diff --git a/src/calibre/ebooks/conversion/plugins/txt_input.py b/src/calibre/ebooks/conversion/plugins/txt_input.py
index d5d6814830..81b5543d6b 100644
--- a/src/calibre/ebooks/conversion/plugins/txt_input.py
+++ b/src/calibre/ebooks/conversion/plugins/txt_input.py
@@ -110,7 +110,7 @@ class TXTInput(InputFormatPlugin):
             ienc = options.input_encoding
             log.debug('Using user specified input encoding of %s' % ienc)
         else:
-            det_encoding = detect(txt)
+            det_encoding = detect(txt[:4096])
             det_encoding, confidence = det_encoding['encoding'], det_encoding['confidence']
             if det_encoding and det_encoding.lower().replace('_', '-').strip() in (
                     'gb2312', 'chinese', 'csiso58gb231280', 'euc-cn', 'euccn',