diff --git a/include/mqttclient.h b/include/mqttclient.h
index 6e351e2a..5e595cca 100644
--- a/include/mqttclient.h
+++ b/include/mqttclient.h
@@ -17,7 +17,7 @@
 #define MQTT_KEEPALIVE 10 // keep alive interval in seconds
 
 #ifndef MQTT_CLIENTNAME
-#define MQTT_CLIENTNAME clientId.c_str()
+#define MQTT_CLIENTNAME clientId
 #endif
 
 extern TaskHandle_t mqttTask;
diff --git a/src/mqttclient.cpp b/src/mqttclient.cpp
index 8e1d5aec..6dd5a89a 100644
--- a/src/mqttclient.cpp
+++ b/src/mqttclient.cpp
@@ -41,7 +41,10 @@ esp_err_t mqtt_init(void) {
 
 int mqtt_connect(const char *my_host, const uint16_t my_port) {
   IPAddress mqtt_server_ip;
-  static String clientId = "paxcounter-" + ETH.macAddress();
+  const uint16_t hashed = hash(&ETH.macAddress()[0], 4); // hash MAC to 4 digits
+  char clientId[16];
+  snprintf(clientId, 16, "paxcounter_%d", hashed);
+
   ESP_LOGI(TAG, "MQTT name is %s", MQTT_CLIENTNAME);
 
   // resolve server host name