I used the free version of the Mr. Malty pitching rate calculator to determine the pitching rate for a 1.064 ale. I entered “simple starter, 1 Wyeast activator pack, 5.25 volume, and 80% viability”. It showed that i would need a 3.6 liter starter. I then used the calculator at fermentationriot.com, entered exactly the same info and it calculated that I would need a 4.6 liter starter. Which one is correct, and why would there be such a difference in starter size from two different calculators?

Neither is correct. They’re just making an estimate, and 3.6 L and 4.6 L are pretty close, really.

Although the Fermentationriot page does say this:

Which the MrMalty calculator would call “intermittent shaking”.

The Fermentationriot calculator also says to use 187 g of DME for the 4.6 L starter, so it may be recommending a larger starter because it’s very low gravity, ~1.015.