diff --git a/src/borg/archive.py b/src/borg/archive.py
index a0bde175e..91239bdcd 100644
--- a/src/borg/archive.py
+++ b/src/borg/archive.py
@@ -25,7 +25,7 @@
 from .crypto.key import key_factory
 from .compress import Compressor, CompressionSpec
 from .constants import *  # NOQA
-from .hashindex import ChunkIndex, ChunkIndexEntry
+from .hashindex import ChunkIndex, ChunkIndexEntry, CacheSynchronizer
 from .helpers import Manifest
 from .helpers import hardlinkable
 from .helpers import ChunkIteratorFileWrapper, open_item
@@ -478,30 +478,22 @@ def save(self, name=None, comment=None, timestamp=None, additional_metadata=None
 
     def calc_stats(self, cache):
         def add(id):
-            count, size, csize = cache.chunks[id]
-            stats.update(size, csize, count == 1)
-            cache.chunks[id] = count - 1, size, csize
+            entry = cache.chunks[id]
+            archive_index.add(id, 1, entry.size, entry.csize)
 
-        def add_file_chunks(chunks):
-            for id, _, _ in chunks:
-                add(id)
-
-        # This function is a bit evil since it abuses the cache to calculate
-        # the stats. The cache transaction must be rolled back afterwards
-        unpacker = msgpack.Unpacker(use_list=False)
-        cache.begin_txn()
-        stats = Statistics()
+        archive_index = ChunkIndex()
+        sync = CacheSynchronizer(archive_index)
         add(self.id)
+        pi = ProgressIndicatorPercent(total=len(self.metadata.items), msg='Calculating statistics... %3d%%')
         for id, chunk in zip(self.metadata.items, self.repository.get_many(self.metadata.items)):
+            pi.show(increase=1)
             add(id)
             data = self.key.decrypt(id, chunk)
-            unpacker.feed(data)
-            for item in unpacker:
-                chunks = item.get(b'chunks')
-                if chunks is not None:
-                    stats.nfiles += 1
-                    add_file_chunks(chunks)
-        cache.rollback()
+            sync.feed(data)
+        stats = Statistics()
+        stats.osize, stats.csize, unique_size, stats.usize, unique_chunks, chunks = archive_index.stats_against(cache.chunks)
+        stats.nfiles = sync.num_files
+        pi.finish()
         return stats
 
     @contextmanager
diff --git a/src/borg/cache_sync/cache_sync.c b/src/borg/cache_sync/cache_sync.c
index 70f568f82..53b615522 100644
--- a/src/borg/cache_sync/cache_sync.c
+++ b/src/borg/cache_sync/cache_sync.c
@@ -38,6 +38,7 @@ cache_sync_init(HashIndex *chunks)
     unpack_init(&ctx->ctx);
     /* needs to be set only once */
     ctx->ctx.user.chunks = chunks;
+    ctx->ctx.user.num_files = 0;
     ctx->buf = NULL;
     ctx->head = 0;
     ctx->tail = 0;
@@ -56,11 +57,17 @@ cache_sync_free(CacheSyncCtx *ctx)
 }
 
 static const char *
-cache_sync_error(CacheSyncCtx *ctx)
+cache_sync_error(const CacheSyncCtx *ctx)
 {
     return ctx->ctx.user.last_error;
 }
 
+static uint64_t
+cache_sync_num_files(const CacheSyncCtx *ctx)
+{
+    return ctx->ctx.user.num_files;
+}
+
 /**
  * feed data to the cache synchronizer
  * 0 = abort, 1 = continue
diff --git a/src/borg/cache_sync/unpack.h b/src/borg/cache_sync/unpack.h
index 4a0ba1d41..8332fcff9 100644
--- a/src/borg/cache_sync/unpack.h
+++ b/src/borg/cache_sync/unpack.h
@@ -50,6 +50,8 @@ typedef struct unpack_user {
 
     HashIndex *chunks;
 
