diff --git a/sys/include/phydat.h b/sys/include/phydat.h
index 1433e65579..5205da9625 100644
--- a/sys/include/phydat.h
+++ b/sys/include/phydat.h
@@ -117,7 +117,9 @@ enum {
     UNIT_TIME,      /**< the three dimensions contain sec, min, and hours */
     UNIT_DATE,      /**< the 3 dimensions contain days, months and years */
     /* mass concentration */
-    UNIT_GPM3       /**< grams per cubic meters */
+    UNIT_GPM3,      /**< grams per cubic meter */
+    /* number concentration */
+    UNIT_CPM3       /**< count per cubic meter */
     /* extend this list as needed */
 };
 
diff --git a/sys/phydat/phydat_str.c b/sys/phydat/phydat_str.c
index 956cedb6f2..7add91d282 100644
--- a/sys/phydat/phydat_str.c
+++ b/sys/phydat/phydat_str.c
@@ -106,6 +106,8 @@ const char *phydat_unit_to_str(uint8_t unit)
         case UNIT_GPM3:     return "g/m^3";
         case UNIT_F:        return "F";
         case UNIT_PH:       return "pH";
+        case UNIT_CPM3:     return "#/m^3";
+
         default:            return "";
     }
 }