Here's a quick reference guide for substituting olive oil for butter or margarine in baking.
Butter/Margarine.........Extra Virgin Olive Oil
1 tsp.................................3/4 tsp.
1 tbsp.............................. 2 1/4 tsp.
1/4 cup ............................3 Tbsp.
1/3 cup ............................1/4 cup
1/2 cup.............................1/4 cup + 2 Tbsp.
2/3 cup.............................1/2 cup
3/4 cup ............................1/2 cup + 1 Tbsp.
1 cup.................................3/4 cup