The calculator uses Bayes' Theorem to calculate the probability that God exists, given the inputs you enter. It is a merely a tool to aid thought on some major arguments surrounding God's existence. It requires you to enter 7 probabilities between 0 and 1, as follows:

The first 2 numbers represent your view on whether God helps explain the existence of the universe. So in the top left box, you should enter (your best guess of!) the probability that God would create the universe. In the top right box, you should enter the probability that the universe would exist if there were no God. Note that these numbers don't need to covary in any systematic way: you might think that the existence of the universe was certain either way, say, and enter 1 in both boxes. Certainly, putting a high number on one doesn't require you to put a low number on the other.

The boxes on the second row represent your view on whether God helps explain the existence of life. So in the first box on the second row, you should enter the probability that God would create life.* In the second box on the second row, you should enter the probability that life would come into existence in a Godless universe. Again, these numbers are independent and you should be happy with two numbers which are both high, or both low, or whatever else you think.

The boxes on the third row represent your view on whether the existence of suffering is evidence that God doesn't exist. So in the first box on the third row, you should enter the probability that God would permit suffering. In the second box on the third row, you should enter the probability that suffering would exist in a Godless universe. Yet again, these numbers are independent of one another.

And finally, the last number represents the probability that God exists given any other factors that you think are relevant. If you are any in doubt, just leave it at 0.5.

*As you enter successive numbers, assume that the previous thing is already taken into account. So when you estimate the probability that life would exist, you shouldn't worry about the possibility that there might have been no universe at all. And when you enter the probability that suffering would exist, you shouldn't take into account the probability that there might have been no life.