diff --git a/src/layer/diag.cpp b/src/layer/diag.cpp
index 936b32b2366..f6f9c64a2ac 100644
--- a/src/layer/diag.cpp
+++ b/src/layer/diag.cpp
@@ -37,8 +37,7 @@ int Diag::forward(const Mat& bottom_blob, Mat& top_blob, const Option& opt) cons
     if (dims == 1)
     {
         int w = bottom_blob.w;
-        int top_w = w + std::abs(diagonal);
-        int stride = top_w + 1;
+        int top_w = w + ((diagonal >= 0) ? diagonal : -diagonal);
 
         top_blob.create(top_w, top_w, elemsize, opt.blob_allocator);
         if (top_blob.empty())
@@ -58,10 +57,16 @@ int Diag::forward(const Mat& bottom_blob, Mat& top_blob, const Option& opt) cons
     {
         int w = bottom_blob.w;
         int h = bottom_blob.h;
-        float tmp = (w - h) / 2.0;
 
-        int len = std::min(w, h) - (int)std::max(std::abs(diagonal - tmp) - std::abs(tmp), 0.0f);
-        len = std::max(len, 0);
+        int len = 0;
+        int minimum = std::min(w - h, 0);
+        int maximum = std::max(w - h, 0);
+        if (diagonal <= maximum && diagonal >= minimum)
+            len = std::min(w, h);
+        else if (diagonal > -h && diagonal < minimum)
+            len = diagonal + h;
+        else if (diagonal > maximum && diagonal < w)
+            len = -diagonal + w;
 
         top_blob.create(len, elemsize, opt.blob_allocator);
         if (top_blob.empty())