Revision History:

• As of Nov 12 2018, this now comes with a SiLabs CP2104 instead of FTDI chipset for USB-to-serial. We also now do not solder in the DC jack as well, so its even skinnier!

Detailed specifications:

• ATmega328 microcontroller with Optiboot (UNO) Bootloader
• USB Programming and debugging via the well-supported genuine CP2104
• Input voltage: 7-9V (a 9VDC power supply is recommended)
• 5V regulator can supply peak ~800mA as long as the die temp of the regulator does not exceed 150*C
• 3.3V regulator can supply peak ~150mA as long as the die temp of the regulator does not exceed 150*C
• 5V logic with 3.3V compatible inputs, can be converted to 3.3V logic operation
• 20 Digital I/O Pins: 6 are also PWM outputs and 6 are also Analog Inputs
• 6-pin ICSP Header for reprogramming
• 32KB Flash Memory - 0.5K for bootloader, 31.5KB available after bootloading
• 16MHz Clock Speed
• Compatible with "Classic" and "R3" Shields
• Adafruit Black PCB with gold plate on pads
• 53mm x 68.5mm / 2.1" x 2.7"
• Height (w/ barrel jack): 13mm / 0.5"
• Weight: 16.5g
• Derivative of "Arduino UNO R3 Reference design" 
• Open source hardware files on github!
• Fritzing object in the Adafruit Fritzing Library
• ATmega328P product page