When you set up your , you picked a mix: maybe 80% and 20% , or whatever balance of growth and stability fit your timeline. (If you never consciously picked one, Risk and Diversification is the place to start; this article assumes that groundwork.) The catch is that no portfolio holds its shape on its own. is the small annual chore that puts it back.

What drift is

Stocks usually grow faster than bonds. That's the whole reason you own them, and it's also why your mix won't stay where you set it. Say you start with $10,000 split 80/20. After a strong three-year run for stocks, the stock side may have grown enough to make the split 88/12. You didn't buy anything, sell anything, or decide anything. Your portfolio became more aggressive by itself.

That matters because the mix was the decision. An 88/12 portfolio falls harder in a crash than the 80/20 you signed up for. And drift runs furthest after long bull markets, which is exactly when a downturn does the most damage. Rebalancing means trimming what grew and adding to what lagged until you're back at your target, so the risk level stays the one you chose.

The calendar method

You don't need software or a spreadsheet. The simplest approach is a date on the calendar:

  1. 1Pick one day a year you'll remember: your birthday, New Year's week, the day after you file taxes.
  2. 2Log in and check what percentage of your portfolio each holding makes up. Most brokerages show this on the main screen.
  3. 3Compare those percentages to your targets. If nothing is off by more than a few points, close the tab. You're done for the year.
  4. 4If something is meaningfully off (five percentage points is a common threshold), move money until the mix matches your targets again.

The whole session runs about 15 minutes, and in many years it ends at step three.

Rebalance with new money before you sell

How you get back to target matters for taxes. In a regular , selling your winners creates a capital gain, and the IRS taxes gains you take. The gentler move is to rebalance with new money: point your next several contributions entirely at whatever is underweight (usually bonds, after a good stretch for stocks) until the mix corrects itself. Same destination, no tax bill.

Inside a or , none of that caution applies. Trades in retirement accounts aren't taxable events, so you can sell and buy freely and rebalance in one sitting. If you hold similar investments in both kinds of accounts, do your selling inside the retirement account and let new money do the work in the taxable one.

Or let a fund handle it

If this sounds like a chore you'll never do, you can buy your way out of it. A target-date fund rebalances internally and gradually shifts more conservative as your retirement year approaches. A robo-advisor rebalances your account automatically, often whenever it drifts past a set band. If your entire investing life is a in a 401(k), rebalancing is already being done for you, and you can skip the calendar reminder.

Tip
More is not better here. Checking monthly invites tinkering, and tinkering is how long-term plans die. A yearly rebalance captures nearly all the benefit; the discipline matters more than the frequency.

Put a repeating date in your phone now. Those 15 minutes a year are what keep the risk level you chose from quietly becoming one you didn't.