Recipe calls for tablespoons of butter but all I have is whipped butter. What is the conversion?
It depends on the brand of whipped butter mainly. A common volume ratio is 4:1. To be exact though I would suggest comparing the fat and calorie content of your whipped butter to normal butter.
Normal unsalted butter has 100 calories and 11g of fat per tablespoon.
Whipped butter typically has either air or nitrogen whipped into it, increasing its volume four-fold. If you check the nutrition information of your whipped butter and see 25 calories and 2-3g of fat per tablespoon then you can be sure it is a 4:1 ratio.
Note however that if you're baking the air whipped in can have a significant effect on your end result. It will likely be much airier and could even collapse.
You could also rely on any package marketing for an idea. e.g. 50% less fat than butter!
I would expect this to vary by how much air the manufacturer whipped into their butter.
I would recommend going by weight. Butter weighs about the same as water (obviously not exactly the same as it floats but close enough.) It is about .5oz per Tbs.
Either use a scale or use the weight/serving info on the label to measure out .5oz.