cache.h       |  3 ++-
 environment.c |  2 +-
 sha1_file.c   | 43 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/cache.h b/cache.h
index 6e33f2f28..a022e1bd2 100644
--- a/cache.h
+++ b/cache.h
@@ -1186,8 +1186,9 @@ static inline int hex2chr(const char *s)
 }
 
 /* Convert to/from hex/sha1 representation */
+extern int get_default_abbrev(void);
 #define MINIMUM_ABBREV minimum_abbrev
-#define DEFAULT_ABBREV default_abbrev
+#define DEFAULT_ABBREV get_default_abbrev()
 
 struct object_context {
 	unsigned char tree[20];
diff --git a/environment.c b/environment.c
index c1442df9a..fd6681e46 100644
--- a/environment.c
+++ b/environment.c
@@ -16,7 +16,7 @@ int trust_executable_bit = 1;
 int trust_ctime = 1;
 int check_stat = 1;
 int has_symlinks = 1;
-int minimum_abbrev = 4, default_abbrev = 7;
+int minimum_abbrev = 4, default_abbrev = -1;
 int ignore_case;
 int assume_unchanged;
 int prefer_symlink_refs;
diff --git a/sha1_file.c b/sha1_file.c
index ca149a607..28ba04b65 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -3720,3 +3720,46 @@ int for_each_packed_object(each_packed_object_fn cb, void *data, unsigned flags)
 	}
 	return r ? r : pack_errors;
 }
+
+static int init_default_abbrev(void)
+{
+	unsigned long count = 0;
+	struct packed_git *p;
+	struct strbuf buf = STRBUF_INIT;
+	DIR *dir;
+	char *name;
+	int ret;
+
+	prepare_packed_git();
+	for (p = packed_git; p; p = p->next) {
+		if (open_pack_index(p))
+			continue;
+		count += p->num_objects;
+	}
+
+	strbuf_addstr(&buf, get_object_directory());
+	strbuf_addstr(&buf, "/42/");
+	name = strbuf_detach(&buf, NULL);
+	dir = opendir(name);
+	free(name);
+	if (dir) {
+		struct dirent *de;
+		while ((de = readdir(dir)) != NULL) {
+			count += 256;
+		}
+		closedir(dir);
+	}
+	for (ret = 7; ret < 15; ret++) {
+		unsigned long expect_collision = 1ul << (ret * 2);
+		if (count < expect_collision)
+			break;
+	}
+	return ret;
+}
+
+int get_default_abbrev(void)
+{
+	if (default_abbrev < 0)
+		default_abbrev = init_default_abbrev();
+	return default_abbrev;
+}