Open your brokerage app right now and look at your asset allocation. If you set up a 70/30 split between stocks and bonds two years ago and haven't touched it since, there's a good chance it's closer to 78/22 today — not because you did anything wrong, but because stocks quietly outran bonds while you were busy living your life. That drift is invisible until you actually check, and by the time most people notice, they're carrying more risk than they signed up for right before a market pullback finds them.
Why fall is the natural checkpoint
Q4 carries several deadlines that don't care whether you've looked at your portfolio lately. Tax-loss harvesting — selling a losing position to offset gains elsewhere — only counts if the trade settles by December 31. Required minimum distributions for anyone over 73 are due by the same date, and missing one triggers a 25% IRS penalty on the amount you should have withdrawn. And if your employer's open enrollment window falls in November, your HSA and 401(k) contribution elections for next year get locked in before you've even finished this year's returns. Flexible spending accounts add a fourth pressure point for anyone who has one, since most FSAs are use-it-or-lose-it by year-end and unspent balances simply disappear on January 1. September through October is the window where you can still act on all four without rushing, and it's also before the holiday season eats every spare hour you'd otherwise spend on it.
None of this requires becoming a market-watcher. A fall check-in is closer to rotating your tires than day-trading — a scheduled, boring task that prevents an expensive surprise. Set a recurring date, do the fifteen-minute version below, and you're done until spring.
How drift actually happens
Say you started 2024 with $40,000 in a total stock market fund like VTI and $17,000 in a bond fund like BND — a clean 70/30 split. VTI has returned roughly 24% cumulatively since then while BND has barely moved. Do the math and your stock sleeve is now worth around $49,600 against $17,300 in bonds, which puts you at 74/26 without a single trade. Nobody chose that allocation. The market chose it for you, one green day at a time.
This is where the two most common reactions both go wrong. The anxious reaction is to check daily and second-guess every dip, which burns energy without improving outcomes — Vanguard's own research on investor behavior during volatile years found that people who checked balances most frequently also made the most costly, panic-driven trades. The opposite reaction is to ignore it entirely on the theory that "time in the market beats timing the market," which is true for staying invested but says nothing about whether your risk level still matches your actual life. A 74/26 portfolio isn't wrong. It's just not the one you decided on, and if a recession hits while you're sitting on 74% equities instead of the 70% you meant to hold, that extra 4% is real money exposed to a drop you didn't choose. Four percentage points sounds small until you attach a dollar figure to it — on a $250,000 portfolio, that's $10,000 sitting in the stock market that your original plan put in bonds instead.
The fifteen-minute check
Pull up your accounts — brokerage, 401(k), Roth IRA, HSA if it's invested — and total them by category rather than by account. A target-date fund inside your 401(k) already blends stocks and bonds internally, so you'll need to look at its underlying allocation, usually listed in the fund's fact sheet, rather than counting the whole balance as one bucket.
- Add up total equity value across every account.
- Add up total bond and cash-equivalent value.
- Calculate the percentage split and compare it to your target.
- If any category is off by more than 5 percentage points, rebalance. If it's off by 2 or 3, leave it — trading costs and, in taxable accounts, capital gains taxes usually outweigh a small correction, and you'll likely be back in range within a quarter or two anyway.
The 5% band is the detail most people skip, and it's the one that actually matters. Rebalancing every time your portfolio moves at all turns a set-and-forget strategy into a part-time job, and in a taxable brokerage account it also means realizing gains you didn't need to realize yet. Use the band as your rule, not your mood.
Where to rebalance without triggering a tax bill
Here's the part that trips people up: rebalancing inside a taxable brokerage account by selling winners to buy losers creates a taxable event, even if you're just restoring your original 70/30 split. Selling $5,000 of an appreciated stock fund to buy bonds means paying capital gains tax on the profit — short-term rates if you've held it under a year, which can run as high as your ordinary income bracket.
The workaround is to rebalance with new money first. If you're still contributing to a 401(k), IRA, or brokerage account, direct fresh contributions toward whichever category has fallen behind — in the example above, that's bonds — instead of selling anything. This gets you back toward target with zero tax consequence. Only sell existing positions to rebalance inside tax-advantaged accounts like a 401(k) or IRA, where trades don't trigger a taxable event at all, and save taxable-account selling for situations where you're also harvesting a loss to offset the gain.
Two moves worth making before December, one worth skipping
Check whether any position in your taxable account is sitting at a loss and consider harvesting it — selling to lock in the loss for tax purposes, then buying a similar (not identical, to avoid the IRS wash-sale rule) fund to stay invested. This is one of the few tax strategies that costs you nothing but fifteen minutes and actually reduces what you owe.
Second, if you've had a raise, a bonus, or a paid-off debt this year, revisit your contribution percentages before open enrollment locks them in for 2027. Bumping a 401(k) contribution from 8% to 10% now, while it's fresh in your mind, beats discovering in March that you meant to and never got around to it.
What's not worth doing: trying to time a rebalance around a Fed announcement, an election, or a headline about an impending correction. Predicting short-term market direction is a losing game even for professional fund managers — the SPIVA scorecard has shown for over a decade that most actively managed funds fail to beat a simple index over 10-year periods, and that's with teams of analysts doing it full-time. A fixed rebalancing rule beats a market opinion because it doesn't require being right about anything except your own target allocation.
What to leave alone
Your emergency fund and any cash you'll need within the next twelve months don't belong in this exercise at all — they're not part of your investment allocation, they're liquidity, and moving them into anything with market risk defeats their purpose. Same goes for a house down payment fund earmarked for next year. The fifteen-minute check is for money that's actually invested for the long term, not for cash that happens to be sitting in a high-yield savings account.
Set a recurring reminder for the last week of September and the last week of March — twice a year is enough for almost every investor, and it's often enough for someone managing a portfolio well into seven figures. Anything more frequent starts working against you, because the temptation to react to short-term noise grows every time you look.