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 (if you can go into a store) and Ammonia Clear (available on Amazon). Ace ammonia is 10% concentration; Ammonia Clear is 5% concentration.