The arithmetic behind mega backdoor Roth headroom is not hard once every input is right. What goes wrong in a spreadsheet or a mental estimate is almost always one of two omissions, and both push the estimate the same direction: they make it look like there is more room than the plan will actually allow.
A saver turning 61 this year, with $12,000 already withheld from their own paycheck, a $6,000 employer match and profit-sharing contribution so far, another $6,000 of employer money expected before year-end, and no after-tax contributions yet.
| Step | Typical by-hand estimate | What this calculator does |
|---|---|---|
| Which limit applies | Anchors to the $24,500 figure everyone has seen in headlines, and treats that as the ceiling for after-tax room too | Uses the 2026 overall 415(c) limit ($72,000 base), not the smaller 402(g) elective-deferral limit -- the two are different numbers for a different purpose |
| Catch-up amount | Applies the standard 50+ catch-up ($8,000 for 2026) because that is the number most articles quote | Recognizes age 61 falls in the SECURE 2.0 enhanced catch-up band (60-63) and applies $11,250 instead, for an $83,250 ceiling rather than $80,000 |
| What already counts against it | Subtracts only the after-tax dollars contributed so far ($0), leaving an apparent $83,250 of headroom | Nets out the elective deferral already withheld ($12,000) and every employer dollar already deposited or still expected ($12,000), for real headroom of $59,250 |
| Result | Overstates headroom by $24,000 -- the exact amount of the employer money the estimate forgot to count | Reports the figure the plan will actually honor, plus the per-paycheck amount to use it up by year-end |
Figures use the 2026 limits this calculator ships with; see the calculator for the 2025 figures.
Subtracting only the after-tax dollars already contributed from the big 415(c) number, and forgetting that the elective deferral and every employer match or profit-sharing dollar deposited this year also count against that same ceiling. That single omission overstates headroom by whatever the employer has already put in — often several thousand dollars — and the saver finds out only when the plan rejects a contribution or, worse, does not and creates an excess annual addition.
SECURE 2.0 gives savers who turn 60, 61, 62, or 63 during the year a larger "enhanced" catch-up than everyone else 50 and older gets. A spreadsheet built once and reused every year, or copied from a generic article about the standard catch-up, applies the smaller number to someone in that four-year band and understates their real ceiling — leaving real tax-advantaged room unused, which cannot be made up after December 31.
The $23,500 (2025) / $24,500 (2026) figure everyone quotes is the 402(g) elective deferral limit — your own payroll withholding. The after-tax headroom this strategy uses comes from the much larger 415(c) overall limit, $70,000-$81,250 (2025) or $72,000-$83,250 (2026) depending on age. An estimate that anchors to the smaller, more famous number treats most of the real headroom as if it did not exist.
Only if it is rebuilt correctly every year with that year's IRS figures, the saver's exact catch-up band, every dollar already contributed by both the saver and the employer at this specific employer's plan, and the plan's own after-tax cap if it has one below the IRS ceiling. A formula copied from a prior year, or from a generic template that does not separate 402(g) from 415(c), reproduces the same mistakes a purely mental estimate makes — it is just wrong with more decimal places.
It applies the correct year's IRS figures and age-adjusted catch-up automatically, nets every dollar already counted — elective deferral, employer match and profit sharing, and after-tax contributions already made — against the 415(c) ceiling, and separately checks the plan's own after-tax percentage cap when one is entered, reporting whichever of the two actually binds. Nothing is uploaded; it runs the same arithmetic a correct spreadsheet would, without the two steps a hand estimate most often skips.
This page names no specific plan provider, spreadsheet, or competing product — it compares this calculator against estimating the same arithmetic yourself, not against another tool.