$18,000, plus a catch-up of $6,000 if age 50 or over, is the maximum elective deferral or Roth 401(k) contribution permitted by an individual across all plans in which the individual participates. A separate employer contribution is permitted to the solo 401(k) (if net profit is sufficient). For someone age 50 or over the maximum total to the solo 401(k) cannot exceed $60,000.
