diff --git a/src/grits.py b/src/grits.py
index c9d4828..eba104c 100644
--- a/src/grits.py
+++ b/src/grits.py
@@ -227,12 +227,9 @@ def iou(bbox1, bbox2):
     Compute the intersection-over-union of two bounding boxes.
     """
     intersection = Rect(bbox1).intersect(bbox2)
-    union = Rect(bbox1).include_rect(bbox2)
-    
-    union_area = union.get_area()
+    union_area = Rect(bbox1).get_area() + Rect(bbox2).get_area() - intersection.get_area()
     if union_area > 0:
-        return intersection.get_area() / union.get_area()
-    
+        return intersection.get_area() / union_area
     return 0
 
 
diff --git a/src/postprocess.py b/src/postprocess.py
index 25feaee..3bfd68c 100644
--- a/src/postprocess.py
+++ b/src/postprocess.py
@@ -5,6 +5,8 @@
 
 from fitz import Rect
 
+from grits import iou
+
 
 def apply_threshold(objects, threshold):
     """
@@ -31,20 +33,6 @@ def apply_class_thresholds(bboxes, labels, scores, class_names, class_thresholds
     return bboxes, scores, labels
 
 
-def iou(bbox1, bbox2):
-    """
-    Compute the intersection-over-union of two bounding boxes.
-    """
-    intersection = Rect(bbox1).intersect(bbox2)
-    union = Rect(bbox1).include_rect(bbox2)
-    
-    union_area = union.get_area()
-    if union_area > 0:
-        return intersection.get_area() / union.get_area()
-    
-    return 0
-
-
 def iob(bbox1, bbox2):
     """
     Compute the intersection area over box area, for bbox1.