Sinric Pro Water Tank (Water Level Monitor) using ultrasonic sensor : https://help.sinric.pro/pages/tutorials/custom-device-types/ultrasonic-sensor/HC-SR04.html

HC-SR04-ultrasonic-sensor.ino

#if defined(ESP8266)
  const int trigPin = 12;
  const int echoPin = 14;
#elif defined(ESP32) 
  const int trigPin = 5;
  const int echoPin = 18;
#elif defined(ARDUINO_ARCH_RP2040)
  const int trigPin = 15;
  const int echoPin = 14;
#endif
 
long duration;
float distanceInCm; 

void setup() {
  Serial.begin(115200);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  // Clears the trigPin
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  duration = pulseIn(echoPin, HIGH);

  // convert duration to CM 
  distanceInCm = duration/29 / 2;

  // inches 
  // distanceInInches = duration / 74 / 2;

  Serial.print("Distance (cm): ");
  Serial.println(distanceInCm); 
  
  delay(1000);
}