I wonder if I should use are or is in the following sentence:
The cost savings ____ 50%?
Or should I just say the cost savings percentage is 50%. Thanks
I would say definitely don't rephrase to include the word "percentage". Bear in mind that "50%" is pronounced "fifty percent" - saying the saving percentage is fifty percent would sound incredibly awkward to any native speaker, and discarding the actual word "percent" wouldn't be much better (nor would it match the written form).
Idiomatically, the usual form is the cost savings are fifty percent, but there's nothing at all wrong with the cost saving is fifty percent - it's just less common, particularly in more recent decades.
Mixing a plural subject with a singular verb is rarely acceptable in English; this isn't one of the rare exceptions. As to whether the saving(s) should be singular or plural, I would say that if you're definitely referring to the cost of one single thing you can't use the plural form. In (almost?) all other contexts you can use either singular or plural.
I say "almost" because I can't think of a context where you can't use the singular form (it's probably always possible to conceptualize all costs or savings as one cost or one saving), but it's at least feasible someone can think of a context where only the plural form works.
@discodane is right. Saving is only used when it refer to one instance of reducing something. Remember when you put "ing" at the end of a verb, you change the verb into a noun. That's the only case where saving is used. See the following example:
You can enjoy a year's membership for just $28 – a saving of $7 off the regular rate.
In the case above, you can change saving to reduction or reducing.
As for the case where the word savings is used, bear in mind savings is a word/term, instead of the plural form of saving. So I believe the answer to your question should be
The cost savings is 50%.
"Is" is the correct answer. What you are referring to is a singular percentage of a singular cost. What the number is before the percentage is irrelevant. It is a singular percent so the cost savings IS 50%.
A counter example would be if there were multiple costs.
"The costs(plural) savings are 50%"