Can I make a contribution to my 401k until 4/15 for 2018? I already have my W2. Where do I list it to get credit for the contribution? Will this reduce my taxable income?

[wearel3gendary4m]

I want to make a contribution to my 401k to reduce my taxable income. I already have my W2.  If I am permitted to make that contribution up until 4/15, where is it reflected on my tax return? Will my employer have to file an amended W2?  

[Ashby · Accepted solution]

No, you cannot make an additional elective deferral to your 401(k) for 2018.  Your elective deferrals to your 401(k) needed to have been made from your pay by payroll deduction by year end.  Money cannot be deposited to your 401(k) from your savings.