You will have to look at the sucralose package and see what the conversion rate is for a cup of sugar. Sucralose can be all over the map as far as strength, depending on what it's cut with.
I actually mix my own sweetener using a tablespoon of pure powdered sucralose dissolved in a cup of distilled water. It only takes about 2-3 drops of this mixture to sweeten a cup of coffee. A cup of this stuff lasts me a couple of months.
On the other hand, if you use something like Splenda, it's sucralose mixed with dextrose and maltodextrin to bulk it up. So the ratio will be completely different.