Can I max. out a 401k and SEP?

earned 40k profit the same year. So can I contribute an additional 8k to a SEP on top of my 16.5 401k contribution?

You can max out both, but your SEP max is less than $8,000. Your SEP limit is $8,000 minus 1/2 of your self-employment tax.

In addition to the 401(k) and the SEP you can also make an IRA contribution of $5,000 ($6,000) if you're 50 at the end of the year.

See Pubs 560 and 590.

Phil Marti VITA/TCE Volunteer Clarksburg, MD

True, but note that you can only take a deduction for the IRA contribution if you were not covered by a plan at work (and here we have both a 401k and sep), or if your income was low enough (for single filers, the deduction is phased out as your income rises from 58k to 68k).