I'm planning on making some cookies tonight but only need a half batch of royal icing. How much icing sugar would I need in cups?

I believe a full 1kg bag of icing sugar is 8 cups which is what the recipe calls for.....so half would be cups....you could just eyball and pour half the bag in....you will be thinning it down with water so even if you have a bit too much icing sugar, you can add a bit more water...

Thanks