A tub of butter? Are you really using a tub of butter? We think you're actually using a tub of margarine – or worse – "spread!"*
A stick of butter – formerly a mystery to our readers in Europe, Australia, South Africa, etc., – is a quarter of a pound, or 4 ounces, or 8 tablespoons, or half a cup (or 125 g). If you had a kitchen scale, you would weigh out 4 ounces. If you had a kitchen scale, you would probably not be asking this question.
The manufacturers of tubs do not necessarily make it easy. Some are 7.5 ounces, 13 ounces, 15 ounces, 16 ounces, 45 ounces, etc. – in other words, a jumble of sizes. If you have an 8-ounce tub, you would use half. If you have a 16-ounce (1 pound) tub, you would use a quarter. This is assuming your tubs are full and your estimating skills are good.
If not, fill a half-cup measure, making sure there are no voids hidden in the butter/margarine/spread you are using. That is the equivalent of 1 stick.
Now, you must be sure not to use whipped butter (the butter you are most likely to find in a tub) or any of the "lite" margarines or spreads, because they are not formulated for cooking or baking projects. The margarines and spreads often have extra water and/or fillers that do not respond correctly in baking. The whipped butter, of course, is about one-third air, so if you fill a half-cup measure, you are only using about a third of cup of actual butter.
*We totally support your free choice, but we never use the word butter as a generic term for any old shortening. When we butter a pan, we butter the pan. We occasionally let our prejudices show (we cannot think of any circumstance under which we would ever use "spread"), and for this we apologize profusely. If, however, you are a spread user, you have our full support.