What you'll learn
- How to measure allocation drift with a simple formula (not guesswork)
- Three drift types: asset class, position-level, and hidden ETF overlap
- The 5/25 rule and when a 60/40 portfolio actually needs rebalancing
- Why multi-account and held-away 401(k) drift stays invisible in broker dashboards
- Manual vs automated monitoring: what breaks and what scales
What Is Portfolio Drift?
Portfolio drift (allocation drift) is the gap between your current asset weights and your target allocation. It happens whenever holdings perform differently, you add new contributions, or dividends reinvest into winners. No trade is required. A 60/40 portfolio can become 68/32 after a strong equity year without you touching anything.
The cost is not the rebalancing trade itself. It is months of unintended risk: more equity exposure than you planned, higher drawdown in the next correction, and larger tax bills if you rebalance late in a taxable account.
Three ways drift shows up
Asset class drift: stocks vs bonds vs cash move away from your 60/40 (or 70/30) target.
Position drift: one holding grows from 5% to 15% through price appreciation alone. That is also concentration risk.
Hidden ETF drift: fund names look diversified but underlying holdings overlap. Use an ETF overlap check to see true single-stock weights.
How to Calculate Portfolio Drift
For each asset class or holding i, compare current weight to target weight:
| wcurrent,i | Current weight = market value of asset i / total portfolio value |
| wtarget,i | Target weight from your investment policy (e.g. 60% stocks) |
| Di | Drift for asset i in percentage points (pp) |
To summarize drift across the whole portfolio:
$1M 60/40 Portfolio After One Strong Equity Year
Assume you started at target and held for 12 months with no rebalancing. Stocks returned +24%, bonds +3%.
Target 60/40 drifts to 64.4/35.6
Target: 60% US stocks, 40% US bonds
Starting balance: $1,000,000 ($600,000 stocks, $400,000 bonds)
Stocks +24%: $600,000 → $744,000
Bonds +3%: $400,000 → $412,000
Ending balance: $1,156,000
Stock weight: 744,000 / 1,156,000 = 64.4% (target 60%, drift = +4.4 pp)
Bond weight: 412,000 / 1,156,000 = 35.6% (target 40%, drift = -4.4 pp)
Dtotal = (|+4.4| + |-4.4|) / 2 = 4.4 percentage points
Below the 5 pp band. No rebalance required yet.Run the same math after year two and drift typically crosses 6-8 pp. That is when most investors under the 5/25 rule would rebalance.
60/40 target vs reality after one equity rally
The 5/25 Drift Threshold
Rebalance when: any asset class drifts more than 5 absolute percentage points or more than 25% of its target weight, whichever is smaller.
For a 60% stock target, act when stocks cross 55% or 65%. For a 10% international target, act at 7.5% or 12.5% because 25% of 10% is only 2.5 points. Full breakdown in the rebalancing guide.
| Target weight | 5 pp band | 25% of target band | Effective trigger |
|---|---|---|---|
| 60% stocks | 55% - 65% | ±15 pp | 55% or 65% (5 pp wins) |
| 40% bonds | 35% - 45% | ±10 pp | 35% or 45% (5 pp wins) |
| 10% international | 5% - 15% | 7.5% - 12.5% | 7.5% or 12.5% (25% wins) |
| 5% REITs | 0% - 10% | 3.75% - 6.25% | 3.75% or 6.25% (25% wins) |
The 3-Step Process to Catch Drift
Aggregate holdings across all accounts
Do not check Fidelity, Vanguard, and Schwab separately. Roll up taxable brokerage, IRA, Roth, HSA, and held-away 401(k) into one household total. A stock at 8% in one account and 7% in another is 15% total exposure, invisible in any single broker view.
Calculate current weights and per-asset drift
Group holdings by asset class (or map each fund to its underlying exposure). Compute wcurrent for each class, then Di = wcurrent − wtarget. For ETF-heavy portfolios, run overlap analysis first so your stock/bond split reflects what you actually own.
Compare to your threshold and rebalance or alert
If any class crosses the 5/25 band, rebalance or set automated drift alerts. In taxable accounts, batch trades when drift is large enough to justify the tax cost. In IRAs and 401(k)s, rebalancing has no immediate tax impact, but drift still changes your risk profile.
What Causes Drift (Beyond Market Moves)
- Uneven returns: equities outperform bonds in most years, pushing stock weight higher automatically
- Payroll contributions: 401(k) deposits often follow a fixed election, which can amplify drift instead of correcting it
- Employer match: match paid as company stock concentrates risk in a single position
- Dividend reinvestment: DRIP buys more of whatever paid the dividend, usually your winners
- Partial rebalancing: selling one overweight position but not the rest leaves residual drift
- ETF overlap: adding a "diversifying" fund that holds the same mega-caps as your existing ETFs
Manual vs Automated Drift Monitoring
| Method | Update frequency | Multi-account view | Missed drift risk |
|---|---|---|---|
| Annual review | 1x/year | Rarely consolidated | Very high |
| Quarterly spreadsheet | 4x/year | Manual, error-prone | High |
| Single-broker dashboard | Daily (one account) | No | High for multi-account |
| Automated monitoring | Continuous | Yes, all accounts | Low (threshold alerts) |
The multi-account blind spot
Spreadsheets updated monthly give you one snapshot. Drift happens every trading day. Multi-account holders face a bigger gap: 8% in Fidelity + 7% in IRA + 6% in an old 401(k) = 21% total exposure to one stock, but each account looks fine on its own. Detect concentration and drift at the household level, not per broker login.
Automated Drift Monitoring
Portfolio monitoring tools close the gaps manual tracking leaves open:
- Connect all accounts via read-only API (including many held-away 401(k) plans)
- Refresh holdings on a regular cadence, not when you remember to open a spreadsheet
- Calculate drift and concentration across the consolidated household portfolio
- Send email or Telegram alerts the moment drift crosses your 5/25 band
- Surface ETF overlap so fund-level weights match economic exposure
The goal is not to rebalance constantly. It is to know exactly when drift has crossed a meaningful threshold so you can act deliberately, not discover it six months late in an annual review.