diff --git a/doc/Makefile b/doc/Makefile
index 2ab5a5ce..5330389e 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -3,11 +3,11 @@
 SHELL := bash -euo pipefail
 
 # You can set these variables from the command line, and also
-# from the environment for the first two.
+# from the environment for the first three.
 SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build
+BUILDDIR      ?= _build
 SOURCEDIR     = .
-BUILDDIR      = _build
 
 # Require stricter builds with
 #   -n: warn on missing references
diff --git a/doc/make.bat b/doc/make.bat
index 2119f510..24c26b22 100644
--- a/doc/make.bat
+++ b/doc/make.bat
@@ -7,8 +7,10 @@ REM Command file for Sphinx documentation
 if "%SPHINXBUILD%" == "" (
 	set SPHINXBUILD=sphinx-build
 )
+if "%BUILDDIR%" == "" (
+	set BUILDDIR=_build
+)
 set SOURCEDIR=.
-set BUILDDIR=_build
 
 if "%1" == "" goto help