diff --git a/observe_main.html b/observe_main.html
index 925ee84..a5170b0 100644
--- a/observe_main.html
+++ b/observe_main.html
@@ -807,7 +807,9 @@
               Math.pow((item.width*real_width_FoV),2.0)*Math.PI/4.0;
             break
           case "Actinobacteria":
-            total.Actinobacteria += item.length*real_width_FoV;
+            // the diameter is estimated to always be 1 um.
+            // A is the (D=1)²/4*pi = pi/4
+            total.Actinobacteria += item.length*real_width_FoV*Math.PI/4.0;
             break
           case "Flagelate":
             total.Flagelate +=1;