Replacing butter with canola oil depends on the use of the butter in the recipe.

If butter is being used to saute vegetables or brown meats it can be replaced in equal quantity with oil.

If it is being used to flavour a dish as in risotto where you would stir it in at the end, then a better option than replacing would be to leave it out all together; use stock, herbs; tomatoes, lemon juice or whatever is appropriate to enhance the flavour.

If butter is being used the thicken a sauce as in a beurre blanc sauce, do not replace with oil, the sauce will not thicken.

Do not replace oil for butter in baked goods. Butter plays a number of roles when baking including enhancing flavour, creating a luxurious mouth feel, creating structure and stability, adding flakiness,etc.

If you are trying to avoid butter, there are many recipes that call for oil rather than butter. Find this type of recipe instead of trying to replace butter with oil in a recipe calling for butter.

Hope this helps out with your cooking.