diff --git a/armsrc/Standalone/lf_em4100emul.c b/armsrc/Standalone/lf_em4100emul.c
index fc399c64a..dce6c455e 100644
--- a/armsrc/Standalone/lf_em4100emul.c
+++ b/armsrc/Standalone/lf_em4100emul.c
@@ -85,7 +85,7 @@ void RunMod() {
     Dbprintf("[=] >>  LF EM4100 simulator started  <<");
 
     int selected = 0; //selected slot after start
-    slots_count = sizeof(low) / sizeof(low[0]);
+    slots_count = ARRAYLEN(low);
     bba = BigBuf_get_addr();
     for (;;) {
         WDT_HIT();
diff --git a/armsrc/Standalone/lf_em4100rwc.c b/armsrc/Standalone/lf_em4100rwc.c
index f4025d1b1..fcd3fe294 100644
--- a/armsrc/Standalone/lf_em4100rwc.c
+++ b/armsrc/Standalone/lf_em4100rwc.c
@@ -129,7 +129,7 @@ void RunMod() {
     //      2 - simulate tag from selected slot
     //      3 - write to T5555 tag
     uint8_t state = 0;
-    slots_count = sizeof(low) / sizeof(low[0]);
+    slots_count = ARRAYLEN(low);
     bba = BigBuf_get_addr();
     LED_Slot(selected);
     for (;;) {