Which pins are safe to use on ESP32? (pinout basics)

ESP32 boards have lots of GPIO pins, but not all pins behave the same. Some are input-only, some are used at boot, and some are connected to on-board flash depending on the module.

Simple rules that prevent most problems

Start with pins labeled GPIO on your board's pinout. Do not guess from random diagrams.
Avoid wiring to boot strap pins until you know what you are doing. Bad pull-ups/pull-downs can stop the board from booting.
Remember input-only pins. Some ESP32 pins can read inputs but cannot drive outputs.

Input-only pins

On classic ESP32, GPIO34 to GPIO39 are typically input-only. Great for sensors, not for LEDs or relays.

Pins that can affect boot

Some pins are sampled during boot to choose modes. If you connect a module that forces one of these pins high/low, the ESP32 may fail to boot or fail to upload.

If uploads are flaky, disconnect external wiring and try uploading with only USB connected.

Best practice

Use your exact board's pinout (the same dev kit can be wired differently across clones).
Test with a simple sketch first (blink or serial print) before adding lots of peripherals.
Keep wires short and share a common ground.

Bottom line

Use your board's pinout, prefer known-safe GPIO pins, and be careful with boot-related pins. If the ESP32 will not upload, disconnect external wiring and try again.