diff --git a/src/core/ngx_slab.c b/src/core/ngx_slab.c
index 4357e8dd9a8..b33f43f8ea3 100644
--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -120,6 +120,7 @@ ngx_slab_init(ngx_slab_pool_t *pool)
     }
 
     p += n * sizeof(ngx_slab_page_t);
+    size -= n * sizeof(ngx_slab_page_t);
 
     pages = (ngx_uint_t) (size / (ngx_pagesize + sizeof(ngx_slab_page_t)));