I included this calculator as part of my article on performing a fishless cycle but wanted to make it a separate post for easier access.
This is a simple calculator. Make sure to select your units (gallons or liters) as a first step (based on your preference). The concentration is dependent on the ammonia you use and will usually be between 5% and 10%.
The output is in mL and in drops (useful for small quantities of nano aquariums).
Source for Ammonia:
For aquariums, make sure to use Ammonia that is free of impurities (soaps, dyes, surfactants). I only know of two that are confirmed suitable: Ace hardware (EDIT: I don’t see that this is sold online any longer – it may be available in a store.) and Austin’s Clear Ammonia (available on Amazon). Ace ammonia is 10% concentration; Ammonia Clear is around 5% concentration.
There is now an aquarium specific product – Dr Tim’s Ammonia. I still prefer the options listed above as they are cheaper and some people have indicated the percent ammonia contained is unclear.