/internal/core/src/index/ScalarIndex.h b/internal/core/src/index/ScalarIndex.h
index d56bddbf29..e549245ca3 100644
--- a/internal/core/src/index/ScalarIndex.h
+++ b/internal/core/src/index/ScalarIndex.h
@@ -147,7 +147,8 @@ class ScalarIndex : public IndexBase {
                index_type_ == milvus::index::HYBRID_INDEX_TYPE ||
                index_type_ == milvus::index::INVERTED_INDEX_TYPE ||
                index_type_ == milvus::index::MARISA_TRIE ||
-               index_type_ == milvus::index::MARISA_TRIE_UPPER;
+               index_type_ == milvus::index::MARISA_TRIE_UPPER ||
+               index_type_ == milvus::index::ASCENDING_SORT;
     }
 
     virtual int64_t
diff --git a/internal/core/src/index/ScalarIndexSort.cpp b/internal/core/src/index/ScalarIndexSort.cpp
index c9035555d2..644190cd24 100644
--- a/internal/core/src/index/ScalarIndexSort.cpp
+++ b/internal/core/src/index/ScalarIndexSort.cpp
@@ -100,6 +100,7 @@ ScalarIndexSort<T>::Build(size_t n, const T* values, const bool* valid_data) {
     is_built_ = true;
 
     setup_data_pointers();
+    ComputeByteSize();
 }
 
 template <typename T>
@@ -143,6 +144,7 @@ ScalarIndexSort<T>::BuildWithFieldData(
     is_built_ = true;
 
     setup_data_pointers();
+    ComputeByteSize();
 }
 
 template <typename T>
@@ -281,6 +283,7 @@ ScalarIndexSort<T>::LoadWithoutAssemble(const BinarySet& index_binary,
     }
 
     is_built_ = true;
+    ComputeByteSize();
 
     LOG_INFO("load ScalarIndexSort done, field_id: {}, is_mmap:{}",
              field_id_,
diff --git a/internal/core/src/index/ScalarIndexSort.h b/internal/core/src/index/ScalarIndexSort.h
index c2ab8ed3e1..d4fa899a18 100644
--- a/internal/core/src/index/ScalarIndexSort.h
+++ b/internal/core/src/index/ScalarIndexSort.h
@@ -116,6 +116,28 @@ class ScalarIndexSort : public ScalarIndex<T> {
         return size_ == 0;
     }
 
+    void
+    ComputeByteSize() override {
+        ScalarIndex<T>::ComputeByteSize();
+        int64_t total = this->cached_byte_size_;
+
+        // idx_to_offsets_: vector<int32_t>
+        total += idx_to_offsets_.capacity() * sizeof(int32_t);
+
+        // valid_bitset_: TargetBitmap
+        total += valid_bitset_.size_in_bytes();
+
+        if (is_mmap_) {
+            // mmap mode: add mmap size and filepath
+            total += mmap_size_;
+        } else {
+            // memory mode: add data vector
+            total += data_.capacity() * sizeof(IndexStructure<T>);
+        }
+
+        this->cached_byte_size_ = total;
+    }
+
     IndexStatsPtr
     Upload(const Config& config = {}) override;
 
diff --git a/internal/core/src/index/StringIndexMarisa.cpp b/internal/core/src/index/StringIndexMarisa.cpp
index 594f9d48ad..2d56f61420 100644
--- a/internal/core/src/index/StringIndexMarisa.cpp
+++ b/internal/core/src/index/StringIndexMarisa.cpp
@@ -57,6 +57,29 @@ StringIndexMarisa::Size() {
     return total_size_;
 }
 
