diff --git a/doc/build_osx.md b/doc/build_osx.md
index fad610544e5..82a2354ac02 100644
--- a/doc/build_osx.md
+++ b/doc/build_osx.md
@@ -57,6 +57,12 @@ export CXX=g++-6
 export DISABLE_MONGODB=1
 ```
 
+## Detect number of threads
+
+```
+export THREADS=$(sysctl -n hw.ncpu)
+```
+
 # Build ClickHouse
 
 ```