diff --git a/file_to_dna.py b/file_to_dna.py
index d86f0da71ce119859286c7df6b1c43fb1c9f8129..e7dcfef2e8f4792c36ad593531ef2e6a92bc2185 100755
--- a/file_to_dna.py
+++ b/file_to_dna.py
@@ -24,7 +24,7 @@ primer_size = 20
 overhang_size = 4
 bsaI_size = 7
 
-extern_extremity_size = primer_size + buffer_size # size of non-payload dna part at the extern extremities (start of first block and end of last block)
+extern_extremity_size = primer_size + overhang_size + bsaI_size + buffer_size # size of non-payload dna part at the extern extremities (start of first block and end of last block)
 intern_extremity_size = overhang_size + bsaI_size + buffer_size # size of non-payload dna part at the extremities between blocks
   
 max_total_block_size = 500 # maximum allowed size for a complete block