Portfolio Rebalancing Calculator

What is rebalancing?

A portfolio drifts because its parts do not move together. Leave a 60/40 split alone through a good year for equities and it is no longer 60/40, and the risk you are carrying is not the risk you chose. Rebalancing is selling some of what grew and buying what lagged to put the weights back. This tool works out how far each holding has drifted and the exact amount to trade, and it will also show you how to close the gap with new money instead of selling. Everything runs in your browser, so nothing about your portfolio is transmitted.

How to use it

1. List each holding with a name, an asset class and what it is worth today
2. Set target percentages, or press a preset and adjust. They have to add up to 100%
3. Press Calculate the rebalance for the drift and the trades
4. To avoid selling, enter the amount you are about to invest in the second section instead

What is the formula?

Target amount = total portfolio × target weight, and the trade is that target minus what the holding is worth now. Positive means buy, negative means sell. Take 100,000 split as 60,000 equity, 30,000 bonds and 10,000 cash, against targets of 50/30/20. Equity should be 50,000, so you sell 10,000. Cash should be 20,000, so you buy 10,000. Bonds are already at their target and are left alone. The buys and sells cancel out, because the total does not change.

How can I rebalance without selling?

Put the money you are about to invest into the second section and it goes only to the holdings that are below target, largest shortfall first. Nothing is sold, so no gain is realised and no tax event is created. Adding 12,000 to the example sends 9,300 to cash and 2,700 to bonds. Equity receives 0, because it is the holding that is already above its target. Cash moves from 10.0% to 17.2%.

Note what did not happen: the target was not reached. When the shortfall is larger than the money available, the contribution is split in proportion to each shortfall and you land closer than you were. This is the normal case for anyone rebalancing out of monthly savings, and it is why the approach is slower than selling. What it buys you is that the tax bill stays at zero.

What do the presets do?

Four starting points, all common rules of thumb rather than recommendations.

• Equal weight. Every holding gets the same share
• Equity heavy. Around 70% equity, 20% bonds, the rest in cash and other
• Bond heavy. Around 30% equity, 55% bonds
• Age based. 100 minus your age into equity. At 40 that is 60% equity, with the remaining 40% split into 32.0% bonds, 6.0% cash and 2.0% other

The presets adapt to what you actually hold. If you have no "other" asset class, the equity heavy preset redistributes that slice across the classes you do have, giving 73.7% equity, 21.1% bonds and 5.3% cash rather than leaving 5% unassigned. Several holdings in the same class split that class evenly.

The age-based rule deserves a caveat. It was coined when bond yields and retirement horizons looked different, and target-date funds today hold considerably more equity at the same age. Treat it as a conversation starter, not a conclusion.

How often should I rebalance?

The tool does not decide this for you, but the two common approaches are worth knowing. A calendar rule means checking on a fixed schedule, typically once or twice a year. A threshold rule means acting only when a holding drifts more than a set distance from its target, often five percentage points, or a quarter of its own weight.

Both beat rebalancing constantly, which produces a lot of trading for very little change in risk. The drift column tells you where you stand against whichever rule you have chosen.

What this does not account for

The arithmetic here is deliberately simple, and a few real-world costs sit outside it.

• Tax. Selling to rebalance in a taxable account can realise a capital gain. Rebalancing inside a tax-sheltered account, or with new contributions, avoids this. It is usually the largest hidden cost of a rebalance
• Trading costs and spreads. Not modelled. They argue for rebalancing less often, and for using contributions where you can
• Whole shares and fund minimums. The tool gives you an amount, not a share count. Rounding to what you can actually buy leaves a small residual drift
• Wash sale rules. If part of the rebalance involves selling at a loss and buying something similar back, check the rules where you live before acting
• Multiple currencies. Figures are unitless, so enter everything in one currency. Mixed-currency portfolios need converting first

Notes

• Targets must add up to 100%. The running total is shown as you type and turns red until it does
• Values can be zero, which is how you add an asset class you intend to start rather than one you already hold. The portfolio as a whole cannot be zero
• Nothing is stored. Reloading the page clears everything, and no figure you enter is sent anywhere
• This is arithmetic, not investment advice. What your targets should be is your decision

Where these figures come from

The rules and figures this page relies on come from the sources below (links checked 22 September 2026).

• Topic no. 409, Capital gains and losses — Internal Revenue Service
• Publication 550, Investment Income and Expenses — Internal Revenue Service
• CG51560: Share identification rules for capital gains tax from 6.4.2008 — HM Revenue & Customs, Capital Gains Manual

What can change: The tool models no tax at all: it returns the trades that reach your target weights and nothing more. Whether a sale creates a taxable gain, and how a repurchase is matched against it, is set by tax law and changes with it.

Related tool

The figures here are what each holding is worth today. If you also want to know what a holding cost you, and what a further purchase would do to that average, the average cost basis calculator works that out from your individual purchases.