This tool analyzes your deck to calculate the color ratio, density, and complexity of your card's mana costs in order to determine the weighted ratio of colored and dual lands you should use. It also offers a rough guess for your "minimun-safe" dual land ratio, meaning the minumum rate of two-color lands you could possibly get away with.
This tool is a great place to start when first creating the mana base for a new deck, but you should always exercise your own judgment. The tool can't know everything, and playtesting is always the best method for final determination. In the end, this is just a good beginning.
To use this tool, simply input your nonland cards below in normal deck format and hit submit. Vrgrb will take care of the rest.
Note: Vrgrb assumes you actually want to cast all of your cards, so if there are cards you have no plans of casting (Force of Will), leave them off of your list.