Whether or not you get a nasty surprise (in terms of a penalty) is entirely dependent on whether you underpay, and if so, whether you meet one of the safe harbors:
- Owe less than $1000
- Paid at least 90% of the amount owed in the current year
- Paid at least 100% (110% if AGI is over $150k) of the amount owed in the previous year
The 2018 tax brackets are
Rate | Single | Married Filing Jointly |
10% | Up to $9,525 | Up to $19,050 |
12% | $9,526 to $38,700 | $19,051 to $77,400 |
22% | 38,701 to $82,500 | $77,401 to $165,000 |
24% | $82,501 to $157,500 | $165,001 to $315,000 |
32% | $157,501 to $200,000 | $315,001 to $400,000 |
35% | $200,001 to $500,000 | $400,001 to $600,000 |
37% | over $500,000 | over $600,000 |
So subtract $12k single / $24k married filing jointly from your expected income after payroll deductions (401k, health insurance, HSA/FSA, etc), find your bracket, subtract the lower end of the bracket from your income, multiply by that bracket, subtract the lower end from the upper end of each lower bracket and add that amount to your total tax due.
If you expect to have more than 12k/24k in itemized deductions subtract that instead of $24k. Remember that many itemized deductions have been eliminated or reduced.
If you're especially high income you may have extra medicare taxes or you may be subject to the Alternative Minimum Tax.