diff --git a/src/GlueXSensitiveDetectorFMWPC.cc b/src/GlueXSensitiveDetectorFMWPC.cc
index a4b709f..1e42eec 100644
--- a/src/GlueXSensitiveDetectorFMWPC.cc
+++ b/src/GlueXSensitiveDetectorFMWPC.cc
@@ -30,6 +30,12 @@ double GlueXSensitiveDetectorFMWPC::TWO_HIT_TIME_RESOL = 400*ns;
 // Minimum photoelectron count for a hit
 double GlueXSensitiveDetectorFMWPC::THRESH_KEV = 0.;
 
+// Coordinate of wire 0, transverse to wire direction
+double GlueXSensitiveDetectorFMWPC::WIRE_OFFSET = -73*cm;
+
+// Minimum photoelectron count for a hit
+double GlueXSensitiveDetectorFMWPC::WIRE_PITCH = 1.0*cm;
+
 int GlueXSensitiveDetectorFMWPC::instanceCount = 0;
 G4Mutex GlueXSensitiveDetectorFMWPC::fMutex = G4MUTEX_INITIALIZER;
 
@@ -184,15 +190,15 @@ G4bool GlueXSensitiveDetectorFMWPC::ProcessHits(G4Step* step,
       int wire = 0;
       if (layer % 2 != 0) {
          // Vertical wires
-         wire = floor(x[0]/cm + 73.0);
+         wire = floor(x[0] - WIRE_OFFSET)/WIRE_PITCH;
       }
       else {
          // Horizontal wires
-         wire = floor(x[1]/cm + 73.0);
+         wire = floor(x[1] - WIRE_OFFSET)/WIRE_PITCH;
       }
       if (layer == 6) {
          // Vertical wires
-         wire = floor(x[0]/cm + 73.0);
+         wire = floor(x[0] - WIRE_OFFSET)/WIRE_PITCH;
       }
       
       //cout<<"MWPC: layer/wire = "<<layer<<" / "<<wire<<endl;
diff --git a/src/GlueXSensitiveDetectorFMWPC.hh b/src/GlueXSensitiveDetectorFMWPC.hh
index 4ad6eab..b9ebfde 100644
--- a/src/GlueXSensitiveDetectorFMWPC.hh
+++ b/src/GlueXSensitiveDetectorFMWPC.hh
@@ -43,6 +43,8 @@ class GlueXSensitiveDetectorFMWPC : public G4VSensitiveDetector
    static int MAX_HITS;
    static double TWO_HIT_TIME_RESOL;
    static double THRESH_KEV;
+   static double WIRE_OFFSET;
+   static double WIRE_PITCH;
 
    static int instanceCount;
    static G4Mutex fMutex;