diff --git a/flight/PiOS/STM32F10x/pios_sys.c b/flight/PiOS/STM32F10x/pios_sys.c
index 04d79c45b..6b59afbad 100644
--- a/flight/PiOS/STM32F10x/pios_sys.c
+++ b/flight/PiOS/STM32F10x/pios_sys.c
@@ -118,8 +118,8 @@ uint32_t PIOS_SYS_getCPUFlashSize(void)
 
 /**
 * Returns the serial number as a string
-* param[out] str pointer to a string which can store at least 32 digits + zero terminator!
-* (24 digits returned for STM32)
+* param[out] uint8_t pointer to a string which can store at least 12 bytes
+* (12 bytes returned for STM32)
 * return < 0 if feature not supported
 */
 int32_t PIOS_SYS_SerialNumberGetBinary(uint8_t *array)
@@ -127,7 +127,7 @@ int32_t PIOS_SYS_SerialNumberGetBinary(uint8_t *array)
 	int i;
 
 	/* Stored in the so called "electronic signature" */
-	for (i = 0; i < 12; ++i) {
+	for (i = 0; i < PIOS_SYS_SERIAL_NUM_BINARY_LEN; ++i) {
 		uint8_t b = MEM8(0x1ffff7e8 + i);
 
 		array[i] = b;
@@ -148,7 +148,7 @@ int32_t PIOS_SYS_SerialNumberGet(char *str)
 	int i;
 
 	/* Stored in the so called "electronic signature" */
-	for (i = 0; i < 24; ++i) {
+	for (i = 0; i < PIOS_SYS_SERIAL_NUM_ASCII_LEN; ++i) {
 		uint8_t b = MEM8(0x1ffff7e8 + (i / 2));
 		if (!(i & 1))
 			b >>= 4;
diff --git a/flight/PiOS/inc/pios_sys.h b/flight/PiOS/inc/pios_sys.h
index 9407fa720..7f183a892 100644
--- a/flight/PiOS/inc/pios_sys.h
+++ b/flight/PiOS/inc/pios_sys.h
@@ -32,12 +32,15 @@
 #ifndef PIOS_SYS_H
 #define PIOS_SYS_H
 
+#define PIOS_SYS_SERIAL_NUM_BINARY_LEN 12
+#define PIOS_SYS_SERIAL_NUM_ASCII_LEN (PIOS_SYS_SERIAL_NUM_BINARY_LEN * 2)
+
 /* Public Functions */
 extern void PIOS_SYS_Init(void);
 extern int32_t PIOS_SYS_Reset(void);
 extern uint32_t PIOS_SYS_getCPUFlashSize(void);
-extern int32_t PIOS_SYS_SerialNumberGetBinary(uint8_t *array);
-extern int32_t PIOS_SYS_SerialNumberGet(char *str);
+extern int32_t PIOS_SYS_SerialNumberGetBinary(uint8_t array[PIOS_SYS_SERIAL_NUM_BINARY_LEN]);
+extern int32_t PIOS_SYS_SerialNumberGet(char str[PIOS_SYS_SERIAL_NUM_ASCII_LEN+1]);
 
 #endif /* PIOS_SYS_H */