diff --git a/cores/arduino/HardwareSerial.h b/cores/arduino/HardwareSerial.h
index 1beafc5..8a5bf95 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -40,14 +40,14 @@
 // often work, but occasionally a race condition can occur that makes
 // Serial behave erratically. See https://github.com/arduino/Arduino/issues/2405
 #if !defined(SERIAL_TX_BUFFER_SIZE)
-#if (RAMEND < 1000)
+#if ((RAMEND - RAMSTART) < 1023)
 #define SERIAL_TX_BUFFER_SIZE 16
 #else
 #define SERIAL_TX_BUFFER_SIZE 64
 #endif
 #endif
 #if !defined(SERIAL_RX_BUFFER_SIZE)
-#if (RAMEND < 1000)
+#if ((RAMEND - RAMSTART) < 1023)
 #define SERIAL_RX_BUFFER_SIZE 16
 #else
 #define SERIAL_RX_BUFFER_SIZE 64
diff --git a/cores/arduino/USBAPI.h b/cores/arduino/USBAPI.h
index 4072772..f22ab6a 100644
--- a/cores/arduino/USBAPI.h
+++ b/cores/arduino/USBAPI.h
@@ -75,7 +75,7 @@ extern USBDevice_ USBDevice;
 struct ring_buffer;
 
 #ifndef SERIAL_BUFFER_SIZE
-#if (RAMEND < 1000)
+#if ((RAMEND - RAMSTART) < 1023)
 #define SERIAL_BUFFER_SIZE 16
 #else
 #define SERIAL_BUFFER_SIZE 64