Ultrasonic Water Level

#include <SPI.h>
#include <NewPing.h>
#include "SSD1306Ascii.h"
#include "SSD1306AsciiWire.h"
// 0X3C+SA0 - 0x3C or 0x3D
#define I2C_ADDRESS 0x3C
SSD1306AsciiWire oled;

#define TRIGGER_PIN  9  // Arduino pin tied to trigger pin on the ultrasonic sensor.
#define ECHO_PIN     10  // Arduino pin tied to echo pin on the ultrasonic sensor.
#define MAX_DISTANCE 400 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE); // NewPing setup of pins and maximum distance.

#include "OneButton.h"
int ButtonPin = 12;                          //we need the OneButton library
OneButton button(ButtonPin, true);                         //attach a button on pin D12 to the library

int temperature = 0;
long distance = 0;
float pingtime = 0;
float pi = 3.1415926;
int base = 0;  //distance to bottom of container (offset)
int height = 0;  //actual height of liquid.
float diameter = 93.11;  //diameter in mm of container
float radius = diameter/2;  //Area = pi * r^2
float volume = 0;           //volume of liquid. π * radius² * height
float liters = 0;

void setup() {
  // Start the Serial communication
  Serial.begin(115200);
  pinMode(ButtonPin, INPUT_PULLUP);
  button.attachClick(singleclick);                  // link the function to be called on a singleclick event.

  Wire.begin();
  Wire.setClock(400000L);
  oled.begin(&Adafruit128x32, I2C_ADDRESS);
  oled.setFont(TimesNewRoman16_italic);   //(Adafruit5x7);
  oled.clear();

  oled.setCursor(0,0); //(col,row)
  //oled.set2X();
  oled.setInvertMode(2);
  oled.println("      CAL     ");

  delay(2000);   //delay to calibrate base
  pingtime = sonar.ping();
  pingtime = pingtime + (.6 * temperature);
  pingtime = pingtime/2;
  distance = pingtime * (0.343);
  base = distance;
  delay(1000);
  oled.setCursor(0,0); //(col,row)
  //oled.set2X();
  oled.setInvertMode(2);
  oled.println("      LITERS     ");

}

void loop() {
  delay(250);

  pingtime = sonar.ping();
  pingtime = pingtime + (.6 * temperature);
  pingtime = pingtime/2;
  distance = pingtime * (0.343);
  height = base - distance;
 //Volume = π * radius² * height
  volume = pi * sq(radius) * height;  //cubic mm
  liters = volume * 1.0E-6;           //convert to liters
  // Display the distance
  oled.setFont(TimesNewRoman16);   //(Adafruit5x7);
  oled.clearToEOL();
  oled.setCursor(40,10); //(col,row)
  oled.setInvertMode(0);
  oled.println(liters);

  Serial.print("$");    //send to process app
  Serial.print(liters);
  Serial.print(";");

//  Serial.println(liters);
//  Serial.println(distance);
//  Serial.println();
}

void singleclick(){
  base = distance;    //set the base in mm Bottom of container
}