Portfolio Rebalancing Calculator

An allocation you set once does not stay put

Choosing a target asset allocation — a specific split between stocks, bonds, and cash — is a decision made once, but the portfolio does not stay at that split automatically. Different asset classes grow at different rates, and over time the better-performing ones naturally become a larger share of the total portfolio than originally intended, purely through differential growth rather than any deliberate action. Rebalancing is the process of periodically buying and selling to bring the actual allocation back in line with the original target.

This calculator takes your current holdings and target percentages and shows exactly what needs to be bought and sold to restore the original allocation.

Why drift happens even without touching the account

Consider a portfolio that starts at a clean 60% stocks, 40% bonds split. Over a strong year for stocks and a flat year for bonds, the stock portion grows faster, and the portfolio can drift to something like 68% stocks, 32% bonds — without a single trade being made. The investor did nothing wrong; this is simply the mathematical consequence of two asset classes growing at different rates.

Left unaddressed indefinitely, this drift compounds over multiple years, and a portfolio that started at a deliberately chosen risk level can gradually become considerably riskier than intended, purely through inaction rather than any conscious decision to take on more risk.

Two common approaches to deciding when to rebalance

There is no single correct rebalancing frequency, but two approaches dominate in practice. Calendar-based rebalancing checks and corrects the allocation on a fixed schedule — commonly annually, sometimes semi-annually or quarterly — regardless of how much drift has actually occurred by that date.

Threshold-based rebalancing instead waits until an asset class drifts beyond a specified band — often 5 percentage points from its target — and only then triggers a correction, checking more frequently but trading less often overall. Both approaches are reasonable and used widely; the threshold approach tends to reduce unnecessary trading during periods of modest, temporary drift that might correct itself without any action, while calendar-based rebalancing is simpler to implement consistently without requiring ongoing monitoring.

The tax cost that only applies in a taxable account

Rebalancing inside a tax-advantaged account — a 401(k), a traditional or Roth IRA — triggers no current tax consequence at all, since trades inside these accounts are not taxable events until money is eventually withdrawn. Rebalancing in an ordinary taxable brokerage account is a different matter entirely: selling an appreciated position to fund the purchase of an underweight asset class realizes a capital gain, which is taxable in the year the sale occurs.

This is a genuine cost that should factor into the rebalancing decision for a taxable account specifically — a modest amount of drift might reasonably be left alone if correcting it would trigger a meaningful, avoidable tax bill, particularly for a position with a large unrealized gain relative to its cost basis.

Rebalancing with new contributions instead of selling

For anyone still actively contributing to a portfolio, there is a more tax-efficient alternative to selling appreciated positions: directing new contributions specifically toward whichever asset classes have fallen below their target, rather than spreading new money proportionally across every holding.

This approach gradually pulls the portfolio back toward its target allocation using only new money, without selling any existing position and without triggering any capital gains tax at all. It works best for portfolios receiving regular ongoing contributions large enough, relative to the degree of drift, to meaningfully close the gap within a reasonable timeframe — for a portfolio with minimal new contributions relative to its total size, or drift too large to close with new money alone, some direct selling and buying may still be necessary.

Rebalancing across multiple accounts, not just one

Many households hold investments across several accounts simultaneously — a 401(k), an IRA, a taxable brokerage account — and it is worth thinking about the overall combined allocation across all of them together, rather than maintaining an identical target allocation independently within each individual account.

This household-level view often makes tax-efficient rebalancing easier, since trades inside tax-advantaged accounts carry no tax cost — meaning larger rebalancing trades can often be concentrated inside a 401(k) or IRA, while a taxable account is left largely undisturbed to avoid unnecessary capital gains, achieving the same overall target allocation with a meaningfully lower total tax cost than rebalancing each account separately and identically.

Connecting rebalancing back to the original target

Rebalancing only matters if the original target allocation itself was reasonable for your situation — the asset allocation calculator provides a starting-point target based on age and risk tolerance, which this rebalancing calculator then helps maintain over time as markets move the actual holdings away from that target.