diff --git a/site_ansto/instrument/hrpd/config/hmm/hmm_configuration.tcl b/site_ansto/instrument/hrpd/config/hmm/hmm_configuration.tcl
index 07307f51..be3837e4 100644
--- a/site_ansto/instrument/hrpd/config/hmm/hmm_configuration.tcl
+++ b/site_ansto/instrument/hrpd/config/hmm/hmm_configuration.tcl
@@ -53,7 +53,7 @@ proc ::histogram_memory::isc_initialize {} {
     ::histogram_memory::two_theta -boundaries
 
     detector_active_height_mm 335
-    detector_active_width_mm 3463.4
+    detector_active_width_mm 3490.64
     detector_radius_mm 1250.0
 
   # hmm configure FAT_SIMULATED_EVENT_Y0 $y_bb0