diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index e9d21321e7..6aa71ba011 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -2750,6 +2750,8 @@ int avcodec_parse_frame(AVCodecContext *avctx, uint8_t **pdata,
  * @param[in] samples the input buffer containing the samples
  * The number of samples read from this buffer is frame_size*channels,
  * both of which are defined in \p avctx.
+ * For PCM audio the number of samples read from \p samples is equal to
+ * \p buf_size * input_sample_size / output_sample_size.
  * @return On error a negative value is returned, on success zero or the number
  * of bytes used to encode the data read from the input buffer.
  */