+void
+StringIndexMarisa::ComputeByteSize() {
+    StringIndex::ComputeByteSize();
+    int64_t total = cached_byte_size_;
+
+    // Size of the trie structure (marisa trie uses io_size() for serialized/memory size)
+    total += trie_.io_size();
+
+    // str_ids_: vector<int64_t>
+    total += str_ids_.capacity() * sizeof(int64_t);
+
+    // str_ids_to_offsets_: map<size_t, vector<size_t>>
+    for (const auto& [key, vec] : str_ids_to_offsets_) {
+        total += sizeof(size_t);                   // key
+        total += vec.capacity() * sizeof(size_t);  // vector capacity
+        total += sizeof(std::vector<size_t>);      // vector object overhead
+    }
+    // Map node overhead (rough estimate: ~40 bytes per node for std::map)
+    total += str_ids_to_offsets_.size() * 40;
+
+    cached_byte_size_ = total;
+}
+
 int64_t
 StringIndexMarisa::CalculateTotalSize() const {
     int64_t size = 0;
@@ -135,6 +158,7 @@ StringIndexMarisa::BuildWithFieldData(
 
     built_ = true;
     total_size_ = CalculateTotalSize();
+    ComputeByteSize();
 }
 
 void
@@ -161,6 +185,7 @@ StringIndexMarisa::Build(size_t n,
 
     built_ = true;
     total_size_ = CalculateTotalSize();
+    ComputeByteSize();
 }
 
 BinarySet
@@ -247,6 +272,7 @@ StringIndexMarisa::LoadWithoutAssemble(const BinarySet& set,
     fill_offsets();
     built_ = true;
     total_size_ = CalculateTotalSize();
+    ComputeByteSize();
 }
 
 void
diff --git a/internal/core/src/index/StringIndexMarisa.h b/internal/core/src/index/StringIndexMarisa.h
index 18bf0545b5..c07f87ca5a 100644
--- a/internal/core/src/index/StringIndexMarisa.h
+++ b/internal/core/src/index/StringIndexMarisa.h
@@ -35,6 +35,9 @@ class StringIndexMarisa : public StringIndex {
     int64_t
     Size() override;
 
+    void
+    ComputeByteSize() override;
+
     BinarySet
     Serialize(const Config& config) override;
 
diff --git a/internal/core/src/index/StringIndexSort.cpp b/internal/core/src/index/StringIndexSort.cpp
index d546e28f04..00b32dd3f1 100644
--- a/internal/core/src/index/StringIndexSort.cpp
+++ b/internal/core/src/index/StringIndexSort.cpp
@@ -139,6 +139,7 @@ StringIndexSort::Build(size_t n,
 
     is_built_ = true;
     total_size_ = CalculateTotalSize();
+    ComputeByteSize();
 }
 
 void
@@ -182,6 +183,7 @@ StringIndexSort::BuildWithFieldData(
 
     is_built_ = true;
     total_size_ = CalculateTotalSize();
+    ComputeByteSize();
 }
 
 BinarySet
@@ -335,6 +337,7 @@ StringIndexSort::LoadWithoutAssemble(const BinarySet& binary_set,
 
     is_built_ = true;
     total_size_ = CalculateTotalSize();
+    ComputeByteSize();
 }
 
 const TargetBitmap
@@ -413,6 +416,26 @@ StringIndexSort::CalculateTotalSize() const {
     return size;
 }
 
+void
+StringIndexSort::ComputeByteSize() {
+    StringIndex::ComputeByteSize();
+    int64_t total = cached_byte_size_;
+
+    // Common structures (always in memory)
+    // idx_to_offsets_: vector<int32_t>
+    total += idx_to_offsets_.capacity() * sizeof(int32_t);
+
+    // valid_bitset_: TargetBitmap
+    total += valid_bitset_.size_in_bytes();
+
+    // Add impl-specific memory usage
+    if (impl_) {
+        total += impl_->ByteSize();
+    }
+
+    cached_byte_size_ = total;
+}
+
 void
 StringIndexSortMemoryImpl::BuildFromMap(
     std::map<std::string, PostingList>&& map,
@@ -855,6 +878,36 @@ StringIndexSortMemoryImpl::Size() {
     return size;
 }
 
