diff --git a/README.md b/README.md
index 6835611..53b7e6c 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,11 @@ traversing lists while deleting/freeing nodes.
 Helper Macros
 -------------
 
+- `atonum(str)`
+
+  Convert string to natural number, works for 32-bit non-negative
+  integers.  Returns -1 on error.
+
 - `blkdev(dev)`
 
 Create block device
diff --git a/lite.h b/lite.h
index 8541628..2e6aace 100644
--- a/lite.h
+++ b/lite.h
@@ -101,6 +101,20 @@ static inline int fisslashdir(char *dir)
                          return 0;
 }
 
+/* Convert string to natural number (0-2147483647), returns -1 on error. */
+static inline int atonum(const char *str)
+{
+	int val = -1;
+	char *errstr;
+
+	if (str) {
+		val = strtonum(str, 0, INT32_MAX, &errstr);
+		if (errstr)
+			return -1;
+	}
+
+	return val;
+}
 
 /* Validate string, non NULL and not zero length */
 static inline int string_valid (const char *s)