Current through a Uno R3 Digital output

I know it's low, but how much power can I run through the Uno R3 digital output?

The same as you can "run through" any AVR 328's digital output.

Hansen70:
I know it’s low, but how much power can I run through the Uno R3 digital output?

Do you mean how much “power” a pin can deliver.
Output pins should not switch more than about 20mA, although there is an absolute max rating of 40mA.
At 20mA, if… you would get the full 5volt from a pin at that current, power is 5volt * 0.02Amp= 0.1watt.
Not much, so normally an external transistor is used to power things.
Leo…

Thanks for your response. Are you certain it's .1 watts? Is there literature showing that somewhere?

See:
https://playground.arduino.cc/Main/ArduinoPinCurrentLimitations

"Are you certain . . . "
That’s a bit rude.

.

The literature showing the details would be the datasheet for the atmega328.

Hansen70: Thanks for your response. Are you certain it's .1 watts? Is there literature showing that somewhere?

I added the formula for you to understand how I got that number.

There is also a total current limit for the MCU, and maybe other limits. Read the datasheet (I did).

Or explain WHY you want to know this. Leo..

Wawa: I added the formula for you to understand how I got that number.

There is also a total current limit for the MCU, and maybe other limits. Read the datasheet (I did).

Or explain WHY you want to know this. Leo..

I would have hoped to connect it to the digital output and avoid using a relay.

Hansen70: I would have hoped to connect it to the digital output and avoid using a relay.

What is "IT" and how much current does it require?

...R

Are you hoped to connect the 12vdc 100ma pump (or similar) to the digital output and avoid using a relay.

https://forum.arduino.cc/index.php?topic=478425.msg3267622#msg3267622

the digital output only operate at 5V so you have to use relay or MOSFET to control it.