diff --git a/src/CelerisLab/lbm/kernels/blocks.cu b/src/CelerisLab/lbm/kernels/blocks.cu
index 141d23a..b8c7ef4 100644
--- a/src/CelerisLab/lbm/kernels/blocks.cu
+++ b/src/CelerisLab/lbm/kernels/blocks.cu
@@ -168,10 +168,8 @@ extern "C" __global__ void BlockStep(const fpxx* in, fpxx* out, unsigned int hal
         stream_pull_load(k, f, in + base, j);
         if (cell_kind == KIND_ACTIVE) {
             if (d_block_wall.nx != 0) {
-                const unsigned int bx = NX - 2 * halo, by = NY - 2 * halo;
-                const unsigned int nbx = d_block_wall.nx / bx;
-                const unsigned int gx = (blockIdx.z % nbx) * bx + x - halo;
-                const unsigned int gy = (blockIdx.z / nbx) * by + y - halo;
+                unsigned int gx, gy;
+                block_global_xy(x, y, halo, d_block_wall.nx, gx, gy);
                 if (gy == 1)
                     reconstruct_bottom_velocity(f, 0.0f, bottom_wall_normal_velocity(
                         gx, d_block_wall.nx, d_block_wall.slot_width, d_block_wall.amplitude));
diff --git a/src/CelerisLab/lbm/kernels/core/layout.cuh b/src/CelerisLab/lbm/kernels/core/layout.cuh
index 56a6d61..3b0f76a 100644
--- a/src/CelerisLab/lbm/kernels/core/layout.cuh
+++ b/src/CelerisLab/lbm/kernels/core/layout.cuh
@@ -53,6 +53,18 @@ __device__ __forceinline__ void index_from_thread(unsigned int& x,
     k = (unsigned long)y * NX + x;
 }
 
+// Global cell of window cell (x, y) in block blockIdx.z of a row-major,
+// full-domain block layout domain_nx cells wide. NX/NY are the local strides.
+__device__ __forceinline__ void block_global_xy(unsigned int x, unsigned int y,
+                                                unsigned int halo, unsigned int domain_nx,
+                                                unsigned int& gx, unsigned int& gy)
+{
+    const unsigned int bx = NX - 2 * halo, by = NY - 2 * halo;
+    const unsigned int nbx = domain_nx / bx;
+    gx = (blockIdx.z % nbx) * bx + x - halo;
+    gy = (blockIdx.z / nbx) * by + y - halo;
+}
+
 // ---------------------------------------------------------------------------
 // Neighbor list (periodic BCs via modular arithmetic)
 // ---------------------------------------------------------------------------
diff --git a/src/CelerisLab/lbm/kernels/step/block_channel.cu b/src/CelerisLab/lbm/kernels/step/block_channel.cu
index 67a613d..d31bd0d 100644
--- a/src/CelerisLab/lbm/kernels/step/block_channel.cu
+++ b/src/CelerisLab/lbm/kernels/step/block_channel.cu
@@ -15,10 +15,8 @@ extern "C" __global__ void BlockChannelStep(
     unsigned int x, y; unsigned long k;
     index_from_thread(x, y, k);
     if (x < halo || y < halo || x >= NX - halo || y >= NY - halo) return;
-    const unsigned int bx = NX - 2 * halo, by = NY - 2 * halo;
-    const unsigned int nbx = d_block_channel.nx / bx;
-    const unsigned int gx = (blockIdx.z % nbx) * bx + x - halo;
-    const unsigned int gy = (blockIdx.z / nbx) * by + y - halo;
+    unsigned int gx, gy;
+    block_global_xy(x, y, halo, d_block_channel.nx, gx, gy);
     const unsigned long base = (unsigned long)blockIdx.z * NQ * TOTAL_CELLS;
     const unsigned long cell = (unsigned long)blockIdx.z * TOTAL_CELLS + k;
     float f[NQ];