diff --git a/docs/env_vars.md b/docs/env_vars.md
index 3f1926d3..b28849cc 100644
--- a/docs/env_vars.md
+++ b/docs/env_vars.md
@@ -12,6 +12,8 @@ wlroots specific
   wayland, x11, headless)
 * *WLR_WL_OUTPUTS*: when using the wayland backend specifies the number of outputs
 * *WLR_X11_OUTPUTS*: when using the X11 backend specifies the number of outputs
+* *WLR_HEADLESS_OUTPUTS*: when using the headless backend specifies the number
+  of outputs
 
 rootston specific
 ------------------