diff --git a/build/shared/lib/keywords.txt b/build/shared/lib/keywords.txt
index fa4df1c20..d77a4b10c 100644
--- a/build/shared/lib/keywords.txt
+++ b/build/shared/lib/keywords.txt
@@ -177,6 +177,7 @@ read	KEYWORD2	Serial_Read
 print	KEYWORD2	Serial_Print
 println	KEYWORD2	Serial_Println
 available	KEYWORD2	Serial_Available
+availableForWrite	KEYWORD2
 flush	KEYWORD2	Serial_Flush
 setTimeout	KEYWORD2
 find	KEYWORD2
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
index ed29641b6..29a336649 100644
--- a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
@@ -176,6 +176,21 @@ int HardwareSerial::read(void)
   }
 }
 
+int HardwareSerial::availableForWrite(void)
+{
+#if (SERIAL_TX_BUFFER_SIZE>256)
+  uint8_t oldSREG = SREG;
+  cli();
+#endif
+  tx_buffer_index_t head = _tx_buffer_head;
+  tx_buffer_index_t tail = _tx_buffer_tail;
+#if (SERIAL_TX_BUFFER_SIZE>256)
+  SREG = oldSREG;
+#endif
+  if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
+  return tail - head - 1;
+}
+
 void HardwareSerial::flush()
 {
   // If we have never written a byte, no need to flush. This special
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.h b/hardware/arduino/avr/cores/arduino/HardwareSerial.h
index b96e5d058..935934b19 100644
--- a/hardware/arduino/avr/cores/arduino/HardwareSerial.h
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial.h
@@ -112,6 +112,7 @@ class HardwareSerial : public Stream
     virtual int available(void);
     virtual int peek(void);
     virtual int read(void);
+    int availableForWrite(void);
     virtual void flush(void);
     virtual size_t write(uint8_t);
     inline size_t write(unsigned long n) { return write((uint8_t)n); }