 # How many volts input is an arduino input pin?

Just wondering how many volts makes a digital input pin on the Arduino Uno register as ‘HIGH’ and if its possible to actually fry the connection with too many volts/amps.

From what i understand it should be 0 volts = low while 0< is high (not sure about current).

The Arduino Uno uses the ATmega328P microcontroller, you can read the specifications in the datasheet.

If I remember it right, about 3.5V to 5.0V is HIGH.

You may not apply a voltage that is lower than 0V or higher than 5V to a pin. If you do, the current may not be higher than 1mA. That 1mA can be used to calculate an extra protection resistor. Suppose you have a signal that can be 0V or 5V, but a pulse of 10V might occur. The protection resistor is (10V-5V)/1mA=5k. So the protection resistor is 5k6 (between the signal and the Arduino input pin).

If you have a wire with 12V and let that accidently touch the Arduino board somewhere, that will fry your Arduino board.

With Vcc = 5V:
Low is <0.3 x Vcc = 1.5V
High is ? 0.6 x Vcc = 3V

Lows must be above -0.5V
High must be below Vcc + .5V.

Absolute Max: 6V

See the datasheet: Section 29. Electrical Characteristics

With proper current limiting to under 1mA as Caltoa says, the internal clamping diodes will limit the voltage to the specified ranges.
Atmel takes advantage of that in this AC voltage zero crossing detector Application Note:

Atmel AVR182 Zero Cross Detector.pdf (95.1 KB)