<prompt> Human: Sorry if this is a dumb question but I keep getting different answers from my coworkers. Im not married and have no kids. Ive heard putting 0 will mean they take the most from me each check and at the end of they year I may get something back? But Im confused because my gf always puts zero and always owes. This is my first year of having a full time job  <response>: The withholding algorithm looks at your paycheck and multiplies it by number of paychecks youd get in a full year. Then it subtracts an amount it will consider not taxed.not taxed 2300 4050 number of allowancesIf youre single you get a 6350 standard deduction and a 4050 personal exemption, meaning 10400 of your income is not taxed. So using 2 allowances is just the right thing to do in order to convey that. Then the algorithm calculates tax on the remaining taxable income, and divides by number of paychecks in a full year, and uses that as the withheld amount on the given paycheck.If you use 0 allowances, its like telling the algorithm that 8100 more of your income will be taxed than actually will be. If youre in the 15 tax bracket, that would mean it thinks you owe 1215 more in taxes than you really will, so it will take out 1215 more in taxes than it really needs to about 47 more withheld every two weeks. Youd get the money back as a refund of 1215 at tax time, but only because it would be returning to you money you overpaid all along during the year.If your GF always puts zero and always owes it sounds like perhaps she has other sources of income investments, self-employment that cause her tax to be greater than otherwise. Or maybe she inadvertently used the Married checkbox on her W-4 and not enough tax is being taken. 