+int64_t
+StringIndexSortMemoryImpl::ByteSize() const {
+    int64_t total = 0;
+
+    // unique_values_: vector<string>
+    // sizeof(std::string) includes the SSO buffer
+    // For heap-allocated strings (capacity > SSO threshold), we need to add external buffer
+    const size_t sso_threshold = GetStringSSOThreshold();
+    total += unique_values_.capacity() * sizeof(std::string);
+    for (const auto& str : unique_values_) {
+        // Only add capacity for heap-allocated strings (non-SSO)
+        if (str.capacity() > sso_threshold) {
+            total += str.capacity();
+        }
+    }
+
+    // posting_lists_: vector<PostingList>
+    // PostingList is folly::small_vector<uint32_t, 4>
+    // sizeof(PostingList) includes the inline buffer for 4 elements
+    total += posting_lists_.capacity() * sizeof(PostingList);
+    for (const auto& list : posting_lists_) {
+        // If the capacity exceeds inline capacity (4), it allocates on heap
+        if (list.capacity() > 4) {
+            total += list.capacity() * sizeof(uint32_t);
+        }
+    }
+
+    return total;
+}
+
 StringIndexSortMmapImpl::~StringIndexSortMmapImpl() {
     if (mmap_data_ != nullptr && mmap_data_ != MAP_FAILED) {
         munmap(mmap_data_, mmap_size_);
@@ -1147,4 +1200,10 @@ StringIndexSortMmapImpl::Size() {
     return mmap_size_;
 }
 
+int64_t
+StringIndexSortMmapImpl::ByteSize() const {
+    // mmap size (O(n) - the mapped index data)
+    return mmap_size_;
+}
+
 }  // namespace milvus::index
diff --git a/internal/core/src/index/StringIndexSort.h b/internal/core/src/index/StringIndexSort.h
index a1e2c44ec0..998aadfcd5 100644
--- a/internal/core/src/index/StringIndexSort.h
+++ b/internal/core/src/index/StringIndexSort.h
@@ -123,6 +123,11 @@ class StringIndexSort : public StringIndex {
     int64_t
     Size() override;
 
+    // Computes and caches the total memory usage in bytes.
+    // For mmap mode, this includes both memory-resident structures and mmap size.
+    void
+    ComputeByteSize() override;
+
  protected:
     int64_t
     CalculateTotalSize() const;
@@ -199,6 +204,10 @@ class StringIndexSortImpl {
 
     virtual int64_t
     Size() = 0;
+
+    // Returns the memory usage in bytes for this impl
+    virtual int64_t
+    ByteSize() const = 0;
 };
 
 class StringIndexSortMemoryImpl : public StringIndexSortImpl {
@@ -273,6 +282,9 @@ class StringIndexSortMemoryImpl : public StringIndexSortImpl {
     int64_t
     Size() override;
 
+    int64_t
+    ByteSize() const override;
+
  private:
     // Helper method for binary search
     size_t
@@ -387,6 +399,9 @@ class StringIndexSortMmapImpl : public StringIndexSortImpl {
     int64_t
     Size() override;
 
+    int64_t
+    ByteSize() const override;
+
  private:
     // Binary search for a value
     size_t
diff --git a/internal/core/src/index/Utils.h b/internal/core/src/index/Utils.h
index cef0ab0f66..ad102bab65 100644
--- a/internal/core/src/index/Utils.h
+++ b/internal/core/src/index/Utils.h
@@ -234,4 +234,12 @@ void inline SetBitsetGrowing(void* bitset,
     }
 }
 
+// Get the SSO (Small String Optimization) threshold for std::string.
+// Strings with capacity <= this threshold store data inline (no heap allocation).
+inline size_t
+GetStringSSOThreshold() {
+    static const size_t threshold = std::string().capacity();
+    return threshold;
+}
+
 }  // namespace milvus::index