Rebalancing drift calculator
Model how a simple 2-asset portfolio drifts over time when one side outperforms. Compare no rebalancing, annual, and drift bands side-by-side — with a live chart and cost estimate.
What this calculator tells you
Rebalancing is a risk control decision, not a return optimisation one. This tool helps you choose a rule you can actually stick to.
- None — let it drift. Simple, but risk creeps up over time.
- Annual — check and reset once a year. Predictable, low attention.
- Bands — only act when drift exceeds your threshold. Fewer trades, same control.
- Max drift — how far from target you can get (risk creep).
- Rebalance count — how hands-on the rule really is.
- Cost estimate — at small portfolio sizes, transaction fees matter more than “perfect” weights.
Model your drift & rebalance frequency
Results update live. The chart shows Asset A allocation over time for all three methods simultaneously — so you can see the difference at a glance.
Uses constant returns (not a market simulation). New contributions are invested at target weights, which reduces drift vs doing nothing. Band threshold applies to all three methods — only the “bands” scenario uses it actively.
| Method | Final value | Final allocation (A/B) | Max drift | Rebalances | Est. cost |
|---|---|---|---|---|---|
| No rebalancing | — | — | — | 0 | €0 |
| Annual | — | — | — | — | — |
| Bands (±5 pp) | — | — | — | — | — |
Transaction cost applies to traded value at each rebalancing event. Final values differ because each method deducts different cumulative costs.
Each line shows how far Asset A drifts from the target under each method. Rebalancing events snap the allocation back.
How to read the results
How far Asset A's weight got from your target at any point. Under "no rebalancing" with different return assumptions this grows continuously. With annual or bands, it resets. Drift is not inherently bad — it's only a problem when it pushes your risk beyond what you're comfortable with.
Bands usually produce fewer rebalances than annual over the same period — especially when returns are similar across assets. Fewer rebalances means lower total cost and less decision fatigue. The right answer is the one you'll actually execute consistently.
Transaction costs compound. At 0.1% per trade, the difference between annual and bands can be meaningful over 15+ years — especially when bands delay rebalancing by 1–2 years at a time. Try changing the cost % to see how sensitive your results are.
This uses constant returns — no volatility, no sequence risk, no taxes. Real markets are bumpier. The chart is useful for understanding structure (how each rule behaves), not for precise forecasting. Use rough estimates, not precise projections.
Rebalancing without selling
The cheapest rebalancing move is no sale at all. Three approaches that correct drift without triggering a transaction cost or a tax event.
When you make a regular monthly contribution, direct it entirely toward the underweight asset. If your 80/20 portfolio has drifted to 85/15, the next contribution goes entirely to bonds. Over time this closes the gap with zero friction — no sale, no tax event, no spread paid on the outgoing leg.
Check on a schedule, act only if drift is meaningful. Set a calendar reminder (quarterly, semi-annually) and review your allocation — but only execute a trade if drift has exceeded your band. This prevents unnecessary trades on a fixed date while ensuring you don't go years without looking. Annual + bands, combined.
A more precise framework for setting bands: act when an asset drifts by 5 pp in absolute terms OR 25% of its target weight in relative terms — whichever is smaller. For a 40% equity position the relative trigger is 10 pp, so the 5 pp limit fires first. For a 10% bond position the relative trigger is 2.5 pp — smaller, so you'd act sooner. It scales bands to position size automatically.
If your broker pays dividends or interest in cash, turn off automatic reinvestment and direct the proceeds manually toward whichever asset is underweight. Small amounts add up. It's not a substitute for formal rebalancing in a large drifted portfolio, but it reduces the drift that builds between rebalancing events at no extra cost.
When rebalancing may not make sense
Most rebalancing content tells you to do it. Here's when you probably shouldn't — or when the cost outweighs the benefit.
A ±2 pp deviation on a 2-fund portfolio is not a meaningful risk shift. The expected benefit of correcting it is smaller than the certain friction of trading. Let it run.
Paying a €5 flat commission on a €2,000 rebalancing trade is 0.25% in friction — before any spread. At small portfolio sizes, contribution routing costs less and achieves the same effect over a few months.
If you've been directing new money toward the underweight asset for 2–3 months, give it time before pulling the trigger on a sell. Check whether drift is still widening or already narrowing.
In a taxable account, selling an appreciated position triggers capital gains tax. If your equity position is sitting on a large unrealised gain, wider bands or contribution-only correction can be cheaper than a formal rebalance — even if the allocation stays off-target longer.
