Asset allocation rebalancing.

Why rebalancing is the rare free lunch in investing

Portfolio rebalancing — periodically adjusting your allocation back to target percentages — does something mathematically unusual: it systematically sells high and buys low. When equities outperform bonds, your equity allocation drifts above target; rebalancing sells some equities (at their elevated prices) and buys bonds (at relatively lower prices). The reverse happens in downturns. Done consistently, rebalancing improves risk-adjusted returns by capturing the rebalancing bonus — typically 0.3-0.6% annually over long periods. This calculator shows you what rebalancing a specific portfolio would look like; the commentary below is about how often and how precisely.

The rebalancing bonus math

Consider two assets with perfectly anticorrelated returns, each averaging 5% annually. If one doubles in year 1 and halves in year 2, and vice versa for the other, both end exactly where they started after 2 years. A 50/50 unrebalanced portfolio: ends at the same place too. A 50/50 portfolio rebalanced at end of year 1: captures half of the big winner's gain through rebalancing, then doesn't suffer the big loser's full loss. Net: rebalanced portfolio outperforms unrebalanced despite identical underlying returns. This is the rebalancing bonus in idealised form. In reality, assets aren't perfectly anticorrelated and returns aren't perfectly periodic, so the bonus is smaller but real — historically around 0.3-0.6% annually for well-constructed diversified portfolios.

The three rebalancing triggers

Three common approaches to deciding when to rebalance:

Calendar-based: Rebalance on a fixed schedule — monthly, quarterly, semi-annually, or annually. Simple, consistent, easily automated. Doesn't respond to market conditions.

Threshold-based: Rebalance when any asset drifts 5% or more from target (sometimes stated as 20% relative drift — e.g., 60% target moving to 72% or 48%). Responds to market movement; less frequent in calm periods, more frequent in volatile ones.

Combined: Rebalance if either the calendar trigger arrives OR a threshold is breached — whichever comes first. Captures benefits of both.

Research suggests threshold-based rebalancing at 20% relative drift captures most of the rebalancing bonus with minimal transaction costs. Annual calendar-based rebalancing is nearly as effective and simpler to execute. Monthly rebalancing adds transaction costs without proportional benefits.

The tax-drag consideration for investors

Rebalancing inside tax-advantaged wrappers (tax-advantaged savings account, tax-advantaged pension account, pension) has no tax consequences — buy and sell freely. Rebalancing in general investment accounts triggers capital gains tax on realised gains. This can meaningfully reduce the rebalancing bonus or even eliminate it. For investors, the optimal approach often combines:

Rebalance inside tax-advantaged savings accounts and pensions freely.
Rebalance general investment accounts using tax-smart techniques (allocate new contributions to the underweight asset to avoid selling, harvest losses when available).
Coordinate rebalancing across accounts — the total portfolio can be rebalanced without triggering tax in any single account if you're strategic.

Ignoring tax implications and mechanically rebalancing general accounts every year often eliminates the theoretical rebalancing bonus entirely through tax drag.

The new-money rebalancing approach

For investors in accumulation phase (contributing monthly or annually), the cheapest rebalancing method is: direct new contributions to the underweight asset class. No selling, no tax consequences, no transaction costs. Target 60/40 stocks/bonds; stocks have grown to 65% of portfolio; direct 100% of new contributions to bonds until the percentages rebalance naturally. This technique works for 5-10 years of typical accumulation before portfolio size outgrows new contribution amounts. For most people under 45, new-money rebalancing is the primary rebalancing mechanism.

Transaction costs that erode the bonus

Each rebalancing trade costs:

Dealing fees: 0-12 per trade depending on platform.
Bid-ask spread: 0.1-0.5% for liquid ETFs and funds; higher for less liquid investments.
Stamp duty: 0.5% on individual shares (but not ETFs or funds).
Tax drag (as discussed above).

For small portfolios (under 100,000), frequent rebalancing can cost more than it gains. For large portfolios, costs are relatively smaller and more frequent rebalancing makes sense. The portfolio size break-even for quarterly rebalancing is roughly 50,000-100,000; below that, annual rebalancing is typically sufficient.

The behavioural value beyond the math

Rebalancing requires selling winners and buying losers — the opposite of intuitive behaviour. Most investors want to sell losers and buy winners. Disciplined rebalancing protects against this behavioural mistake by enforcing a mechanical rule. Over multiple market cycles, this discipline is worth more than the mathematical rebalancing bonus. Investors who rebalance consistently tend to stay invested through downturns; those who don't often ride winners into bubbles and then panic-sell at bottoms. The psychological benefit of having a rule is hard to quantify but real.

Rebalancing during bear markets

The highest-value rebalancing often happens during major market declines, when the rebalancing trigger says "buy more of the asset that's falling". This is psychologically difficult — buying into a falling market feels wrong. It's also mathematically where the rebalancing bonus is largest. Investors who abandon rebalancing during bear markets lose most of the long-term bonus; those who maintain it through fear-driven periods capture disproportionate value. This is arguably the most important habit in long-term portfolio management: mechanical rebalancing through emotion-driven market periods.

The age-based glide path variant

Target-date funds implement a specific rebalancing approach: gradually shift allocation from equity-heavy (early in career) toward bond-heavy (near retirement). The target allocation changes over time. Rebalancing enforces both the drift-back-to-target correction AND the progression toward the age-appropriate allocation. For hands-off investors, target-date funds automate both aspects. For self-managed investors, the concept is useful: your rebalancing target should shift over time, not stay at 60/40 forever.

How much rebalancing is too much

Research consistently shows diminishing returns from frequent rebalancing:

Annual rebalancing: captures most of the bonus.
Quarterly rebalancing: marginal improvement, noticeable transaction costs.
Monthly rebalancing: often worse than annual due to costs and noise.
Daily rebalancing: destroys the bonus entirely through costs.

Unless transaction costs are near zero (institutional investors, or very large portfolios with sophisticated access), annual rebalancing is the practical sweet spot. Combined with threshold-based triggers for large market moves, it captures nearly all the available bonus.

What this calculator shows

The tool calculates the buy/sell amounts needed to bring a specific portfolio back to target allocations. It doesn't automatically account for tax consequences, transaction costs, or the new-money alternative. Use the output as the mechanical rebalancing plan; apply the tax and cost considerations to decide whether and how to execute each rebalance.