diff --git a/include/nuttx/lib/float.h b/include/nuttx/lib/float.h
index b15f52c3604..d130022d9c0 100644
--- a/include/nuttx/lib/float.h
+++ b/include/nuttx/lib/float.h
@@ -42,6 +42,12 @@
 
 #define FLT_RADIX 2
 
+/* Addition rounds to 0: zero, 1: nearest, 2: +inf, 3: -inf, -1: unknown.  */
+
+#ifndef FLT_ROUNDS  /* May be defined in a toolchain header */
+#  define FLT_ROUNDS 1
+#endif
+
 /* Number of base-FLT_RADIX digits in the floating-point significand, p. */
 
 #ifndef FLT_MANT_DIG  /* May be defined in a toolchain header */