What is the difference between BCM and physical pin numbers?

Physical numbers count the header's pins 1 to 40. BCM numbers are the GPIO names the software uses. GPIO17 is physical pin 11, for example. Most libraries, gpiozero included, take BCM numbers.

Which Raspberry Pi pins are I²C, SPI and UART?

I²C1 is GPIO2 and GPIO3 (pins 3 and 5). SPI0 is GPIO10, 9 and 11 with chip selects on GPIO8 and GPIO7 (pins 19, 21, 23, 24, 26). The UART is GPIO14 and GPIO15 (pins 8 and 10).

Is the Raspberry Pi 5 GPIO pinout the same as the Pi 4?

Power, ground and every GPIO number sit on the same pins, and I²C1, SPI0 and the UART are where they were. Some alternate functions differ (GPIO12 and GPIO18 are separate PWM channels on the Pi 5), and software needs libraries that support its RP1 I/O chip: gpiozero through lgpio, or libgpiod. RPi.GPIO does not work.

Raspberry Pi GPIO Pinout

Raspberry Pi 40-pin header · 40-pin 2.54 mm header

Every 40-pin Pi (2, 3, 4, 5, Zero) puts power, ground and each GPIO on the same pins. Physical pin numbers and BCM GPIO numbers are different schemes — mixing them up is the usual cause of a circuit that does nothing.

Checked against the KiCad library and datasheet on 16 October 2026

Raspberry Pi 40-pin header pinout: KiCad footprint Connector_PinHeader_2.54mm:PinHeader_2x20_P2.54mm_Vertical seen from the top, beside the KiCad library symbol Connector:Raspberry_Pi_2_3 with every pin number and name — Left, the 2×20 header footprint: odd pins down one row, even pins down the other, pin 1 the square pad. Right, KiCad's Raspberry Pi symbol, which groups the pins by function and numbers each by its physical position. Exported from the KiCad 9 libraries (Connector:Raspberry_Pi_2_3, Connector_PinHeader_2.54mm:PinHeader_2x20_P2.54mm_Vertical). Open full size

Pin by pin

Pin	Name	Function
1	3V3	3.3 V power from the Pi's regulator
2	5V	5 V power, straight from the supply
3	GPIO2	I²C1 SDA (1.8 kΩ pull-up on the board)
4	5V	5 V power
5	GPIO3	I²C1 SCL (1.8 kΩ pull-up on the board)
6	GND	Ground
7	GPIO4	GPCLK0; default 1-Wire pin
8	GPIO14	UART TXD
9	GND	Ground
10	GPIO15	UART RXD
11	GPIO17	General purpose
12	GPIO18	PWM0, PCM clock
13	GPIO27	General purpose
14	GND	Ground
15	GPIO22	General purpose
16	GPIO23	General purpose
17	3V3	3.3 V power
18	GPIO24	General purpose
19	GPIO10	SPI0 MOSI
20	GND	Ground
21	GPIO9	SPI0 MISO
22	GPIO25	General purpose
23	GPIO11	SPI0 SCLK
24	GPIO8	SPI0 CE0
25	GND	Ground
26	GPIO7	SPI0 CE1
27	GPIO0	ID_SD: HAT ID EEPROM data. Reserved
28	GPIO1	ID_SC: HAT ID EEPROM clock. Reserved
29	GPIO5	GPCLK1
30	GND	Ground
31	GPIO6	GPCLK2
32	GPIO12	PWM0
33	GPIO13	PWM1
34	GND	Ground
35	GPIO19	SPI1 MISO, PWM1, PCM frame sync
36	GPIO16	SPI1 CE2
37	GPIO26	General purpose
38	GPIO20	SPI1 MOSI, PCM data in
39	GND	Ground
40	GPIO21	SPI1 SCLK, PCM data out

Things that catch people out

GPIO is 3.3 V only. 5 V on any GPIO pin will damage the SoC — there is no tolerance.
Pins 3 and 5 already carry 1.8 kΩ pull-ups; do not add your own.
Ground is on pins 6, 9, 14, 20, 25, 30, 34 and 39.
Pins 27 and 28 belong to a HAT's ID EEPROM. Leave them free unless you are building that EEPROM.
The Pi 5 drives the header through its RP1 chip. The pins are the same, but libraries that write the old SoC's registers directly, such as RPi.GPIO, do not work on it; gpiozero (through its lgpio pin factory) and libgpiod do.
The PWM channels in the table are the Pi 2, 3, 4 and Zero's. On the Pi 5, GPIO12, 13, 18 and 19 are four separate PWM0 channels, and some other alternate functions differ too.

Putting the Raspberry Pi 40-pin header on a PCB

A HAT-sized board is 65 × 56 mm (65 × 56.5 mm with a surface-mount header) with the header along one long edge and four M2.5 holes, drilled 2.75 mm, on a 58 × 49 mm pattern. The original HAT specification has since been superseded by HAT+.
Use a 2×20 female header on your board. Pin 1 is the square pad; check that it lands at the corner the Pi's pin 1 does, or the board plugs in mirrored.
Drawing power from pins 2 and 4 takes it straight from the Pi's supply. Budget the Pi's own draw before hanging motors or LED strips off them.

Common questions

What is the difference between BCM and physical pin numbers?: Physical numbers count the header's pins 1 to 40. BCM numbers are the GPIO names the software uses. GPIO17 is physical pin 11, for example. Most libraries, gpiozero included, take BCM numbers.
Which Raspberry Pi pins are I²C, SPI and UART?: I²C1 is GPIO2 and GPIO3 (pins 3 and 5). SPI0 is GPIO10, 9 and 11 with chip selects on GPIO8 and GPIO7 (pins 19, 21, 23, 24, 26). The UART is GPIO14 and GPIO15 (pins 8 and 10).
Is the Raspberry Pi 5 GPIO pinout the same as the Pi 4?: Power, ground and every GPIO number sit on the same pins, and I²C1, SPI0 and the UART are where they were. Some alternate functions differ (GPIO12 and GPIO18 are separate PWM channels on the Pi 5), and software needs libraries that support its RP1 I/O chip: gpiozero through lgpio, or libgpiod. RPi.GPIO does not work.

Making a HAT? Describe what sits on top of the 40-pin header and PCBEditor builds the board with the 2x20 header in place.

Design a Pi add-on board →

Sources

Raspberry Pi documentation: GPIO and the 40-pin header
Raspberry Pi HAT specification
KiCad 9 libraries: symbol Connector:Raspberry_Pi_2_3, footprint Connector_PinHeader_2.54mm:PinHeader_2x20_P2.54mm_Vertical (CC-BY-SA 4.0).