401(k) vs. Roth IRA vs. Solo 401(k): A Decision Guide to Every Retirement Calculator on This Site

Why This Site Has 12 Separate Retirement Calculators

Search "401k calculator," "roth ira calculator," or "solo 401k vs sep ira" and you'll get a different tool for each, because each account has genuinely different rules — contribution limits, income phase-outs, and tax treatment all vary. This site's Investment & Retirement cluster has grown to 12 dedicated calculators: 401(k), Roth IRA, Traditional IRA, Solo 401(k), SEP IRA, Mega Backdoor Roth, RMD, CD, Annuity, FIRE, Coast FIRE, and Medicare IRMAA.

The question most people actually have isn't "how does account X work" — it's "which one should I use, and in what order." This guide answers that directly, using the same IRS-sourced 2026 limits that power each calculator (no new figures introduced here).

401(k) vs. Roth IRA vs. Traditional IRA: The Priority Order

For most W-2 employees, the standard priority order is:

1. 401(k) up to the employer match. If your employer matches 50% up to 6% of pay, that's an immediate, guaranteed 50% return — no other step on this list beats it. Run the 401(k) Calculator with your actual match formula to see the year-by-year effect.

2. Max a Roth or Traditional IRA ($7,500 for 2026, $8,600 if 50+). Which one depends on income and tax-bracket expectations: Roth IRA contributions phase out between $153,000-$168,000 MAGI (single/HoH) or $242,000-$252,000 (married filing jointly) for 2026 — above those ranges, direct contributions aren't allowed at all. Traditional IRA contributions are always allowed regardless of income, but the deduction phases out between $81,000-$91,000 (single, if covered by a workplace plan) or $129,000-$149,000 (married joint, contributor covered) — if neither spouse is covered by a workplace plan, the full Traditional IRA contribution is deductible at any income. Compare your own numbers with the Roth IRA and Traditional IRA calculators, both of which apply these exact phase-out ranges automatically based on your filing status and MAGI.

3. Back to the 401(k) up to the full $24,500 employee limit (age 50+ adds an $8,000 catch-up; ages 60-63 get a larger $11,250 "super catch-up" under SECURE 2.0).

4. Mega backdoor Roth, if your plan allows it. Combined employee + employer + after-tax 401(k) contributions can reach the overall IRC Section 415(c) limit of $72,000 (2026) — far above the $24,500 employee-only limit. This only works if your specific plan permits after-tax contributions AND in-plan Roth conversions or in-service withdrawals; most plans don't. Check the Mega Backdoor Roth Calculator to see your remaining room after your standard deferral and employer contributions.

The Traditional-vs-Roth choice at every step comes down to one question: is your tax rate today higher or lower than you expect it to be in retirement? Higher today favors Traditional (deduct now, pay tax later at a lower rate); lower today favors Roth (pay tax now at a low rate, withdraw tax-free later).

Self-Employed: Solo 401(k) vs. SEP IRA — Solo 401(k) Almost Always Wins

If you're a sole proprietor, single-member LLC, or partner with no employees, the "solo 401k vs sep ira" comparison has a clear, calculable answer: a Solo 401(k) lets you contribute more than a SEP IRA at every income level, because it adds a $24,500 employee elective deferral on top of the same ~20%-of-net-earnings employer contribution a SEP IRA uses — the SEP IRA has no employee-deferral option at all.

Running both this site's Solo 401(k) Calculator and SEP IRA Calculator formulas directly across a range of incomes confirms exactly how much this matters: at $150,000 of net self-employment income, a Solo 401(k) allows roughly $53,400 total ($24,500 employee + ~$28,900 employer) versus about $28,900 for a SEP IRA — nearly double. A Solo 401(k) reaches the overall $72,000 (2026) 415(c) cap at around $245,000 of net self-employment income; a SEP IRA doesn't reach that same $72,000 cap until roughly $369,000 — because every dollar of the SEP's contribution has to come from the 20%-of-earnings employer side, with nothing contributed "for free" the way the employee deferral is in a Solo 401(k). (The SEP's crossover point lands close to the plan's own $360,000 compensation limit, since 20% of $360,000 is $72,000 almost exactly.)

The one scenario where a SEP IRA can make more sense: if you have common-law employees. A SEP requires contributing the same percentage of pay for eligible employees as you contribute for yourself, but a Solo 401(k) is only available if you have no employees other than a spouse — so a growing business with staff may not have the Solo 401(k) option at all, regardless of the math above.

The Retiree Trap: How RMDs and Roth Conversions Can Trigger IRMAA

"Roth conversion irmaa calculator" and "roth conversion irmaa exception" are both real, frequently searched questions, because the interaction is genuinely easy to miss: Medicare Part B premiums use your Modified AGI from two years earlier — your 2026 premium is based on your 2024 tax return, not your current income. A large Required Minimum Distribution or a lump-sum Roth conversion in one year can push that year's MAGI into a higher IRMAA bracket, raising your Medicare premium two years later, even though your income has since dropped back down.

Worked example using the actual 2026 CMS bracket table: a single retiree with a normal MAGI of $95,000 (below the $109,000 first-bracket threshold, so no surcharge) converts $20,000 to a Roth in one year, pushing that year's MAGI to $115,000 — just over the threshold. Two years later, that pushes their Part B premium from the standard $202.90/month into the next bracket, adding an $81.20/month surcharge ($974.40/year) they wouldn't otherwise have owed. The fix isn't to avoid conversions — it's to size them deliberately: run the RMD Calculator and Medicare IRMAA Calculator together before a large conversion or distribution, and consider spreading conversions across multiple years to stay under a bracket threshold rather than doing one large lump sum.

RMDs themselves start at age 73 (rising to 75 in 2033 under SECURE 2.0) and are calculated by dividing your prior year-end balance by the IRS Uniform Lifetime Table factor for your age — the RMD Calculator applies that table directly rather than a rough percentage estimate.

Skipping the Traditional Milestones: FIRE and Coast FIRE

The 401(k)/IRA priority order above assumes a traditional retirement age. If your goal is retiring well before 65, the math changes: the FIRE Calculator uses the "25x rule" (annual expenses ÷ a 4% safe withdrawal rate, from the 1998 Trinity Study) to find your target invested-asset number regardless of age, and the Coast FIRE Calculator answers a different, often more useful question for people in their 30s and 40s: how much do I need invested *today* so growth alone — with zero further contributions — reaches my full retirement number by a normal retirement age? Reaching Coast FIRE doesn't mean you can stop working; it means you can stop prioritizing retirement savings specifically and redirect that money elsewhere (a home down payment, a career change, more current spending) while your existing balance compounds untouched.

Where CDs and Annuities Fit — and Where They Don't

Neither a CD nor an Annuity is a substitute for the tax-advantaged accounts above — they're generally used for money that needs to stay accessible and low-risk, not for long-run retirement growth. A CD locks a fixed rate for a fixed term (with an early-withdrawal penalty if you break it early) and is FDIC-insured up to $250,000, making it a reasonable home for cash you'll need within a few years. A fixed-period annuity converts a lump sum into guaranteed periodic payments, which some retirees use specifically to create predictable income once they've already funded their 401(k)/IRA accounts — but the underlying growth rate is typically far lower than a diversified stock/bond portfolio over a multi-decade horizon, so using an annuity too early in a career trades away most of the growth that makes retirement accounts powerful in the first place.