diff --git a/image_assembler.py b/image_assembler.py
index fd58245..3ff5259 100644
--- a/image_assembler.py
+++ b/image_assembler.py
@@ -15,7 +15,7 @@ class ImageAssembler:
         self.output = Image.new('RGBA', (metadata['width'], metadata['height']), (255, 255, 255, 0))
 
         # TODO double check this logic
-        self.output_width_cells = math.ceil(float(metadata['width']) / cell_size)
+        self.output_width_cells = 1 + math.ceil(float(metadata['width'] - cell_size) / (cell_size - 2*padding))
 
     def atlas_get_cell(self, cell_no: int):
         cell_x_pos = cell_no % self.atlas_width_cells