If you're moving from 80/20 toward 70/30 as you approach retirement, drift in the right direction isn't a problem — it's progress. Rebalancing back to 80/20 would be working against your intended glide path.
| Your situation | Suggested band | Why |
|---|---|---|
| Small portfolio + fixed-fee broker | ±8–10 pp | Friction is proportionally larger — trade less often |
| Large portfolio + commission-free broker | ±3–5 pp | Low cost to act — tighter control is affordable |
| Taxable account (capital gains apply) | ±7–10 pp | Tax friction raises the effective cost of each rebalance |
| Tax-sheltered account (ISA, pension, PEA) | ±3–5 pp | No tax friction — tighter bands cost less to maintain |
| Highly volatile assets | ±7–10 pp | Volatility triggers bands faster — wider bands reduce over-trading |
| Near retirement / drawdown phase | ±3–5 pp | Protecting allocation matters more than minimising trades |
Want to explore more tools?
See how fees compound, how to model broker costs, or how cash drag affects long-term returns.
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Frequently asked questions
What is portfolio drift?
Drift is when your actual allocation moves away from your target because assets grow at different rates. A portfolio that starts at 80/20 equities/bonds will naturally tilt toward equities over time if equities outperform. The question is how much drift you're comfortable tolerating before resetting.
Are drift bands better than annual rebalancing?
Bands reduce unnecessary rebalancing by only triggering when drift is meaningful. Annual is simpler and more predictable — you check once a year regardless of what happened. For most passive investors a ±5 percentage point band or annual check is more than sufficient. The performance difference between the two is typically small; the execution difference is larger.
Do monthly contributions reduce drift?
Yes. Directing new contributions toward your underweight asset is a cost-free way to reduce drift — you rebalance through buying rather than selling. This is especially effective when your portfolio is smaller relative to your contributions, and it avoids triggering any transaction costs or tax events.
How wide should my rebalancing bands be?
For a simple 2–3 fund portfolio, ±5 percentage points is a common starting point. Wider bands mean fewer trades and lower costs; tighter bands mean closer tracking but more friction. The right width depends on your broker's transaction costs, the tax implications of selling in your country, and your personal tolerance for deviation from target. There's no universal correct answer.
Does rebalancing improve returns?
Not reliably. Rebalancing is primarily a risk management tool — it keeps your portfolio at the intended risk level rather than letting one asset class dominate. In trending markets (equities persistently outperforming bonds) it can reduce returns relative to doing nothing. The real value is discipline: you stay within your risk tolerance without needing to make active judgement calls each time.
Should you rebalance during a market crash?
In a crash, your equity allocation will likely have fallen below target — meaning your portfolio is now less risky than intended, not more. Rebalancing into equities during a crash is mathematically consistent with a rule-based approach and has historically added value. The challenge is execution: it feels like buying into a falling market. This is exactly why having a pre-set rule matters — you follow the rule, not the news.
What is the 5/25 rebalancing rule?
The 5/25 rule says: rebalance when an asset drifts by 5 percentage points in absolute terms OR 25% of its target weight in relative terms — whichever is smaller. For a 40% equity allocation the relative trigger is 10 pp (40% × 25%), so the 5 pp absolute limit applies first. For a 10% bond allocation the relative trigger is 2.5 pp — so you'd act before hitting the 5 pp absolute limit. It's a way to make bands proportional to position size, which matters most when you hold smaller satellite allocations alongside a core portfolio.
What is the hybrid rebalancing approach?
The hybrid approach combines a calendar schedule with a threshold trigger: check your portfolio on a fixed date (quarterly or semi-annually), but only execute a rebalance if drift has exceeded your band. If the allocation is still within range, do nothing and schedule the next check. This prevents unnecessary trades from a rigid calendar rule while ensuring you review the portfolio regularly — rather than relying on market alerts or memory to flag drift.
Can rebalancing reduce portfolio volatility?
Rebalancing doesn't directly reduce return volatility, but it prevents your portfolio from silently becoming riskier than intended. An 80/20 portfolio that drifts to 90/10 has a different risk profile — it will fall harder in an equity crash. By resetting the allocation you keep risk within the range you originally accepted. That's not volatility reduction in general; it's risk containment within your chosen band.
QuantRoutine provides educational content only. This calculator uses a simplified deterministic model with constant returns — it does not simulate market volatility, sequence risk, taxes, tracking difference, or real-world execution effects. Results are illustrative only and should not be used as a basis for investment decisions. Always review your broker's current fee schedule and the tax rules that apply in your country before making rebalancing decisions.