diff --git a/fichero_printer/fichero/printer.py b/fichero_printer/fichero/printer.py
index c424b42..1614776 100644
--- a/fichero_printer/fichero/printer.py
+++ b/fichero_printer/fichero/printer.py
@@ -31,6 +31,11 @@ NOTIFY_UUID = "00002af0-0000-1000-8000-00805f9b34fb"
 # Printer name prefixes to auto-discover
 PRINTER_NAME_PREFIXES = ("FICHERO_", "D11s_")
 
+# --- Constants ---
+PRINTHEAD_PX = 96  # Fichero/D11s printhead width in pixels
+BYTES_PER_ROW = PRINTHEAD_PX // 8  # 12 bytes per row (96 pixels / 8)
+DOTS_PER_MM = 8  # 203 DPI / 25.4 mm/inch ≈ 8 dots/mm
+
 # --- Timing constants ---
 CHUNK_SIZE_BLE = 200  # BLE MTU-limited
 CHUNK_SIZE_CLASSIC = 4096  # RFCOMM can handle larger chunks