diff --git a/src/main/vcp_hal/usbd_cdc_interface.c b/src/main/vcp_hal/usbd_cdc_interface.c
index f1577643b1..b44576f8a1 100644
--- a/src/main/vcp_hal/usbd_cdc_interface.c
+++ b/src/main/vcp_hal/usbd_cdc_interface.c
@@ -409,11 +409,17 @@ uint32_t CDC_Receive_BytesAvailable(void)
 
 uint32_t CDC_Send_FreeBytes(void)
 {
-    // return the bytes free in the circular buffer
+    /*
+        return the bytes free in the circular buffer
+
+        functionally equivalent to:
+        (APP_Rx_ptr_out > APP_Rx_ptr_in ? APP_Rx_ptr_out - APP_Rx_ptr_in : APP_RX_DATA_SIZE - APP_Rx_ptr_in + APP_Rx_ptr_in)
+        but without the impact of the condition check.
+    */
     uint32_t freeBytes;
 
     ATOMIC_BLOCK(NVIC_BUILD_PRIORITY(6, 0)) {
-        freeBytes = APP_RX_DATA_SIZE - rxAvailable;
+        freeBytes = ((UserTxBufPtrOut - UserTxBufPtrIn) + (-((int)(UserTxBufPtrOut <= UserTxBufPtrIn)) & APP_TX_DATA_SIZE)) - 1;
     }
 
     return freeBytes;