diff --git a/utils/utils.py b/utils/utils.py
index cc314102..18ccb514 100755
--- a/utils/utils.py
+++ b/utils/utils.py
@@ -313,15 +313,12 @@ def box_iou(boxes1, boxes2):
     return iou
 
 
-def wh_iou(box1, box2):
-    # Returns the IoU of wh1 to wh2. wh1 is 2, wh2 is 2xn
-    w1, h1 = box1[0], box1[1]
-    w2, h2 = box2[0], box2[1]
-
-    # Intersection area
-    inter = torch.min(w1, w2) * torch.min(h1, h2)
-
-    return inter / (w1 * h1 + w2 * h2 - inter)  # iou = inter / (area1 + area2 - inter)
+def wh_iou(wh1, wh2):
+    # Returns the nxm IoU matrix. wh1 is nx2, wh2 is mx2
+    wh1 = wh1[:, None]  # [N,1,2]
+    wh2 = wh2[None]  # [1,M,2]
+    inter = torch.min(wh1, wh2).prod(2)  # [N,M]
+    return inter / (wh1.prod(2) + wh2.prod(2) - inter)  # iou = inter / (area1 + area2 - inter)
 
 
 class FocalLoss(nn.Module):
@@ -445,9 +442,8 @@ def build_targets(model, targets):
         # iou of targets-anchors
         t, a = targets, []
         gwh = t[:, 4:6] * ng
-        gwht = gwh.t()
         if nt:
-            iou = torch.stack([wh_iou(x, gwht) for x in anchor_vec], 0)
+            iou = wh_iou(anchor_vec, gwh)
 
             if use_all_anchors:
                 na = len(anchor_vec)  # number of anchors