<prompt> Human: My SO 35M is trying to file his tax return. We are filing separately and single. I did mine a couple weeks ago.The first W-2 he put in to TurboTax was about 60000 gross income and 9500 for federal income tax withheld. This put him close to 1000 tax refund, which is about the same that I got we have the same main job.However, he works part-time at another job. This W-2 puts 12000 gross income and 900 federal income tax withheld. Now we are watching the number on the left and it now says he owes 1100.I guess my question is, why did this second job bump the amount of taxes he owe by close to 2000? We want to understand before we pay it.Edit: thank you guys so much! You were very helpful and it now makes complete sense. He will definitely be changing the W-4 for the second job  <response>: 60k puts him in the 25 tax bracket, so each additional dollar he earns gets taxed at 25.He made an additional 12k, so the additional tax is 3k. He withheld 900, so hes short 2,100. His refund before this w-2 was 1,000; so hes short 1,100 