diff --git a/photobook/photobook.py b/photobook/photobook.py
index 9c7e1ae..5c4c389 100644
--- a/photobook/photobook.py
+++ b/photobook/photobook.py
@@ -248,7 +248,7 @@ class Photobook(FPDF):
         height_unit = (pg_size[1] - (len(layout) - 1) * sep) / total_ratio
 
         for (r, row) in enumerate(layout):
-            width_unit = (pg_size[0] - (len(row) - 1) * sep) / sum(row)
+            width_unit = (pg_size[0] - (sum(row) - 1) * sep) / sum(row)
             row_height = height_unit * ratios[r]
 
             for (c, weight) in enumerate(row):
@@ -309,7 +309,7 @@ class Photobook(FPDF):
         width_unit = (pg_size[0] - (len(layout) - 1) * sep) / total_ratio
 
         for (c, column) in enumerate(layout):
-            height_unit = (pg_size[1] - (len(column) - 1) * sep) / sum(column)
+            height_unit = (pg_size[1] - (sum(column) - 1) * sep) / sum(column)
             column_width = width_unit * ratios[c]
 
             for (r, weight) in enumerate(column):