+    uint64_t num_files;
+
     /*
      * We don't care about most stuff. This flag tells us whether we're at the chunks structure,
      * meaning:
@@ -358,6 +360,7 @@ static inline int unpack_callback_raw(unpack_user* u, const char* b, const char*
         if(length == 6 && !memcmp("chunks", p, 6)) {
             u->expect = expect_chunks_begin;
             u->inside_chunks = 1;
+            u->num_files++;
         } else {
             u->expect = expect_map_item_end;
         }
diff --git a/src/borg/hashindex.pyx b/src/borg/hashindex.pyx
index 084518f96..bf84d6d49 100644
--- a/src/borg/hashindex.pyx
+++ b/src/borg/hashindex.pyx
@@ -9,7 +9,7 @@ from libc.errno cimport errno
 from cpython.exc cimport PyErr_SetFromErrnoWithFilename
 from cpython.buffer cimport PyBUF_SIMPLE, PyObject_GetBuffer, PyBuffer_Release
 
-API_VERSION = '1.1_03'
+API_VERSION = '1.1_04'
 
 
 cdef extern from "_hashindex.c":
@@ -38,7 +38,8 @@ cdef extern from "cache_sync/cache_sync.c":
         pass
 
     CacheSyncCtx *cache_sync_init(HashIndex *chunks)
-    const char *cache_sync_error(CacheSyncCtx *ctx)
+    const char *cache_sync_error(const CacheSyncCtx *ctx)
+    uint64_t cache_sync_num_files(const CacheSyncCtx *ctx)
     int cache_sync_feed(CacheSyncCtx *ctx, void *data, uint32_t length)
     void cache_sync_free(CacheSyncCtx *ctx)
 
@@ -329,6 +330,48 @@ cdef class ChunkIndex(IndexBase):
 
         return size, csize, unique_size, unique_csize, unique_chunks, chunks
 
+    def stats_against(self, ChunkIndex master_index):
+        """
+        Calculate chunk statistics of this index against *master_index*.
+
+        A chunk is counted as unique if the number of references
+        in this index matches the number of references in *master_index*.
+
+        This index must be a subset of *master_index*.
+
+        Return the same statistics tuple as summarize:
+        size, csize, unique_size, unique_csize, unique_chunks, chunks.
+        """
+        cdef uint64_t size = 0, csize = 0, unique_size = 0, unique_csize = 0, chunks = 0, unique_chunks = 0
+        cdef uint32_t our_refcount, chunk_size, chunk_csize
+        cdef const uint32_t *our_values
+        cdef const uint32_t *master_values
+        cdef const void *key = NULL
+        cdef HashIndex *master = master_index.index
+
+        while True:
+            key = hashindex_next_key(self.index, key)
+            if not key:
+                break
+            our_values = <const uint32_t*> (key + self.key_size)
+            master_values = <const uint32_t*> hashindex_get(master, key)
+            if not master_values:
+                raise ValueError('stats_against: key contained in self but not in master_index.')
+            our_refcount = _le32toh(our_values[0])
+            chunk_size = _le32toh(master_values[1])
+            chunk_csize = _le32toh(master_values[2])
+
+            chunks += our_refcount
+            size += <uint64_t> chunk_size * our_refcount
+            csize += <uint64_t> chunk_csize * our_refcount
+            if our_values[0] == master_values[0]:
+                # our refcount equals the master's refcount, so this chunk is unique to us
+                unique_chunks += 1
+                unique_size += chunk_size
+                unique_csize += chunk_csize
+
+        return size, csize, unique_size, unique_csize, unique_chunks, chunks
+
     def add(self, key, refs, size, csize):
         assert len(key) == self.key_size
         cdef uint32_t[3] data
@@ -420,3 +463,7 @@ cdef class CacheSynchronizer:
             error = cache_sync_error(self.sync)
             if error != NULL:
                 raise ValueError('cache_sync_feed failed: ' + error.decode('ascii'))
+
+    @property
+    def num_files(self):
+        return cache_sync_num_files(self.sync)
diff --git a/src/borg/helpers.py b/src/borg/helpers.py
index f65aac9bb..fdacd1418 100644
--- a/src/borg/helpers.py
+++ b/src/borg/helpers.py
@@ -131,7 +131,7 @@ class MandatoryFeatureUnsupported(Error):
 
 def check_extension_modules():
     from . import platform, compress, item
-    if hashindex.API_VERSION != '1.1_03':
+    if hashindex.API_VERSION != '1.1_04':
         raise ExtensionModuleError
     if chunker.API_VERSION != '1.1_01':
         raise ExtensionModuleError