LCD in this picture has 2×16 characters, so in quick way horizontal bar could have 16 steps resolution, but it’s not enough.  Each character is formed from 5×8 pixels. Every character can be sliced in to 5 pieces. After that we can have 5*16 = 80 steps.

First step is to create 5 custom characters. More about  createChar() please read at arduino.cc.

Each custom character is specified by an array of eight bytes. Byte represents character’s row.

byte p1[8] = {
  0x10,
  0x10,
  0x10,
  0x10,
  0x10,
  0x10,
  0x10,
  0x10};

byte p2[8] = {
  0x18,
  0x18,
  0x18,
  0x18,
  0x18,
  0x18,
  0x18,
  0x18};

byte p3[8] = {
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C};

byte p4[8] = {
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E};

byte p5[8] = {
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F};

These characters look like this:

In progress bar demo ADC conversion value is showed

All code

//https://www.electronicsblog.net/
//Arduino LCD horizontal progress bar using custom characters
#include <LiquidCrystal.h>

#define lenght 16.0

double percent=100.0;
unsigned char b;
unsigned int peace;

// custom charaters

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
byte p1[8] = {
  0x10,
  0x10,
  0x10,
  0x10,
  0x10,
  0x10,
  0x10,
  0x10};

byte p2[8] = {
  0x18,
  0x18,
  0x18,
  0x18,
  0x18,
  0x18,
  0x18,
  0x18};

byte p3[8] = {
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C,
  0x1C};

byte p4[8] = {
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E,
  0x1E};

byte p5[8] = {
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F,
  0x1F};

void setup()   {

  delay(100);
  lcd.createChar(0, p1);
  lcd.createChar(1, p2);
  lcd.createChar(2, p3);
  lcd.createChar(3, p4);
  lcd.createChar(4, p5);

  lcd.begin(16, 2);

}

void loop()
{

  lcd.setCursor(0, 0);

  //ADC conversion

  unsigned int value = analogRead(0);

  percent = value/1024.0*100.0;

  lcd.print(value);

  lcd.print(" - ");

  lcd.print(percent);
  lcd.print(" %   ");

  lcd.setCursor(0,1);

  double a=lenght/100*percent;

 // drawing black rectangles on LCD

  if (a>=1) {

    for (int i=1;i<a;i++) {

      lcd.write(4);

      b=i;
    }

    a=a-b;

  }

  peace=a*5;

// drawing charater's colums

  switch (peace) {

  case 0:

    break;

  case 1:
    lcd.write(0);

    break;

  case 2:
    lcd.write(1);
    break;

  case 3:
    lcd.write(2);
    break;

  case 4:
    lcd.write(3);
    break;

  }

//clearing line
  for (int i =0;i<(lenght-b);i++) {

    lcd.print(" ");
  }

  ;
}

&#91;/code&#93;

<strong>Update for Arduino IDE 1.0  and newer version users</strong>

lcd.write(0);

leads to "call of overloaded 'write(int)' is ambiguous" error, so please replace it to

lcd.print((char)0);