If I contribute to an employer sponsored 401k plan at work, how much can I contribute to an IRA in 2019 to lower my AGI as a single taxpayer?

https://www.irs.gov/retirement-plans/2019-ira-deduction-limits-effect-of-modified-agi-on-deduction-i...

<a rel="nofollow" target="_blank" href="https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-ira-contribution-limits">https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-ira-contribution-limits</a>

Depending on your MAGI, your deduction might be limited or eliminated.

For 2019, $6,000, or $7,000 if you’re age 50 or older by the end of the year; or
    your taxable compensation for the year.

(Taxable compensation is generally wages that you worked for - W-2 or net self-employed income minus the deducible part of the SE tax, but can include commissions, alimony and separate maintenance, and nontaxable combat pay ).

See this IRS link for Traditional IRA deduction limits when covered by a retirement plan at work.

https://www.irs.gov/Retirement-Plans/IRA-Deduction-Limits