How it works
The calculator works out your 2026 federal tax twice, with and without each move: money put into a 401(k), a deductible IRA or an HSA comes off your taxable income, and investment losses offset your gains first, then up to $3,000 of other income. The difference is what each move saves. State tax is estimated with a flat rate.
Tax saved = 2026 tax now − 2026 tax after the move
Worked example
A single saver earning $100,000, with $10,000 in their 401(k) so far, puts in $5,000 more before December 31. In the 22% bracket with a 5% state tax, that saves $1,350.
They also took $10,000 of long-term gains this year and hold funds now worth $13,000 less than they paid. Selling them wipes out the gain's 15% tax and takes $3,000 off their other income: $2,810 saved. Together the two moves save $4,160.
The moves, and the rules
- 401(k). Up to $24,500 in 2026, plus $8,000 from age 50, or $11,250 at 60 to 63. It has to come out of a 2026 paycheck, so ask payroll early.
- Traditional IRA. Up to $7,500, plus $1,100 from age 50, until April 15, 2027. It's deductible unless you or a spouse have a workplace plan and earn above the IRS limits. A Roth IRA saves nothing now; it's taxed going in and tax-free coming out.
- HSA. $4,400 self-only or $8,750 family, plus $1,000 from 55, until April 15, 2027, if you had an HSA-eligible health plan. See the HSA calculator.
- Selling at a loss. Losses offset gains first, then up to $3,000 of other income ($1,500 married filing separately); the rest carries forward. If you buy the same or a substantially identical investment within 30 days before or after, the wash-sale rule disallows the loss.
For the whole year's picture, see the income tax calculator; for Roth accounts, the Roth IRA calculator. This is an estimate, not tax advice: it doesn't include tax credits, itemized deductions or how your state taxes investment gains.
Questions people ask
What can I do before December 31 to lower my taxes?
The biggest levers for most people are raising 401(k) contributions from your last paychecks and selling investments that are down to offset gains. IRA and HSA contributions for 2026 can wait until April 15, 2027.
Is tax-loss harvesting worth it?
If you have gains to offset, or income to reduce by up to $3,000, usually yes, and unused losses carry forward. It lowers your cost basis, so part of the saving can come back as tax when you sell later.
Can I still contribute to a 401(k) for 2026 in January?
No. 401(k) contributions count for the year they come out of your pay, so 2026's must come from a 2026 paycheck. IRAs and HSAs are the ones you can fund until the April deadline.
Sources
- IRS: 401(k) limit increases to $24,500 for 2026, IRA limit increases to $7,500
- IRS: Topic 409, capital gains and losses ($3,000 loss limit and carryover)
- IRS: Publication 550, investment income and expenses (wash sales)
- IRS: Rev. Proc. 2025-19 (2026 HSA limits)
- IRS: Rev. Proc. 2025-32 (2026 brackets and capital gains thresholds)
Reviewed September 29, 2026 by The Dollars World team. How we build and check our calculators · Our data and formulas. This is general information, not financial advice.