Why do I owe money when I claim single, 0 dependents on my two jobs and have an additional $15 taken.?

I did my taxes last year, I claimed 0 dependents on one job and 1 dependent on another all year long and I was shocked when I owed $600 at the end of the year. I always thought as long as you claim 0 you should be fine. So I adjusted my w-4 so I had 0 dependents on both and had them take an additional $15 from each... show more I did my taxes last year, I claimed 0 dependents on one job and 1 dependent on another all year long and I was shocked when I owed $600 at the end of the year. I always thought as long as you claim 0 you should be fine. So I adjusted my w-4 so I had 0 dependents on both and had them take an additional $15 from each job on every check. This was to calculate a good hundred over what I owed last year. Last year i made about $33000. I went to the IRS calculator and now it is trying to say that it's estimated that I will owe $1400 this year in taxes. I only made $21000 this year, (loss of pay is because I have been in college all year). I know this is just an estimate, but what gives? I want to understand why they aren't pulling enough money from my paychecks. How does this all work. I would like to know so I can re-adjust additional with holdings on my w-4 beginning of January. Thanks
7 answers 7