How do you calculate SEP IRA? Would you be able to share the formula? Is it applicable to a person with a full time job who already contributes to 401K and ROTH IRA and is ineligible for a tax-deductible IRA?
Turbo Tax should be able to calculate how much you are eligible to contribute.
I think with a 401K and Roth IRA you can still have an SEP IRA, but, the exact effect of your primary employer 401k on the limit you can contribute I don't know... I'm not close enough to the maximums to worry about it myself, so I haven't looked it up.
IIRC: Take your total earnings for the year from your 1099 Misc, Box 7. (Starting in 2018, I think you can first subtract 20% as a new "pass-through deduction", but I haven't gotten clarity yet on whether that applies to tradeline earnings). Then I estimate federal taxes owed based on my marginal federal rate, and state taxes I'll owe from my marginal state tax rate. Then, take 92.35% of your total from your 1099 Misc Box 7, and multiply that by 15.3%, to get what you owe in FICA. Half of that FICA can be your
deduction for an SEP IRA.
To calculate the total you can actually
contribute to the SEP IRA, take your 1099 Misc Box 7, subtract half of the above FICA to get a net income, and then 20% of that can be contributed to an SEP IRA (all the way up to $55,000, if you made like $300k in tradeline sales :P ).
All that should be checked with a tax advisor, yada yada, but the numbers I got in Excel matched what Turbo Tax gave me for 2017 and the Vanguard calculator (
https://personal.vanguard.com/us/SbsCalculatorController), so, I think I'm right
for me. You mileage may vary and all of that.