Portfolio Rebalancing Calculator

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 may improve risk-adjusted returns by capturing a 'rebalancing bonus', though estimates of its size vary and it is often modest. 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

Take two assets moving in opposite directions: one doubles in year one and halves in year two, the other halves then doubles. Each ends exactly where it started, and so does an unrebalanced 50/50 split of the two, at 100 for every 100 put in. Rebalancing at the end of year one changes that. The portfolio is worth 125 at that point, so rebalancing puts 62.50 into each side; year two then halves one side to 31.25 and doubles the other to 125, ending at 156.25. That is the rebalancing bonus in idealised form, and its size here is an artefact of the extreme moves. Real assets do not move in exact opposition on an exact schedule, so the effect is far smaller, and estimates of how much smaller vary by study and period.

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 a tax-advantaged wrapper carries no tax on the trades themselves. Rebalancing in a general investment account can trigger tax on realised gains, which reduces the rebalancing bonus and in some cases removes it. Approaches that come up in practice combine several things:

Trades inside tax-advantaged accounts and pensions are typically free of immediate tax.
General investment accounts can be moved toward target by directing new contributions at the underweight side rather than selling the overweight one, and by realising losses where they exist.
Rebalancing considered across all accounts together can sometimes bring the whole portfolio back to target without a taxable trade in any one of them.

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 often the main 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.
Transfer tax: some markets levy a transfer tax on individual shares (often not on 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 applies. 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. The instinct runs the other way, towards selling losers and buying winners. Disciplined rebalancing works against that instinct by enforcing a mechanical rule. Over multiple market cycles that discipline may matter more than the arithmetic does, though it is not something the calculation can measure. 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. It is also the point at which a mechanical rule does the most work, since it removes the decision at the moment the decision is hardest.

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 often shifts over time rather than staying at 60/40 indefinitely.

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: erases the bonus entirely through costs.

Unless transaction costs are near zero (institutional investors, or very large portfolios with sophisticated access), annual rebalancing is a common, low-friction choice. 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. The output is the mechanical rebalancing figure; the tax and cost considerations above sit on top of it.