diff --git a/fs_mgr/libsnapshot/dm_snapshot_internals.h b/fs_mgr/libsnapshot/dm_snapshot_internals.h
new file mode 100644
index 0000000..4903de1
--- /dev/null
+++ b/fs_mgr/libsnapshot/dm_snapshot_internals.h
@@ -0,0 +1,133 @@
+// Copyright (C) 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stdint.h>
+
+#include <vector>
+
+namespace android {
+namespace snapshot {
+
+class DmSnapCowSizeCalculator {
+  public:
+    DmSnapCowSizeCalculator(unsigned int sector_bytes, unsigned int chunk_sectors)
+        : sector_bytes_(sector_bytes),
+          chunk_sectors_(chunk_sectors),
+          exceptions_per_chunk(chunk_sectors_ * sector_bytes_ / (64 * 2 / 8)) {}
+
+    void WriteByte(uint64_t address) { WriteSector(address / sector_bytes_); }
+    void WriteSector(uint64_t sector) { WriteChunk(sector / chunk_sectors_); }
+    void WriteChunk(uint64_t chunk_id) {
+        if (modified_chunks_.size() <= chunk_id) {
+            modified_chunks_.resize(chunk_id + 1, false);
+        }
+        modified_chunks_[chunk_id] = true;
+    }
+
+    uint64_t cow_size_bytes() const { return cow_size_sectors() * sector_bytes_; }
+    uint64_t cow_size_sectors() const { return cow_size_chunks() * chunk_sectors_; }
+
+    /*
+     * The COW device has a precise internal structure as follows:
+     *
+     * - header (1 chunk)
+     * - #0 map and chunks
+     *   - map (1 chunk)
+     *   - chunks addressable by previous map (exceptions_per_chunk)
+     * - #1 map and chunks
+     *   - map (1 chunk)
+     *   - chunks addressable by previous map (exceptions_per_chunk)
+     * ...
+     * - #n: map and chunks
+     *   - map (1 chunk)
+     *   - chunks addressable by previous map (exceptions_per_chunk)
+     * - 1 extra chunk
+     */
+    uint64_t cow_size_chunks() const {
+        uint64_t modified_chunks_count = 0;
+        uint64_t cow_chunks = 0;
+
+        for (const auto& c : modified_chunks_) {
+            if (c) {
+                ++modified_chunks_count;
+            }
+        }
+
+        /* disk header + padding = 1 chunk */
+        cow_chunks += 1;
+
+        /* snapshot modified chunks */
+        cow_chunks += modified_chunks_count;
+
+        /* snapshot chunks index metadata */
+        cow_chunks += 1 + modified_chunks_count / exceptions_per_chunk;
+
+        return cow_chunks;
+    }
+
+  private:
+    /*
+     * Size of each sector in bytes.
+     */
+    const uint64_t sector_bytes_;
+
+    /*
+     * Size of each chunk in sectors.
+     */
+    const uint64_t chunk_sectors_;
+
+    /*
+     * The COW device stores tables to map the modified chunks. Each table
+     * has the size of exactly 1 chunk.
+     * Each row of the table (also called exception in the kernel) contains two
+     * 64 bit indices to identify the corresponding chunk, and this 128 bit row
+     * size is a constant.
+     * The number of exceptions that each table can contain determines the
+     * number of data chunks that separate two consecutive tables. This value
+     * is then fundamental to compute the space overhead introduced by the
+     * tables in COW devices.
+     */
+    const uint64_t exceptions_per_chunk;
+
+    /*
+     * |modified_chunks_| is a container that keeps trace of the modified
+     * chunks.
+     * Multiple options were considered when choosing the most appropriate data
+     * structure for this container. Here follows a summary of why vector<bool>
+     * has been chosen, taking as a reference a snapshot partition of 4 GiB and
+     * chunk size of 4 KiB.
+     * - std::set<uint64_t> is very space-efficient for a small number of
+     *   operations, but if the whole snapshot is changed, it would need to
+     *   store
+     *     4 GiB / 4 KiB * (64 bit / 8) = 8 MiB
+     *   just for the data, plus the additional data overhead for the red-black
+     *   tree used for data sorting (if each rb-tree element stores 3 address
+     *   and the word-aligne color, the total size grows to 32 MiB).
+     * - std::bitset<N> is not a good fit because requires a priori knowledge,
+     *   at compile time, of the bitset size.
+     * - std::vector<bool> is a special case of vector, which uses a data
+     *   compression that allows reducing the space utilization of each element
+     *   to 1 bit. In detail, this data structure is composed of a resizable
+     *   array of words, each of them representing a bitmap. On a 64 bit
+     *   device, modifying the whole 4 GiB snapshot grows this container up to
+     *     4 * GiB / 4 KiB / 64 = 64 KiB
+     *   that, even if is the same space requirement to change a single byte at
+     *   the highest address of the snapshot, is a very affordable space
+     *   requirement.
+     */
+    std::vector<bool> modified_chunks_;
+};
+
+}  // namespace snapshot
+}  // namespace android
diff --git a/fs_mgr/libsnapshot/partition_cow_creator_test.cpp b/fs_mgr/libsnapshot/partition_cow_creator_test.cpp
index cf2d745..f683f5b 100644
--- a/fs_mgr/libsnapshot/partition_cow_creator_test.cpp
+++ b/fs_mgr/libsnapshot/partition_cow_creator_test.cpp
@@ -17,6 +17,7 @@
 #include <liblp/builder.h>
 #include <liblp/property_fetcher.h>
 
+#include "dm_snapshot_internals.h"
 #include "partition_cow_creator.h"
 #include "test_helpers.h"
 
@@ -99,5 +100,31 @@
     ASSERT_TRUE(ret.has_value());
 }
 
+TEST(DmSnapshotInternals, CowSizeCalculator) {
+    DmSnapCowSizeCalculator cc(512, 8);
+    unsigned long int b;
+
+    // Empty COW
+    ASSERT_EQ(cc.cow_size_sectors(), 16);
+
+    // First chunk written
+    for (b = 0; b < 4_KiB; ++b) {
+        cc.WriteByte(b);
+        ASSERT_EQ(cc.cow_size_sectors(), 24);
+    }
+
+    // Second chunk written
+    for (b = 4_KiB; b < 8_KiB; ++b) {
+        cc.WriteByte(b);
+        ASSERT_EQ(cc.cow_size_sectors(), 32);
+    }
+
+    // Leave a hole and write 5th chunk
+    for (b = 16_KiB; b < 20_KiB; ++b) {
+        cc.WriteByte(b);
+        ASSERT_EQ(cc.cow_size_sectors(), 40);
+    }
+}
+
 }  // namespace snapshot
 }  // namespace android