diff --git a/cores/rp2040/SerialPIO.cpp b/cores/rp2040/SerialPIO.cpp
index 32af295ac..ddd9b0a7f 100644
--- a/cores/rp2040/SerialPIO.cpp
+++ b/cores/rp2040/SerialPIO.cpp
@@ -338,7 +338,7 @@ int SerialPIO::available() {
     if (!_running || !m || (_rx == NOPIN)) {
         return 0;
     }
-    return (_writer - _reader) % _fifoSize;
+    return (_fifoSize + _writer - _reader) % _fifoSize;
 }
 
 int SerialPIO::availableForWrite() {