Calculating Output High voltage for a MCU

[loudaslife]

I'd like to drive some LEDs, but I don't know what voltage to expect coming from the MCU. The ATmega328 datasheet lists the expected output high voltage on page 314, but only for 4 sets of conditions. Is there any way I can calculate the expected voltage given an arbitrary current and source voltage? Or would I have to resort to actually getting my hands on one to measure?

[Rufus]

The datasheet you linked has plots of typical output pin voltage vs. current.

[Psi]

5V in 5V out with no load. 

Worst case with 20mA load you are guaranteed to get more than 4.2V @ 85C.

In reality, at room temp and no other pins loaded, you'll probably get ~4.80V at 20mA load.

[dannyf]

Is there any way I can calculate the expected voltage given an arbitrary current and source voltage?

35.7 Pin Driver Strength.

Typicall 20-25ohm is a good number and 50ohm is a conservative number.