How can I find the scaled dimension of a RectTransform?

I am trying to add UI elements at runtime. My canvas is using a CanvasScaler set to "Scale with screen size". I also created a prefab from a RectTransform. I notice that when I instantiate that prefab and request the width, I get the initial width however I want to get the scaled width. I assume that the width hasn't been updated as I haven't run through an update cycle so the canvasScaler hasn't done his work yet however I need to find the new width as I need this for accurate positioning.

This is how my code looks like:

 GameObject go = GameObject.Instantiate(uiPrefab) as GameObject;
 RectTransform rt = go.GetComponent<RectTransform>();
 print(rt.rect.width)