diff --git a/Documentation/config.txt b/Documentation/config.txt
index 7b676710ba..62b697c087 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -808,6 +808,8 @@ pack.threads::
 	warning. This is meant to reduce packing time on multiprocessor
 	machines. The required amount of memory for the delta search window
 	is however multiplied by the number of threads.
+	Specifying 0 will cause git to auto-detect the number of CPU's
+	and set the number of threads accordingly.
 
 pack.indexVersion::
 	Specify the default pack index version.  Valid values are 1 for
diff --git a/Documentation/git-pack-objects.txt b/Documentation/git-pack-objects.txt
index 8353be186f..5c1bd3b081 100644
--- a/Documentation/git-pack-objects.txt
+++ b/Documentation/git-pack-objects.txt
@@ -177,6 +177,8 @@ base-name::
 	This is meant to reduce packing time on multiprocessor machines.
 	The required amount of memory for the delta search window is
 	however multiplied by the number of threads.
+	Specifying 0 will cause git to auto-detect the number of CPU's
+	and set the number of threads accordingly.
 
 --index-version=<version>[,<offset>]::
 	This is intended to be used by the test suite only. It allows
diff --git a/Makefile b/Makefile
index d33a556ffe..2dc82476e5 100644
--- a/Makefile
+++ b/Makefile
@@ -741,6 +741,7 @@ endif
 ifdef THREADED_DELTA_SEARCH
 	BASIC_CFLAGS += -DTHREADED_DELTA_SEARCH
 	EXTLIBS += -lpthread
+	LIB_OBJS += thread-utils.o
 endif
 
 ifeq ($(TCLTK_PATH),)
diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
index d2bb12e574..586ae11975 100644
--- a/builtin-pack-objects.c
+++ b/builtin-pack-objects.c
@@ -16,6 +16,7 @@
 #include "progress.h"
 
 #ifdef THREADED_DELTA_SEARCH
+#include "thread-utils.h"
 #include <pthread.h>
 #endif
 
@@ -1852,11 +1853,11 @@ static int git_pack_config(const char *k, const char *v)
 	}
 	if (!strcmp(k, "pack.threads")) {
 		delta_search_threads = git_config_int(k, v);
-		if (delta_search_threads < 1)
+		if (delta_search_threads < 0)
 			die("invalid number of threads specified (%d)",
 			    delta_search_threads);
 #ifndef THREADED_DELTA_SEARCH
-		if (delta_search_threads > 1)
+		if (delta_search_threads != 1)
 			warning("no threads support, ignoring %s", k);
 #endif
 		return 0;
@@ -2122,10 +2123,10 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		if (!prefixcmp(arg, "--threads=")) {
 			char *end;
 			delta_search_threads = strtoul(arg+10, &end, 0);
-			if (!arg[10] || *end || delta_search_threads < 1)
+			if (!arg[10] || *end || delta_search_threads < 0)
 				usage(pack_usage);
 #ifndef THREADED_DELTA_SEARCH
-			if (delta_search_threads > 1)
+			if (delta_search_threads != 1)
 				warning("no threads support, "
 					"ignoring %s", arg);
 #endif
@@ -2235,6 +2236,11 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	if (!pack_to_stdout && thin)
 		die("--thin cannot be used to build an indexable pack.");
 
+#ifdef THREADED_DELTA_SEARCH
+	if (!delta_search_threads)	/* --threads=0 means autodetect */
+		delta_search_threads = online_cpus();
+#endif
+
 	prepare_packed_git();
 
 	if (progress)
diff --git a/thread-utils.c b/thread-utils.c
new file mode 100644
index 0000000000..55e7e2904e
--- /dev/null
+++ b/thread-utils.c
@@ -0,0 +1,48 @@
+#include "cache.h"
+
+#ifdef _WIN32
+#  define WIN32_LEAN_AND_MEAN
+#  include <windows.h>
+#elif defined(hpux) || defined(__hpux) || defined(_hpux)
+#  include <sys/pstat.h>
+#endif
+
+/*
+ * By doing this in two steps we can at least get
+ * the function to be somewhat coherent, even
+ * with this disgusting nest of #ifdefs.
+ */
+#ifndef _SC_NPROCESSORS_ONLN
+#  ifdef _SC_NPROC_ONLN
+#    define _SC_NPROCESSORS_ONLN _SC_NPROC_ONLN
+#  elif defined _SC_CRAY_NCPU
+#    define _SC_NPROCESSORS_ONLN _SC_CRAY_NCPU
+#  endif
+#endif
+
+int online_cpus(void)
+{
+#ifdef _SC_NPROCESSORS_ONLN
+	long ncpus;
+#endif
+
+#ifdef _WIN32
+	SYSTEM_INFO info;
+	GetSystemInfo(&info);
+
+	if ((int)info.dwNumberOfProcessors > 0)
+		return (int)info.dwNumberOfProcessors;
+#elif defined(hpux) || defined(__hpux) || defined(_hpux)
+	struct pst_dynamic psd;
+
+	if (!pstat_getdynamic(&psd, sizeof(psd), (size_t)1, 0))
+		return (int)psd.psd_proc_cnt;
+#endif
+
+#ifdef _SC_NPROCESSORS_ONLN
+	if ((ncpus = (long)sysconf(_SC_NPROCESSORS_ONLN)) > 0)
+		return (int)ncpus;
+#endif
+
+	return 1;
+}
diff --git a/thread-utils.h b/thread-utils.h
new file mode 100644
index 0000000000..cce4b77bd6
--- /dev/null
+++ b/thread-utils.h
@@ -0,0 +1,6 @@
+#ifndef THREAD_COMPAT_H
+#define THREAD_COMPAT_H
+
+extern int online_cpus(void);
+
+#endif /* THREAD_COMPAT_H */