diff --git a/src/ffi_ZDC_HCAL.cpp b/src/ffi_ZDC_HCAL.cpp
index 336d2124c91588d6f1a2be2fe40778ff52d7f4d9..048d786fc30eb96c91d6c732152b335cc094d96a 100644
--- a/src/ffi_ZDC_HCAL.cpp
+++ b/src/ffi_ZDC_HCAL.cpp
@@ -36,7 +36,7 @@ static Ref_t createDetector(Detector& lcdd, xml_h e, SensitiveDetector sens) {
 	int layer_num = 1;
 	int slice_num = 1;
 	double totWidth = Layering(x_det).totalThickness();
-	Box envelope (pixel_x/2.0, pixel_y/2.0,totWidth);
+	Box envelope (pixel_x/2.0, pixel_y/2.0,totWidth/2.0);
 	Volume envelopeVol(det_name+"_envelope",envelope,air); 
 	PlacedVolume pv;