Raspberry Pi GPIO Pins Explained: 40-Pin Header, Power, Ground and Interfaces

Raspberry Pi GPIO pins let a Python program read switches and sensors or control LEDs, relays and other hardware. The 40-pin header also exposes power, ground and dedicated interfaces such as I²C, SPI and UART. This guide explains how to read the header safely and choose a pin for a project.

For the complete interactive diagram, keep the Raspberry Pi GPIO Pinout reference open while you wire. Pinout.xyz distinguishes the physical position on the header from the BCM GPIO number used by many Python libraries.

The three kinds of pins

Every header position belongs to one of three practical groups:

• GPIO: General-purpose input/output pins that software can configure as digital inputs or outputs. The BCM number is the GPIO identifier, such as GPIO17.
• Power: 3.3V and 5V pins provide a supply rail. They are not ordinary GPIO outputs and must not be driven by software.
• Ground: Ground pins complete the circuit. Connect the device ground to a Pi ground pin before testing signals.

BCM numbers versus physical pin numbers

A physical pin number describes a position on the header: physical pin 11 is the eleventh header position. A BCM number describes the SoC signal assigned to that position: physical pin 11 carries GPIO17. These names are not interchangeable. A wiring diagram may say “pin 11” while Python code expects GPIO17 or 17.

Orient the Pi with the HDMI connectors on the left and the GPIO header on the right. In that view, physical pin 1 is at the top; the square pad on the underside identifies pin 1 on an unmounted board. Always compare the board orientation with the interactive pinout before inserting a jumper wire.

A practical 40-pin reference

Physical pin	BCM / function	Typical use
1	3.3V power	3.3V supply rail
2	5V power	5V supply rail
3	GPIO2 / I²C SDA	I²C data
5	GPIO3 / I²C SCL	I²C clock
6	Ground	Circuit return
7	GPIO4	Digital input/output
8	GPIO14 / UART TX	Serial transmit
10	GPIO15 / UART RX	Serial receive
11	GPIO17	General-purpose input/output
12	GPIO18 / PCM CLK	Digital I/O or audio clock
13	GPIO27	General-purpose input/output
15	GPIO22	General-purpose input/output
16	GPIO23	General-purpose input/output
18	GPIO24	General-purpose input/output
19	GPIO10 / SPI MOSI	SPI controller output
21	GPIO9 / SPI MISO	SPI controller input
23	GPIO11 / SPI SCLK	SPI clock
24	GPIO8 / SPI CE0	SPI chip select
26	GPIO7 / SPI CE1	Second SPI chip select
29	GPIO5	General-purpose input/output
31	GPIO6	General-purpose input/output
32	GPIO12 / PWM0	Hardware PWM option
33	GPIO13 / PWM1	Hardware PWM option
35	GPIO19 / PCM FS	Digital I/O or audio frame sync
36	GPIO16	General-purpose input/output
37	GPIO26	General-purpose input/output
38	GPIO20 / PCM DIN	Digital I/O or audio input
40	GPIO21 / PCM DOUT	Digital I/O or audio output

This is a working-project reference, not a replacement for the board-specific documentation. Physical pins 27 and 28 carry the ID EEPROM signals (GPIO0 and GPIO1), and the remaining header positions are power or ground. Check the complete diagram before using a less common alternate function.

Common GPIO interfaces

Interface	Common pins	What it is for
I²C	GPIO2 (SDA), GPIO3 (SCL)	Multiple addressed sensors and display modules on two signal wires.
SPI	GPIO10, GPIO9, GPIO11 plus GPIO8 or GPIO7	Fast peripherals such as ADCs, displays and storage devices.
UART	GPIO14 (TX), GPIO15 (RX)	Serial consoles and GPS or other serial modules.
PWM	GPIO12, GPIO13 and other software-controlled pins	Brightness, motor or servo control when the hardware and library support it.
1-Wire	Configured GPIO pin	Devices such as DS18B20 temperature probes on a shared data line.

A safe first GPIO test

Use an LED with a suitable resistor, or an LED module designed for 3.3V logic. This example uses BCM GPIO17, which is physical pin 11:

from gpiozero import LED
from time import sleep

led = LED(17)  # BCM GPIO17 = physical pin 11

try:
    while True:
        led.toggle()
        sleep(1)
except KeyboardInterrupt:
    led.off()Code language: Python (python)

• Never connect 5V to a GPIO signal pin. Raspberry Pi GPIO uses 3.3V logic.
• Use a resistor to limit LED current, and use a level shifter or voltage divider when a peripheral outputs more than 3.3V.
• Turn the Pi off while changing wiring, and verify the pin number convention in both the guide and the code.
• Do not draw more current than the board, power supply or peripheral is designed to provide.

GPIO troubleshooting checklist

• Confirm whether the diagram uses physical, BCM or WiringPi numbering.
• Check that the circuit has a shared ground with the Pi.
• Check the pin is not already claimed by I²C, SPI, UART, a HAT or a background service.
• Check the input pull-up or pull-down setting and whether the switch is active-low.
• Return to the interactive Pinout.xyz diagram for alternate functions and the exact header orientation.

Source and next step

Pin names, physical positions, alternate interfaces and orientation in this guide are based on Pinout.xyz’s Raspberry Pi GPIO Pinout reference. Use it as the canonical visual check before wiring a project. Next, try the Raspberry Pi weather dashboard project and map its sensor wires to the GPIO numbers in the code.