The Raspberry Pi HAT and pHAT Ecosystem Explained: Choosing and Stacking Add-On Boards

The Raspberry Pi's 40-pin GPIO header is the reason the platform has such a deep add-on ecosystem, but "just wire it up yourself" isn't always the best answer — a well-designed add-on board handles power regulation, connector strain relief, and mechanical fit in ways a breadboard and jumper wires never will. This guide explains the HAT specification and its variants, how to tell them apart, and how to choose the right add-on board instead of accidentally buying one that doesn't fit your Pi or conflicts with another board you're already using.

What Makes a Board a "HAT"

HAT stands for Hardware Attached on Top, and it's an actual specification, not just a marketing term for "any GPIO board." A board that's genuinely HAT-compliant must meet several requirements the Raspberry Pi Foundation defined specifically to make add-on boards safely auto-configurable:

• Correct board outline and mounting holes matching the official Pi form factor, so the HAT lines up with the Pi's mounting standoffs.
• An EEPROM containing an ID string that tells Raspberry Pi OS what the board is and what GPIO pins and drivers it needs, enabling auto-configuration without manual setup on genuinely compliant boards.
• A defined power budget the board is allowed to draw from the Pi's 5V rail without external power, and clear labeling when it needs its own supply.
• GPIO pin usage documented against the standard pinout so users can tell at a glance which pins a HAT claims before stacking a second board that might conflict.

Many popular "HAT" boards on the market are HAT-shaped (correct outline and header) but skip the ID EEPROM — these work fine but require manual driver setup rather than being auto-detected. That's a meaningful practical difference even though the physical board looks identical.

HAT, pHAT, and the Size Variants

Form factorFitsNotes HATFull-size Pi (3, 4, 5, and full-size Zero-compatible headers)Full 65mm x 56mm outline, standard mounting holes pHATPi Zero and Zero 2 W primarily, also fits full-size PisHalf-size — 65mm x 30mm — designed to keep a Zero-based build compact BonnetAny Pi with a 40-pin headerAdafruit's naming for their HAT-shaped boards; functionally the same idea, not an official spec term Zero HAT / Zero pHATPi Zero form factor specificallySome vendors explicitly size boards to match the Zero's smaller footprint even when electrically identical to a full HAT

The practical rule: a full-size HAT will physically fit on any 40-pin Pi, but a Pi Zero build with a full-size HAT stacked on top loses the whole point of using a Zero in the first place — check for a pHAT-sized version if board footprint matters for your enclosure.

Stacking Multiple HATs

This is where most HAT-related problems actually happen. Two HATs that each use I2C, or that both claim the same GPIO pin for something like a chip-select or reset line, will conflict the moment you try to stack them. Before combining boards:

• Check documented pin usage for every board — most vendors publish a pinout diagram; cross-reference before assuming two boards will coexist.
• I2C and SPI devices can usually share a bus as long as each device has a distinct address (I2C) or its own chip-select line (SPI) — the real conflicts are almost always around dedicated GPIO pins used for interrupts, resets, or enable lines rather than the bus lines themselves.
• Stacking headers exist for a reason. A pass-through (stacking) header lets a second HAT sit above the first while both still reach the Pi's pins — but only works if neither board physically blocks components on the one below it, which is a real issue with boards that have tall components like RTC coin cell holders or large connectors.
• Power budget adds up. Each HAT drawing power from the Pi's 5V rail eats into the same supply — a Pi already running a display HAT and an audio HAT off the header, with no auxiliary power, is a common cause of brownouts and random reboots that look like software bugs but are actually power delivery problems.

Common HAT Categories and What to Look For

CategoryExamplesWhat to check before buying Power (PoE, UPS)Official PoE+ HAT, UPS HATs with 18650 cellsFan compatibility with your Pi's case, and whether it passes GPIO through for further stacking DisplaysOfficial touchscreen, small SPI TFT HATs, e-ink HATsWhether it needs the full header or leaves pins free for other HATs underneath AudioDAC HATs (HiFiBerry, IQaudio), I2S microphone HATsWhether it disables onboard audio/HDMI audio, and driver support for your OS version Motor/relay controlMotor driver HATs, relay HATsWhether it needs a separate motor supply — most motor HATs should not run motors off the Pi's own 5V rail Sensor/GPIO expansionADC HATs, environmental sensor HATs, breakout HATs with prototyping areaBus type (I2C vs SPI) and whether addresses are configurable if you'll want more than one Real-time clockRTC HATs with coin-cell backupPhysical height — coin cell holders often prevent flush stacking of a second board

When to Build Your Own Instead of Buying a HAT

A generic prototyping HAT (a bare board with a breadboard-style GPIO breakout and open row) is often the better answer when your needs are simple and specific — it costs less than a purpose-built HAT and avoids paying for features you won't use. For anything more involved, a custom HAT is a natural extension of the PCB design skills already covered in this site's KiCad and JLCPCB ordering guides — designing your own gives you the exact pin mapping, mounting, and power handling your project needs, and following the official HAT EEPROM spec means your board gets the same auto-configuration convenience as a commercial one.

Choosing an Approach

Buy a purpose-built HAT when a well-supported one already exists for your exact need — PoE, a display, audio, an RTC — since the driver support and documentation save real setup time over a from-scratch build. Reach for a generic prototyping HAT plus your own wiring when your need is narrow enough that a full purpose-built board is overkill. Design a custom HAT when you're combining several functions that no single commercial board covers, or building something meant to be reproduced and shared with others.