The WSJ article What to Do With a 401(k) When Leaving a Job did a good job summarizing the various things to consider when that time comes. Here is a chart showing what happens to the 401k and 403b plans held across Vanguard-sponsored employer plans:
Now, Vanguard-sponsored 401k plans are mostly from big employers and are highly likely to have both low-cost investment options and reasonable fees. I don’t know if these percentages apply to all 401k and 403b plans in general. I’ve seen some pretty bad ones with absurdly high fees, although that was several years ago. Here’s a summary of the right questions to ask:
Investment selection? Are the available investments better in your 401k or in your choice of IRA custodian? Some 401ks offer choices not available to retail investors. For example, access to mutual funds from Dimensional Fund Advisors (DFA). Or institutional shares of certain funds with low expense ratios. Or stable value funds with higher interest rates than retail bond funds. On the other side, you may be itching to buy something like real estate in a self-directed IRA. These days, both probably have some sort of all-in-one Target retirement fund, but is yours a low-cost option from Vanguard (Target Retirement) or Fidelity (Freedom Index)?
Account fees? Part of the total cost picture is the expense ratio of the funds inside, but you may also be subject to overall account management fees or maintenance fees. Some 401k plans have zero or minimal management fees. Other 401k plans have crazy-high fees on the order of 1.75% of assets annually. Nearly all of the major IRA providers now offer no annual fees and commission-free ETF trades.
Need Advice? Some 401k plans offer some level of included advice that you may value. Other people have their own personal advisor that would love to customize that IRA.
Age 55 Rule (Early withdrawal?). Starting in the year that you turn 55, you can make a withdrawal from your 401k without the 10% early withdrawal penalty. You will still owe income taxes for a pre-tax 401k withdrawal, just not 10% penalty. For IRAs, you would have to wait until age 59.5. Potential early retirees may value these 4.5 years of additional flexibility.
Asset protection needs? There is a lot of legal fine print here, but 401k plans in general appear to offer some of the highest levels of asset protection. IRAs do offer a certain level of asset protection as well, just not quite as high as a 401(k). The difference will probably not matter much for the vast majority of workers, but it may matter for high-income professionals with liability concerns like doctors.
Non-deductible / Backdoor IRA contributions? The article didn’t cover why I didn’t roll over my last 401k plan into an IRA when leaving the employer. My situation is admittedly somewhat uncommon, but not unheard of. Call it “finding-yeast-in-April-2020” uncommon.
Due to my higher income level at the time, I contributed to a non-deductible IRA each year and then converted that to a Roth IRA. (This is also known as a Backdoor Roth IRA.) However, when you do the conversion, if you have any other pre-tax “traditional” IRAs, your conversion must include the pre-tax IRA amount as well on a pro-rated basis. For example, if you had a $20,000 pre-tax deducted IRA and a $5,000 non-deductible IRA, and then decided to convert $5,000 into a Roth IRA it would be considered 80% from the pre-tax IRA and only 20% non-deductible. You’d have to pay taxes on the entire pre-tax IRA amount (contribution + gains) since you never paid taxes initially. By keeping my pre-tax 401k at the employer, I am able to take full tax advantage of all annual Backdoor Roth contributions. Thankfully, the old 401k was at Fidelity with solid investment options and no annual fees.
How much do you value simplicity? If you do a lot of job-hopping, then do you really want to juggle five old 401k accounts? Merging them all into one IRA can save you time and hassle. On top of keeping tabs on your investments and asset allocation, you’d have to do all the mundane things like keep all your contact info and beneficiaries up to date. I would worry also that my spouse would forget about an old 401k plan if I something happened to me.
It might be a little painful, but it may be worthwhile to check on your pre-tax IRAs during this dip. If you have been thinking of converting your “Traditional” IRAs over to Roth IRAs, your shrunken gains will lead to a smaller tax bill now, while your (hopefully) future gains from this point onward will be tax-free after 5 years and age 59.5. 
Updated March 20th. The US Treasury has announced taxpayer relief for federal tax in response to COVID-19. Here is the 
I use a Solo 401k plan because it lets you contribute the most tax-deferred money for a modest amount of self-employed income. At the end of each year, I can more clearly estimate my total income for 2019 and thus my maximum contribution limits. There are several online calculators out there (try 
If you own stocks in a taxable brokerage account and make charitable donations, consider donating your stocks this year instead of just writing a check. Why? Given the all-time market highs, your stocks, mutual funds, and/or ETFs probably have unrealized capital gains. When you donate an appreciated security that you’ve held for at least a year, you’ll both avoid paying long-term capital gains tax AND get a tax deduction for the full current market value.
All this talk about portfolio rebalancing is to improve your risk-adjusted return, not to maximize absolute returns. You are trying to squeeze the most return out of a given degree of risk. Otherwise, if you do nothing eventually whatever has a higher return (historically always stocks) will outperform and take over the portfolio. 
T. Rowe Price has an article 
One of the newer work perks that we took advantage of this year was the Dependent Care Flexible Spending Account (DCFSA). This is separate from the Health Care Flexible Spending Account (HCFSA) and the Health Savings Account (HSA). However, they do work in a similar way in that you can pay for eligible expenses with pre-tax money and thus save money by being exempt from income taxes on that amount. For example, we were able to pay for $5,000 in preschool expenses using your DCFSA in 2018. At a a 30% marginal total tax rate (see below), that was a $1,500 savings. 
The Tax Cuts and Jobs Act (TCJA) changed up the personal income tax brackets, exemptions, and deductions. Here is an updated graphical breakdown of a simple scenario for a married filing joint couple with no children in Tax Year 2018. I’ll also try to illustrate the relationship between gross income, taxable income, marginal tax rate, and effective tax rates. See also:
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