HC-SR04 Ultrasonic Sensor with Arduino: Wiring and Code

November 4, 2026

HC-SR04 ultrasonic distance sensor for Arduino

The HC-SR04 ultrasonic sensor measures distance using sound. It is a favourite for obstacle-avoiding robots, water level monitors and parking sensors, and it takes only a few minutes to connect to an Arduino.

What you need

How the HC-SR04 works

The Arduino sends a short pulse to the Trig pin, and the sensor emits a burst of ultrasound. The Echo pin then stays high for as long as the sound takes to reach an object and return. Because that time covers the trip both ways, the code divides it by two.

The sensor typically measures from about 2 cm to 400 cm.

HC-SR04 Arduino wiring

HC-SR04 pin	Arduino Uno pin
VCC	5V
Trig	D9
Echo	D10
GND	GND

Arduino code

Upload this sketch, then open the Serial Monitor at 9600 baud.

const int trigPin = 9;
const int echoPin = 10;

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  Serial.begin(9600);
}

void loop() {
  // Send a 10 microsecond trigger pulse
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  // Wait up to 30 ms for the echo
  long duration = pulseIn(echoPin, HIGH, 30000);

  if (duration == 0) {
    Serial.println("Out of range");
  } else {
    float distanceCm = duration * 0.0343 / 2;
    Serial.print("Distance: ");
    Serial.print(distanceCm);
    Serial.println(" cm");
  }

  delay(200);
}

The distance formula

Distance in cm = echo time in microseconds × 0.0343 ÷ 2. The value 0.0343 is the speed of sound in cm per microsecond at about 20°C. Sound travels faster in hot air, reaching about 0.0355 at 40°C, so a project that needs better accuracy in summer should adjust this number.

Troubleshooting

Always out of range: check that Trig and Echo are not swapped, and that VCC has 5 V.
Unstable readings: soft surfaces such as cloth absorb sound, and angled surfaces reflect it away. Aim at a flat, hard surface.
Wrong values near the sensor: objects closer than about 2 cm cannot be measured reliably.

Using the HC-SR04 with ESP32

The HC-SR04 runs on 5 V, so its Echo output is also 5 V. An ESP32 uses 3.3 V logic, so reduce the Echo signal with a voltage divider, such as 1 kΩ in series and 2 kΩ to ground, or use a level shifter.