Arduino Uno R3 pinout

The classic Arduino board for learning. It uses 5 V logic, so 3.3 V-only sensors need a level shifter.

Top view, USB connector on the left. Colors: red power, grey ground, green GPIO, yellow analog, orange USB serial, blue I2C.

At a glance

Chip: ATmega328P
Processor: 8-bit AVR ATmega328P at 16 MHz, 2 KB SRAM, 32 KB flash, 1 KB EEPROM
Logic level: 5 V. 3.3 V-only sensors need a level shifter.
Flashing tool: avrdude
Arduino board (FQBN): arduino:avr:uno
Live pin view in BoardPilot: Yes (diagnostic agent available)

Pins to avoid or use with care

USB serial: TX→1 (PD1), RX←0 (PD0)

Used for uploading and the serial monitor. Anything connected here can break uploads.

Good pins for LEDs, buttons and signals

2, ~3, 4, ~5, ~6, 7, 8, ~9

Default pins in Arduino

I2C: SDA = A4, SCL = A5. Only these pins work with the Wire library defaults. I2C only works on A4 (SDA) and A5 (SCL); the SDA/SCL pins near AREF are the same signals.
SPI: MOSI = ~11, MISO = 12, SCK = 13, CS = ~10.
Analog inputs: A0, A1, A2, A3, A4, A5 (0–5 V)

All 31 pins

Pin	GPIO	Chip pin	Functions	Notes
SCL	19	PC5	GPIO, ADC5, I2C_SCL (default)	Same signal as A5 (I2C clock).
SDA	18	PC4	GPIO, ADC4, I2C_SDA (default)	Same signal as A4 (I2C data).
AREF	—		ADC_REF	Analog reference input. Leave it unconnected unless your code calls analogReference(EXTERNAL); never power parts from it.
GND	—
13	13	PB5	GPIO, SPI_SCK (default)	The built-in LED "L" is on this pin (also SPI clock).
12	12	PB4	GPIO, SPI_MISO (default)
~11	11	PB3	GPIO, PWM, SPI_MOSI (default)
~10	10	PB2	GPIO, PWM, SPI_CS (default)	SPI chip select. Keep it an OUTPUT when using SPI, or SPI can stop working.
~9	9	PB1	GPIO, PWM
8	8	PB0	GPIO
7	7	PD7	GPIO
~6	6	PD6	GPIO, PWM
~5	5	PD5	GPIO, PWM
4	4	PD4	GPIO
~3	3	PD3	GPIO, INT1, PWM
2	2	PD2	GPIO, INT0
TX→1	1	PD1	GPIO, UART0_TX	Serial TX. Shared with the USB link: using it for parts can break uploads and the serial monitor.
RX←0	0	PD0	GPIO, UART0_RX	Serial RX. Shared with the USB link: using it for parts can break uploads and the serial monitor.
IOREF	—			Tells shields that this board uses 5 V logic. Connected to 5V; not meant to power parts.
RESET	—		RESET	Reset input. Pull it LOW (or press the button) to restart the board.
3.3V	—			3.3 V output, 50 mA max. Enough for one or two small sensors.
5V	—			5 V output from USB or the on-board regulator.
GND	—
GND	—
Vin	—			Power input (7-12 V recommended, 20 V max) from the barrel jack or an external supply. It is not fed from USB, so do not use it to power parts.
A0	14	PC0	GPIO, ADC0
A1	15	PC1	GPIO, ADC1
A2	16	PC2	GPIO, ADC2
A3	17	PC3	GPIO, ADC3
A4	18	PC4	GPIO, ADC4, I2C_SDA (default)	Default I2C SDA (same signal as the SDA pin).
A5	19	PC5	GPIO, ADC5, I2C_SCL (default)	Default I2C SCL (same signal as the SCL pin).

Check your wiring with BoardPilot

BoardPilot shows this board in 3D, flags risky pins as you wire, and flashes it safely with a backup first.

Download BoardPilot

Sources

Arduino UNO Rev3 Eagle design files (UNO-TH_Rev3e), Board outline and header pad coordinates (IOH, IOL, POWER, AD)
Arduino UNO R3 datasheet (A000066), Connector pinouts; IOREF connected to 5V; VIN 6-20 V
Arduino UNO R3 full pinout (A000066)
Microchip ATmega328P datasheet (DS40002061), 1 Pin Configurations; 14.3 Alternate Port Functions; 24 Analog-to-Digital Converter
arduino/ArduinoCore-avr variants/standard/pins_arduino.h, Arduino pin numbers, SPI/I2C defaults, LED_BUILTIN, A0=14
arduino/ArduinoCore-avr boards.txt, USB VID/PID, upload protocol and speed
Board data file and source notes on GitHub

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