Portfolio Rebalancing: When, How, and Why (2026)

Rebalancing means selling whatever has grown past its target weight and buying whatever has fallen below it, returning the portfolio to the allocation you chose. Its purpose is risk control, not extra return: a 60/40 portfolio left alone for a decade of strong equity markets can drift to 80/20, which means you are carrying far more risk than you signed up for without ever deciding to.

What Drift Actually Does to Your Risk

Drift is not symmetrical. Equities outgrow bonds in most decades, so an un-rebalanced portfolio ratchets toward higher risk and then discovers it during the next bear market. An investor who chose 60/40 for a 25% worst-case drawdown finds themselves in an 80/20 portfolio facing a 35% one.

The offsetting cost is real. Rebalancing sells the asset that has been outperforming, which drags on return in a long one-directional market. Over full cycles the trade is roughly neutral on return and clearly positive on risk, which is why it is a risk-management tool rather than a return strategy.

Rebalancing With Contributions Instead of Sales

While you are still contributing, most drift can be corrected without selling anything. Direct the next several contributions entirely into the underweight sleeve. This costs nothing in tax, nothing in spread, and it works because contributions are large relative to drift in the early years of a portfolio.

1. Calculate current weights, then the dollar gap between each sleeve and its target.
2. Add up your next three to six months of contributions. If they cover the largest gap, no sale is needed.
3. Direct the whole contribution to the most underweight sleeve until it is back on target, then move to the next.
4. If the gap exceeds a year of contributions, sell inside tax-deferred accounts first — no tax consequence there.
5. Sell in a taxable account only as a last resort, and prefer lots with the highest cost basis to minimise the realized gain.
6. Record the date and the trigger you used, so next year the decision is mechanical rather than emotional.

                Portfolio                = $240,000
Target                   = 70% equity / 30% bonds
Current                  = 78% equity / 22% bonds

Equity value             = 0.78 x $240,000 = $187,200
Target equity            = 0.70 x $240,000 = $168,000
Bond value               = $52,800
Target bonds             = $72,000
Bond shortfall           = $19,200

Monthly contribution     = $1,600
Months of all-bond contributions to close the gap
  = $19,200 / $1,600 = 12 months

Tax cost of this method  = $0
Tax cost of selling $19,200 of equity up 50%
  = $6,400 gain x 15% + 5% state = $1,280
              

The twelve-month version reaches target more slowly, and during those months the portfolio carries slightly more equity risk than intended. That is the trade: about 1,280 dollars of tax against a year of modest overexposure. For drift under 10 points the contribution route almost always wins.

The Tax Rules That Change the Answer

The IRS treats a rebalancing sale in a taxable account as a realized capital gain, taxed at long-term rates if the shares were held more than one year and at ordinary income rates if not. That one-year line is worth checking before any sale — the rate difference on a 10,000 dollar gain can exceed 1,700 dollars.

Inside a 401(k), 403(b), traditional IRA, or Roth IRA, no rebalancing trade creates a tax event at all. This is why the standard advice is to hold the sleeves you expect to trade most often inside those wrappers, and to keep the taxable account filled with broad, low-turnover equity index funds you rarely need to touch.

Enter your current and target weights in the portfolio rebalancing strategy tool to see both the trades required and the estimated tax cost, then confirm the target itself still fits with the portfolio allocation calculator. Rebalancing to the wrong target is worse than not rebalancing.