Deductions & credits

Thanks for your opinion!

I understand it is a taxable benefit. But why taxes are so high? My employer gave me $4381 for my spouse's tuition. And my withholding was 2000$. It is because the employer's assistance was added to my gross income, which made my income unusually large. It turned out I paid for my spouse's tuition half cost, despite that being supposed to be free. How do I calculate if my withholding tax was right?

Thanks :))