How Accurate Are 401(k) Calculators?
We checked ten popular retirement tools against IRS Notice 2025-67. Half of the eight we could check had at least one wrong 2026 figure. Here is the method.
The arithmetic in a retirement projection tool is rarely the problem. Compound interest hasn't changed. What changes is the figures, every November, when the IRS publishes the following year's limits, and updating them is the sort of chore that's easy to put off until March.
So in August 2026 we took the ten retirement tools most visible in search for 401(k) figures and checked their published figures against the IRS source, one line at a time.
Two of the ten had nothing to check. One keeps its planner behind a login, and the other publishes a general retirement income tool with no contribution figures on it at all. That left eight calculators with published figures we could compare, line by line, against IRS Notice 2025-67 and the IRS cost-of-living tables.
Four of those eight were publishing at least one incorrect 2026 figure. One was a full year out of date on every contribution limit while displaying a recent "last reviewed" stamp. One cited a notice that doesn't exist. One was running a Social Security formula from two years earlier.
We're not naming the four, or the ten.
This site is new, it has no track record, and a page listing other people's mistakes would say more about us than about them. That costs you something, and it's worth being straight about it: with the names removed you can't verify our finding directly. So the method is below in full, with the exact IRS figures we checked. It takes about a minute per calculator. Run it on whichever ones you like, and if you want to publish what you find, names and all, that's yours to do.
Half the retirement tools we could check had a wrong figure on them
| Result | Sites |
|---|---|
| Reviewed in total | 10 |
| No published figures to check — login-gated or no such tool | 2 |
| Checked against the IRS source | 8 |
| Every figure correct | 4 |
| At least one incorrect 2026 figure | 4 |
The errors sit one layer below the headline number
The four calculators with something wrong made four distinct kinds of mistake, and the table below counts error types, not sites. Two of the four carried more than one 2026 error, so the column adds to more than four.
| Error | Sites | Why it happens |
|---|---|---|
| Headline contribution limit a year or more out of date | 2 | The figure is typed into the page. Nobody remembers every place it was typed. |
| Secondary limits stale while the headline is current | 2 | The deferral limit gets updated because everyone asks about it. The IRA catch-up, the §415(c) total and the compensation limit do not. |
| A citation to an IRS notice number that does not exist | 1 | A source was asserted, never checked. The link resolved to a generic page. |
| Social Security formula two years out of date | 1 | Published on the site's own methodology page, the page that exists to demonstrate rigour. |
Look at the second row. Getting the headline number right isn't the same as being current, and most people only ever need the deferral limit, so that's the figure that gets updated. The errors live one layer down, in the §415(c) total and the compensation limit, where nobody looks.
The other pattern is about sourcing. Only three of the eight named an IRS notice at all. Two of those three were among the four with errors, and one of the notices was invented. The four that were entirely correct mostly cite nothing at all, so a reader who wants to confirm those figures has no way to do it either.
Four steps, about a minute per retirement tool
You don't have to trust this page, and with the names removed you shouldn't have to. This is exactly what we did on each of the eight, against IRS Notice 2025-67. It works on any calculator, this one included.
- Verify one number you can look up. The 2026 employee deferral limit is $24,500. A site showing $23,500 is on 2025 figures. A site showing $23,000 is on 2024. This single check caught two of our four.
- Look for a citation. A calculator that names the IRS notice its figures come from can be checked. One that simply asserts a number can't be, not by you and not by anyone.
- Click the citation. This is the step that catches the most. A link that lands on a generic overview page, or on a document that never mentions the figure, isn't a source.
- Check a second-tier number. The IRA catch-up, the §415(c) combined limit and the compensation limit are where staleness hides. All three are in the table below.
Those four steps are the whole method.
We compared these nine figures against the IRS values on every calculator that published them, and counted a site as having an error if any one of them was wrong:
- Employee elective deferral limit
- Catch-up contribution, age 50 and over
- Enlarged catch-up, ages 60 to 63
- Total annual additions under §415(c)
- Annual compensation limit
- Highly compensated employee threshold
- IRA contribution limit
- IRA catch-up contribution
- Qualified charitable distribution limit
Where a site published a Social Security estimate, we also checked its bend points against the SSA table. Figures clearly labelled as a prior year weren't counted as errors. The correct values for all nine are in the next section.
Check any retirement tool against these 2026 figures
All from IRS Notice 2025-67, announced in IR-2025-111 and published in Internal Revenue Bulletin 2025-49. Try them on any calculator.
| Figure | 2026 | Authority |
|---|---|---|
| Employee elective deferral | $24,500 | IRC §402(g) |
| Catch-up, age 50+ | $8,000 | IRC §414(v) |
| Enlarged catch-up, ages 60–63 | $11,250 | SECURE 2.0 §109 |
| Total annual additions | $72,000 | IRC §415(c) |
| Compensation limit | $360,000 | IRC §401(a)(17) |
| Highly compensated employee threshold | $160,000 | IRC §414(q) |
| IRA contribution | $7,500 | IRC §219(b) |
| IRA catch-up | $1,100 | IRC §219(b)(5)(B) |
| Qualified charitable distribution | $111,000 | IRC §408(d)(8) |
One file, one citation, and a build that refuses
Every IRS figure on this site sits in one data file with its citation attached, so no number is typed into a page and the 2026 limits changed in a single edit. A block that can't say when it was last read against the IRS document it cites won't compile at all, and the methodology page sets out the rest of that machinery, check by check.
None of it makes us immune. A system that prevents stale numbers doesn't prevent a wrong one being entered correctly, and we have no more claim on being right than any of the eight in that table. Find an error here and tell us. We'll fix it, date the correction, and say what was wrong, which is written down as a corrections policy rather than left as a good intention.
Checked on 31 August 2026, and the next date is already set
The last full check was 31 August 2026. The next one is due when the IRS publishes the 2027 figures, expected in late October or early November 2026.
We state a date, not a cadence. That is deliberate. One of the four sites in this audit displays a promise to update "within two weeks of IRS COLA announcements" beside figures that were nine months stale, and another claims a quarterly review it plainly has not run. A promise nobody can check is worth less than a date anybody can. If the date above is old when you read it, that's your answer.
Where every figure in this check is published
Every number used here is published, with its source, on the contribution limits page, and every formula behind our own tools is set out on the methodology page. If a figure here disagrees with the IRS document it cites, the IRS document is right and we want to hear about it.
Frequently asked questions
Are 401(k) calculators accurate?
The arithmetic usually is. The figures often aren't. We pulled ten in August 2026, two of which published nothing we could compare. Of the eight left, four were publishing at least one incorrect 2026 number.
How do I know if a 401(k) calculator is using current numbers?
Check one figure you can verify. The 2026 deferral limit is $24,500, and a calculator still showing $23,500 is on 2025 figures. Then look for a citation, and click it.
Does a wrong figure in a calculator actually matter?
Yes. Someone setting a contribution rate from a calculator showing the 2025 limit will under-contribute by $1,000 over the year, or $1,500 if they're 50 or over.
What is IRS Notice 2025-67?
The notice setting the 2026 cost-of-living adjustments for retirement plans, announced in IR-2025-111 and published in Internal Revenue Bulletin 2025-49.