diff --git a/include/libopencm3/usb/usbd.h b/include/libopencm3/usb/usbd.h
index 0baa7efa..83df0300 100644
--- a/include/libopencm3/usb/usbd.h
+++ b/include/libopencm3/usb/usbd.h
@@ -42,6 +42,9 @@ LGPL License Terms @ref lgpl_license
 
 BEGIN_DECLS
 
+#ifndef ARRAY_LENGTH
+#define ARRAY_LENGTH(array) (sizeof(array) / sizeof(*(array)))
+#endif
 
 enum usbd_request_return_codes {
 	USBD_REQ_NOTSUPP	= 0,