diff --git a/lasy/utils/openpmd_output.py b/lasy/utils/openpmd_output.py
index 13c20ee2..d8463060 100644
--- a/lasy/utils/openpmd_output.py
+++ b/lasy/utils/openpmd_output.py
@@ -47,7 +47,8 @@ def write_to_openpmd_file(dim, file_prefix, file_format, grid, wavelength, pol):
     # Define the mesh
     m = i.meshes["laserEnvelope"]
     m.grid_spacing = [
-        (hi - lo) / npoints for hi, lo, npoints in zip(box.hi, box.lo, box.npoints)
+        (hi - lo) / (npoints - 1)
+        for hi, lo, npoints in zip(box.hi, box.lo, box.npoints)
     ][::-1]
     m.grid_global_offset = box.lo
     m.unit_dimension = {