> I was replacing margarine with butter in a recipe for a cake
>for example, would I use exactly the same quantity of butter as I would
>do margarine?


Indeed ..but as an addition just to what Vox stated( they are used in
similar proportion...).but further there is another difference.
Butter fat is composed of fat of different crystalline morphology than
the common margarine.( taking into consideration the traditional
margarine and not the special or diet margarines which contains more
water).
.Margarine exists more of the beta prime type and less of the beta
crystals; while the butter is the opposite..
Hence margarine creams better with sugar than butter. But butter
produces better pie crust than margarine.
This can be discerned by measuring the specific gravity of the creamed
mixture before you add the flour.
The cream made with margarine is lighter than the ones made with butter
which results in better cake volume and texture with the margarine
than with the butter.( although some authors dispute this).
Meanwhile butter compensates its poor creaming performance by its
superior flavor.

Margarine and Butter contains at least 80% fat and the remainder is
composed of water milk sollds and salt. but when used ocmparatively in
cake making such as making a butter cake, the margarine creams better
than butter and therefore is superior in cake performance.
Another thing also margarine usually contains slightly more salt than
butter. and you can even use unsalted butter( or sweet butter) but
unsalted margarine is not common<sigh>

Now its up to you to take your pick...butter or margarine?