So I had a rude awakening about how to calculate the maximum after-tax deduction I'm allowed to take.

Here's the formula:

Net Business Profit (from Schedule C, also line 12 on 1040)

Subtract Deductible Self-employment tax (from line 27 of 1040)

This is the Adjusted Net Business Profit

Determine your Profit Sharing Number ( I take the maximum amount )

Subtract this Profit Sharing contribution from your Adjusted Net Business Profit

This becomes your "Employee Compensation"

From Employee Compensation deduct your Profit Sharing Contribution as well as "Employee Salary Deferral" amount if any.

The net number is the maximum you can contribute as an after tax contribution to your 401k plan.

With the Employee Salary Deferral don't include the $6,000 catch up provision for being 50 or older - that's good cause it means it won't reduce the amount I can contribute to the after tax account.

Here's some numbers:

maximum after-tax contribution should be:

$57,667 Net Business Profit

$ 4,074 Deductible Self-employment tax

$53,593 Adjusted Net Business Profit

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$10,719 Maximum Profit Sharing

$42,874 Employee Compensation

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$18,000 Employee salary deferral ($6,000 >= age 50 is not included in the 415c limit)

$10,719 Employer profit sharing

$14,155 Employee after tax voluntary contributions

The $6,000 catch-up contribution is not included in the annual addition limit. This applies to both the 415c 2017 $54K statutory limit and the 100% of compensation limit.

Harry Sit has updated his excel spreadsheet to help with this on his thefinancebuff blog

Here's the link:

https://docs.zoho.com/sheet/published.do?rid=hd3vb2c79aa2e630443d58a05e8140934898a