diff --git a/wires.js b/wires.js
index 8db858f..4aa6d5d 100644
--- a/wires.js
+++ b/wires.js
@@ -206,7 +206,21 @@ function zoomToBox(xmin,xmax,ymin,ymax){
 	var ymid=(ymin+ymax)/2;
 	var x=(xmid+grChipOffsetX)/grChipSize*600;
 	var y=600-(ymid-grChipOffsetY)/grChipSize*600;
-	var zoom=5;  // pending a more careful calculation
+	// Zoom to fill 80% of the window with the selection
+	var fillfactor=0.80;
+	var dx=xmax-xmin;
+	var dy=ymax-ymin;
+	if (dx < 1) dx=1;
+	if (dy < 1) dy=1;
+	var zx=(800/600)*fillfactor*grChipSize/dx;
+	var zy=fillfactor*grChipSize/dy;
+	var zoom=Math.min(zx,zy);
+	if (zoom < 1) {
+		zoom = 1;
+	}
+	if (zoom > grMaxZoom) {
+		zoom = grMaxZoom;
+	}
 	moveHere([x,y,zoom]);
 }