Key Takeaways
- A portfolio that drifts 10 percentage points from its target allocation can increase annualized volatility by 2 to 4 percentage points, depending on asset class mix.
- Ignoring drift in a $500,000 portfolio during a 30% equity correction costs roughly $15,000 more in losses than a properly rebalanced portfolio at a 60/40 target.
- Calculate each position's current weight as (Position Market Value / Total Portfolio Value), compare it to your target weight, then buy or sell the dollar difference to realign.
- Tool: Run your portfolio allocation math in the CalcMoney Investment Calculator →
Invest Your Spare Change Automatically
Acorns rounds up your purchases and invests the difference into a diversified portfolio automatically.
Percentage Allocation Is the Only Number That Measures Risk Exposure
The dollar value of a position tells you what it is worth. The percentage weight tells you how much risk it contributes. A $40,000 position in a tech ETF inside a $100,000 portfolio carries 40% weight. The same $40,000 position inside a $400,000 portfolio carries 10% weight. Risk exposure is not the same number.
The formula for any single position's weight is:
Weight (%) = (Position Market Value / Total Portfolio Market Value) x 100
Run this calculation for every holding. The results should sum to exactly 100%. If they do not, you have missed a cash position, a pending dividend, or an accrued fee. Find the discrepancy before proceeding.
How to Calculate Portfolio Allocation: A Worked Example
A $250,000 portfolio holds four positions. Here is the full allocation table:
| Position | Market Value | Current Weight | Target Weight | Drift |
|---|---|---|---|---|
| U.S. Large Cap ETF (e.g., S&P 500 index fund) | $112,500 | 45.0% | 40.0% | +5.0% |
| International Equity ETF | $37,500 | 15.0% | 20.0% | -5.0% |
| U.S. Aggregate Bond ETF | $75,000 | 30.0% | 30.0% | 0.0% |
| Cash / Money Market | $25,000 | 10.0% | 10.0% | 0.0% |
| Total | $250,000 | 100.0% | 100.0% |
The U.S. Large Cap ETF has grown 5 percentage points above target. The International Equity ETF has fallen 5 percentage points below target. Everything else is on plan.
To restore the 40/20/30/10 target:
- Sell (5.0% x $250,000) = $12,500 of the U.S. Large Cap ETF.
- Buy $12,500 of the International Equity ETF.
- Make no changes to the bond fund or cash.
Two trades. The portfolio is back on target.
Setting a Rebalancing Threshold That Prevents Excessive Trading
Rebalancing every week destroys returns through transaction costs and, in taxable accounts, short-term capital gains taxes taxed at ordinary income rates up to 37%. The right answer is a threshold-based trigger, not a calendar trigger.
The standard practice among institutional asset managers is a 5-percentage-point absolute drift band. When any position moves more than 5 points from its target, rebalance that position only. Leave positions inside the band alone.
Some advisors use a proportional band instead: rebalance when a position drifts more than 25% of its target weight. A position with a 20% target would trigger at 25% above or below, meaning at 25% or 15% actual weight.
Either method is defensible. The 5-point absolute band is simpler to monitor and works well for portfolios under $2,000,000.
Tax-Efficient Rebalancing: Where to Make the Trades
In a taxable brokerage account, selling an appreciated position to rebalance creates a capital gains tax event. A $12,500 sale of a position held more than 12 months triggers long-term capital gains tax at 0%, 15%, or 20%, depending on taxable income. At the 15% federal rate, that $12,500 sale costs $1,875 in federal tax before state taxes.
The more efficient sequence is:
- Rebalance first inside tax-advantaged accounts: a traditional IRA, Roth IRA, or 401(k). Trades inside these accounts generate no current-year tax liability.
- Direct new contributions to underweight positions in taxable accounts instead of selling overweight positions.
- Use dividend reinvestment from overweight positions into underweight positions where the brokerage platform allows.
Sell in taxable accounts only when the drift is large enough that the tax cost is smaller than the risk cost of leaving the portfolio misaligned.
A Second Worked Example: A Larger Portfolio With Six Positions
A $750,000 portfolio has drifted significantly after an 18-month equity bull market.
| Position | Market Value | Current Weight | Target Weight | Dollar Drift |
|---|---|---|---|---|
| U.S. Large Cap Growth ETF | $270,000 | 36.0% | 25.0% | +$82,500 |
| U.S. Small Cap ETF | $97,500 | 13.0% | 10.0% | +$22,500 |
| International Developed ETF | $75,000 | 10.0% | 15.0% | -$37,500 |
| Emerging Markets ETF | $37,500 | 5.0% | 10.0% | -$37,500 |
| U.S. Treasury Bond ETF | $195,000 | 26.0% | 30.0% | -$30,000 |
| Cash / Money Market | $75,000 | 10.0% | 10.0% | $0 |
| Total | $750,000 | 100.0% | 100.0% |
Dollar drift for each position = (Current Weight - Target Weight) x Total Portfolio Value.
The Large Cap Growth ETF is $82,500 overweight. The Emerging Markets ETF and International Developed ETF are each $37,500 underweight. The Treasury Bond ETF is $30,000 underweight.
Rebalancing trades, prioritized by tax location (IRA or 401(k) first):
- Sell $82,500 of U.S. Large Cap Growth ETF.
- Sell $22,500 of U.S. Small Cap ETF.
- Buy $37,500 of International Developed ETF.
- Buy $37,500 of Emerging Markets ETF.
- Buy $30,000 of U.S. Treasury Bond ETF.
Total trades: five. Total dollars moved: $105,000 out, $105,000 in. Portfolio restored to target.
How Often to Check Allocation Weights
Check portfolio weights once per month. This takes under five minutes with a spreadsheet or a calculator. Do not rebalance on every check. Only trade when the 5-point threshold is breached.
Annual calendar rebalancing is the most common approach among retail investors, and research from Vanguard and T. Rowe Price both show it leaves portfolios out of target for an average of 6 to 8 months per year after fast-moving markets. Threshold-based monitoring closes that gap.
Calculate Your Exact Rebalancing Trades With the CalcMoney Investment Calculator
Doing this math by hand is correct. Doing it faster and with fewer errors is better. The CalcMoney Investment Calculator lets you enter your current position values and target weights, then outputs the exact dollar amount to buy or sell in each position. It handles portfolios with up to 20 positions and accounts for cash drag.
The calculation above is not complicated, but the friction of setting up a spreadsheet every month is why most investors skip the check entirely. Remove that friction and the rebalancing actually happens.
Open the CalcMoney Investment Calculator and enter your portfolio now →You Might Also Like
- How to Calculate a Stock's Beta and What It Actually Tells You About Risk
- How to Calculate Bond Allocation Percentage by Age in Retirement
- How to Calculate an Inflation-Protected Retirement Portfolio
Results are estimates for informational purposes only. Consult a licensed financial professional before making financial decisions.
Put These Numbers to Work
Open a Fidelity brokerage account. $0 commissions, no account minimums, fractional shares available.
Affiliated. We may earn a commission.
Related Guides
Free Tools
Run the actual numbers
Stop estimating. Plug in your numbers and get a precise answer in seconds. Free, no signup required.
Open Free Calculators