diff --git a/src/layer/arm/lrn_arm.cpp b/src/layer/arm/lrn_arm.cpp
index 901bc6e02..a5b5fb483 100644
--- a/src/layer/arm/lrn_arm.cpp
+++ b/src/layer/arm/lrn_arm.cpp
@@ -70,8 +70,6 @@ int LRN_arm::forward_inplace(Mat& bottom_top_blob) const
         }
     }
 
-    float alpha_div_size = alpha / local_size;
-
     if (region_type == NormRegion_ACROSS_CHANNELS)
     {
         Mat square_sum;
@@ -80,6 +78,8 @@ int LRN_arm::forward_inplace(Mat& bottom_top_blob) const
             return -100;
         square_sum.fill(0.f);
 
+        const float alpha_div_size = alpha / local_size;
+
         #pragma omp parallel for
         for (int q=0; q<channels; q++)
         {
@@ -175,6 +175,8 @@ int LRN_arm::forward_inplace(Mat& bottom_top_blob) const
 
         const int maxk = local_size * local_size;
 
+        const float alpha_div_size = alpha / maxk;
+
         // norm window offsets
         std::vector<int> _space_ofs(maxk);
         int* space_ofs = &_space_ofs[0];
diff --git a/src/layer/lrn.cpp b/src/layer/lrn.cpp
index 8268050c9..f3c840695 100644
--- a/src/layer/lrn.cpp
+++ b/src/layer/lrn.cpp
@@ -100,12 +100,12 @@ int LRN::forward(const Mat& bottom_blob, Mat& top_blob) const
         }
     }
 
-    float alpha_div_size = alpha / local_size;
-
     if (region_type == NormRegion_ACROSS_CHANNELS)
     {
         top_blob.fill(0.f);
 
+        const float alpha_div_size = alpha / local_size;
+
         #pragma omp parallel for
         for (int q=0; q<channels; q++)
         {
@@ -149,6 +149,8 @@ int LRN::forward(const Mat& bottom_blob, Mat& top_blob) const
 
         const int maxk = local_size * local_size;
 
+        const float alpha_div_size = alpha / maxk;
+
         // norm window offsets
         std::vector<int> _space_ofs(maxk);
         int* space_ofs = &_space_ofs[0];
@@ -225,8 +227,6 @@ int LRN::forward_inplace(Mat& bottom_top_blob) const
         }
     }
 
-    float alpha_div_size = alpha / local_size;
-
     if (region_type == NormRegion_ACROSS_CHANNELS)
     {
         Mat square_sum;
@@ -235,6 +235,8 @@ int LRN::forward_inplace(Mat& bottom_top_blob) const
             return -100;
         square_sum.fill(0.f);
 
+        const float alpha_div_size = alpha / local_size;
+
         #pragma omp parallel for
         for (int q=0; q<channels; q++)
         {
@@ -278,6 +280,8 @@ int LRN::forward_inplace(Mat& bottom_top_blob) const
 
         const int maxk = local_size * local_size;
 
+        const float alpha_div_size = alpha / maxk;
+
         // norm window offsets
         std::vector<int> _space_ofs(maxk);
         int* space_ofs = &_space_ofs[0];