I have a full-time job with tax already deducted and now starting a part-time job that I will have to pay taxes on. How do I calculate my estimated tax payments?

They ask for all income so a calculation can be made for taxable income, the amounts withheld, and the remaining to be done in ES payments.

 

https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes

 

Form 1040 ES provides an accurate estimate of needed payments based on taxable income, amounts withheld, and what "shortage" there might be to be paid via ES payments

 

https://www.irs.gov/pub/irs-pdf/f1040es

 

You will likely end up in a higher tax bracket due to the $30,000 increase in income.  If it's from Self-Employment, you will also owe SE tax.  As an alternative, you can increase your W-2 withholding to cover the total tax due.  Try here:

Tax Calculator - Refund & Return Estimator 2021-2022 | TurboTax® Official (intuit.com)

I dont want to mess with my W4 for my full time job. Cant i do separately? This is way above my head. Who do I go to/pay for help with this? When I did it it said I'd be making about $5000 in quarterly payments, which doesn't make sense if I'm only paying on the $30,000.

Also be aware that any increase in income can reduce or eliminate some deductions/credits.

I currently don't have any deductions or credits and I checked and I'll be in same tax bracket 

You can try using the IRS W-4 calculator

https://www.irs.gov/individuals/tax-withholding-estimator 

 

Remember the result will likely be your total recommended withholding.  You could subtract your 2021  withholding from your full time job from this total to get the additional withholding recommendation. 

I know you said you don't want to mess with your full-time job's withholding, but that may be the simplest way to deal with this.

 

If you want to, you can also choose to file quarterly estimated tax payments.  But that may be a bit more cumbersome than changing your withholding.

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