The Risk Legacy Odds Calculator can be used to find the odds of single fight (one roll of the dice) or an entire battle in either Risk or Risk Legacy.
The settings which are currently available contain no spoilers - at least nothing that isn't part of the campaign from the beginning of the first game. Our group is only three games into the campaign, so we may encounter new rules which affect the modifiers and checks that are used by the Fighter. If this is the case, we may update the Fighter, but we will not change the version on this page. Links to newer versions will be provided in a Links tab adjacent to this About tab.
There are three modifiers settings at the top of the screen. The left-hand checkbox indicates that the defender is fortified. Note that if the defender is only using one die, the +1 bonus is only applied once. This appears to be the generally agreed functionality of the fortification rules. The other two boxes are the modifiers on the high and low dice. They can be positive or negative. Unlike the fortificaiton rules, if the defender uses only one die both modifiers will be applied to that die.
On the left of the screen are all the odds for a single roll of the dice. One right are two textboxes in which you can enter the number of armies on either side, and below that are the odds for the entire battle. These odds assume that the attacker and defender will always roll 3 and 2 dice respectively, until the time when they have fewer armies remaining, and then they roll as many as they can.
In the Details area at the bottom of the page, odds for every possible result are tabulated.