Year-End Contribution Room Planner

Tells you how much 2026 room is left in your 401(k), HSA, and IRA, what it takes per paycheck to use it before December 31, and roughly what it saves in federal tax.

Date: 2026-11-06 Theme: Taxes Status: Prototype

What it is

A single-page calculator you open in a browser. You enter your age, pay, health plan type, and what you have put into each account so far in 2026. It returns the room left in each account, a per-paycheck amount to fill it, and an estimate of the federal income tax the pre-tax part saves. Eight synthetic households are built in so you can see how it behaves before typing your own numbers.

Who it serves

Someone with a workplace 401(k), maybe an HSA, who suspects they are under-using tax-advantaged room and has until the last paycheck of the year to change it. It also fits anyone 50 or older who is unsure which catch-up amounts apply.

The money problem it addresses

Payroll 401(k) deferrals have a hard stop. They must come out of a paycheck by December 31, 2026, and every paycheck that passes makes the per-check number bigger. In early November a 26-pay worker with four checks left and $4,500 of room needs about $1,125 a check, which is a very different decision from the $173 it would have been with 26 left. People find this out in December, if at all.

Three other details get missed:

• Employer HSA dollars count against the HSA limit, but employer 401(k) match does not count against the 401(k) deferral limit.
• The age 60 to 63 401(k) catch-up ($11,250) replaces the age 50+ catch-up ($8,000). It does not stack on it.
• Only pre-tax money lowers 2026 taxable income. A Roth IRA contribution does not, so "I put in $3,000" and "I saved tax on $3,000" are not the same sentence.

The planner puts the three accounts in one fill order and shows the tax effect per bracket.

How it works

1. Pick an example household or type your own numbers. Paychecks left can be typed, or estimated from your next payday through December 31, 2026.
2. The answer table shows each account's 2026 limit, what you have used, the room left, a progress bar, and the per-paycheck amount your budget buys. Fill order: 401(k) up to your match floor, then HSA, then IRA, then the rest of the 401(k).
3. The tax panel computes federal tax with the 2026 brackets and standard deduction, before and after the plan, and names your marginal bracket. A small table repeats the plan at half and 1.5 times your budget.
4. A "things to check" list flags a thin payroll runway, a Roth IRA phase-out, no HDHP (no HSA), a possible Saver's Credit, and a budget bigger than the available room.

What's in this folder

• README.md: this file.
• requirements.md: functional rules, limits used, out-of-scope items, and acceptance checks.
• index.html: the page. Open it directly, no server.
• style.css: layout and colors.
• script.js: all the math (integer cents), with the sample data inlined so it runs from file://.
• sample-data.json: 2026 limits, brackets, and eight synthetic households. The same data is mirrored inside script.js.
• cover-image.png: cover art.
• metadata.md: generation record.

How to run / read it

Open index.html in any modern browser. No build step, login, or API key. Nothing you type leaves the page.

Estimated impact

The planner does not create savings by itself. It makes a decision visible while it can still be acted on. The tax number it produces is arithmetic you can check: in the "Tomás" example (single, $118,000 pay, 22% marginal bracket), moving $2,800 of the remaining paychecks into the 401(k), HSA, and a deductible traditional IRA lowers 2026 federal tax by about $616, which is 22% of $2,800. Money in a Roth IRA or a 401(k) is not lost to tax, but only the pre-tax portion shrinks this year's bill. Results ignore state tax, FICA, and credits, and use the federal brackets only.

Good to know

This is an organizer and estimator, not tax, investment, or legal advice. The Roth IRA phase-out follows the IRS formula only approximately, the traditional IRA deduction can be limited when you have a workplace plan (the tool assumes it is fully deductible), and plans differ on match true-ups and on how catch-up contributions are treated. Higher earners may have 401(k) catch-up money forced to Roth under newer rules, which the tool does not model. Verify the numbers with your plan administrator or a tax professional before changing a deferral. The HSA 2026 figures come from IRS Rev. Proc. 2025-19; I could not read that PDF directly, so the amounts were confirmed against several secondary sources that agree with each other.

Sources

• https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500 — 401(k) $24,500, catch-ups, IRA $7,500 and $1,100 catch-up, Roth IRA phase-outs, Saver's Credit income limits
• https://www.irs.gov/newsroom/irs-releases-tax-inflation-adjustments-for-tax-year-2026-including-amendments-from-the-one-big-beautiful-bill — 2026 brackets and standard deduction
• https://www.irs.gov/irb/2025-45_IRB — Rev. Proc. 2025-32, including 2026 HDHP deductible and out-of-pocket ranges
• https://www.irs.gov/pub/irs-drop/rp-25-19.pdf — Rev. Proc. 2025-19, 2026 HSA limits ($4,400 self-only, $8,750 family, $1,000 catch-up at 55+); amounts confirmed via secondary sources
• https://support.taxslayer.com/hc/en-us/articles/7878715601933-2026-Contribution-Limits-HSA-401-k-IRA — secondary confirmation of 2026 HSA, 401(k), IRA limits
• https://www.cpapracticeadvisor.com/2025/10/09/irs-adjusts-tax-brackets-standard-deduction-for-2026/170661/ — secondary confirmation of 2026 brackets and standard deduction