Arduino Sensor for Solution Volume

Designed for measuring volume of a cylinder as the volume changes. Designed to replace Vernier drop counter for titration or anything.
const int pingPin = 7;
int  r = 25 
void setup() {
  Serial.begin(9600);
  lcd.begin(16, 2);

}

void loop()
{
  long duration, cm;

 
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW);
  delayMicroseconds(2);
  digitalWrite(pingPin, HIGH);
  delayMicroseconds(5);
  digitalWrite(pingPin, LOW);


  pinMode(9, INPUT);
  duration = pulseIn(9, HIGH);


  cm = microsecondsToCentimeters(duration);

  cm=cm*3.14159265359*r*r;

  
  Serial.print(cm);
  Serial.print("cm");
  lcd.setCursor(0, 1);
  lcd.print(cm);
  Serial.println();
  
  delay(100);
}


long microsecondsToCentimeters(long microseconds)
{

  return microseconds / 29 / 2;
}