(I updated this because I think I wasnt making myself clear in the first part of the question and I clarified what I'm trying to do. I'm just looking to use marginal tax brackets to derive a good estimate of how much federal income tax I'll owe from each paycheck, assuming no deductions. The math I do seems to mostly match what the IRS withholding calculator gives me so I think its right, but I want to check)

I want to estimate the federal income taxes that I owe on each paycheck (my gross salary is 50900 and I live in Illinois) using marginal tax brackets. Just to get a feel for the numbers I'm assuming no deductions (except the standard deduction and 1 personal exemption because I'm single).

I first used a [paycheck calculator](http://www.paycheckcity.com/calculator/salary/) and ran the numbers. I put 0 W4 allowances and no deductions. That calculator gives me:

```
Semi-monthly Gross Pay $2,120.83
Federal Withholding $334.11
Social Security $131.49
Medicare $30.75
Illinois $106.04

```

Gross pay is 50900/24, SS is 6.2% of 50900, Medicare is 1.45% of 50900, so I get all of that. Illinois income tax is 5% so I understand that too. Federal withholding is just an estimate from the IRS, and the number that calculator gives me matches what I get from the IRS table. The [IRS table](http://www.irs.gov/publications/p15/ar02.html#en_US_2014_publink1000254784) says that if the paycheck is between 1631 and 3817, I should do 211.65 plus 25% of the excess over 1631, so

```
211.65 + 0.25 * (2120.83 - 1631) = 334.11

```

so that matches the calculator too. 

When I use marginal [tax brackets](http://www.moneychimp.com/features/tax_brackets.htm) to calculate the *actual* amount of federal income tax I'll owe on each paycheck, I use these brackets:

```
0     - 9075  ==> 10%
9075  - 36900 ==> 15%
36900 - 89350 ==> 25%

```

so I calculate my total federal income tax would be 

```
0.10 * 9075 + 0.15 * (36900 - 9075) + 0.25 * (50900 - 6200 - 3950 - 36900) = 6043.75

```

because I'm single so my standard deduction is 6200 and my personal exemption is 3950. 

**I don't itemize deductions and for this calculation Im not including any pre tax deductions, so healthcare insurance and all that isnt included. On last year's tax return, I had all my usual deductions, so just using the tax I paid last year isn't good enough, and *I want to do this using tax brackets*.**

So based on the 6043.75 I calculate from marginal tax brackets, I'll owe around 6043.75 / 24 = 251.83 in federal income taxes each pay period right?

[This question popped up](https://money.stackexchange.com/questions/21653/why-is-the-2013-withholding-calculator-giving-me-such-incorrect-recommendations) so I went to the [IRS withholding calculator](http://www.irs.gov/Individuals/IRS-Withholding-Calculator), put in 50900 as my gross income and 0 for everything (**so once again, no deductions except standard and personal exempt**). That tells me that my federal income tax is 6050, which is really close to the number I calculated up above, so I think thats a good estimate, **assuming no deductions**.

**Is this a good, rough estimate of the federal income taxes I'll owe for each paycheck, assuming no deductions and not having to use anything from last year (because my financial situation has changed, tax brackets have changed etc)?**