How the 401(k) optimizer works

2026 tax year · Checked Oct 24, 2026

The optimizer walks every paycheck of the year, at every job, in date order. It applies the shared 401(k) deferral limit across all employers, each employer’s own total-additions limit, employer match as it is paid, and the taxes each employer withholds, then finds the contribution rates that fit the most into Roth without losing match.

Open the 401(k) optimizer

Step by step

1. Pay dates

Weekly and biweekly jobs pay on real calendar Fridays, so three-paycheck months show up. Semi-monthly jobs pay on the 15th and the last business day, monthly jobs on the last business day. Bonuses pay on the 1st of their month and RSUs vest on the 20th.

2. The two limits

Pre-tax and Roth deferrals share one $24,500 limit across every job you have, enforced in paycheck order. Total additions (your deferrals, employer match and after-tax contributions) are limited to $72,000 at each employer separately, which is what makes a mega backdoor Roth possible at more than one job.

3. Employer match

Match is figured each paycheck on that paycheck’s contribution, up to the plan’s percentage of that paycheck’s pay. If you hit the deferral limit early and the plan has no true-up, later paychecks get no match, and the optimizer warns you. With a true-up, the missed match is added at year end.

4. Withholding against what you owe

Each employer withholds as if it were your only job, which is how payroll works. The tax you owe comes from all your income together, less pre-tax contributions, half of any self-employment tax and the standard deduction. The difference is what you would owe in April, shown with quarterly estimated payments.

5. Optimizing

At each job, deferrals go only as high as it takes to earn the full match. Any deferral room left goes to the one job with the most room under its own total-additions limit, and after-tax contributions fill what remains at every job. Midway through the year it counts what you have already contributed and plans only the paychecks left.

Figures used

Elective deferral limit (2026): $24,500
Total additions limit, per employer: $72,000
Social Security wage base, per employer: $184,500
Supplemental withholding on bonuses and RSUs: 22%, or 37% above $1M

Assumed

Every plan allows after-tax contributions and in-plan Roth conversion only when you turn them on for that job.
After-tax money is converted to Roth right away, so no earnings on it are taxed.
Self-employment (1099) income pays both halves of FICA on 92.35% of net earnings.

Not modeled

Age-based catch-up contributions (50 and older) and the Roth-only catch-up rule for high earners.
Plan-specific rules such as caps on after-tax percentages, nondiscrimination testing refunds and vesting of match.
Solo 401(k) and SEP plans for 1099 income.

Sources

These are estimates for education, not tax or investment advice. If a number looks wrong, report a problem with this calculation and say what you entered.