Jason FinanceRebalancing Calculator
Once you set an asset allocation, market moves slowly pull each holding away from its target weight. Rebalancing brings them back. This tool works two ways: buy with new cash only, without selling; or add no money, selling what is overweight and buying what is underweight. Enter each holding’s current market value and target weight to see how much to buy or sell of each.
If you hold Taiwan ETFs in New Taiwan dollars and US ETFs in US dollars, pick a currency for each holding and enter a USD to TWD exchange rate you looked up yourself. The tool converts everything to one base currency for the math and shows each trade in the holding’s own currency. It only calculates from the numbers you enter; it does not recommend any product and is not investment advice.
Results
- Holdings now: NT$1,120,000
- New cash: NT$100,000
- The new cash is not enough for a full rebalance. A full rebalance by buying alone would take NT$180,000 of new cash in total, NT$80,000 more than this.
- Largest drift: 8.4 percentage points before, 2.0 after
- Holdings after trading: NT$1,220,000
Scroll the table sideways to see every column.
| Holding | Action | Amount (holding currency) | Shares | Weight after | Drift after (pp) | Market value | Weight now | Target |
|---|---|---|---|---|---|---|---|---|
| Taiwan ETF A | No trade | — | — | 49.2% | −0.8 | NT$600,000 | 53.6% | 50.0% |
| US ETF B | No trade | — | — | 32.0% | +2.0 | NT$390,000 | 34.8% | 30.0% |
| US bond ETF C | Buy | US$3,076.92 | — | 18.9% | −1.1 | NT$130,000 | 11.6% | 20.0% |
Market values are in the base currency; trade amounts are in each holding’s own currency. Drift = weight after the trades − target weight: positive means overweight, negative means underweight.
This tool only calculates from the market values, target weights and exchange rate you enter. It does not know your actual trade prices, fees, taxes or currency spreads, and it is not investment advice. How you set your targets, whether to rebalance and when are your own decisions. Selling may trigger transaction taxes, fees or other taxes, so check your broker’s fees and the tax rules that apply to you before you trade.
How to use it
- Choose how to rebalance: buy with this round of new cash only, or add no money and sell to adjust.
- Choose a base currency. If you hold both TWD and USD, enter a USD to TWD exchange rate.
- For each holding, enter a name, currency, current market value and target weight. The targets must add up to 100%.
- To see whole shares, enter a price per share; leave it blank for amounts only.
- The results show how much to buy or sell of each holding, the weights after trading, and how many percentage points each is still off target.
How the two modes differ
New cash only: nothing is sold, so no transaction tax or realized gain or loss comes from selling. A common way to use it is to adjust the weights each time you invest new money. The limit is that the new cash may not be enough: overweight holdings cannot be sold, only diluted by buying the others. When the cash falls short, the tool first fills the holding with the largest shortfall until it matches the next one, then fills them together, which keeps the drift after buying as small as possible. It also tells you how much more cash a full rebalance would take.
Selling to rebalance: sell the overweight holdings and use the proceeds to buy the underweight ones, landing exactly on target without adding money. The cost is the sale itself: there may be a securities transaction tax and fees, and selling at a gain may have to be reported under the tax rules that apply to you. The tool lists total sells, total buys and the total traded so you can estimate those costs.
You can also combine the two: adjust with new cash most of the time and sell only when the drift gets large. Which one to use and how often to rebalance are your own decisions.
Methodology
- Each market value is first converted to the base currency: with TWD as the base, USD holdings × the rate; with USD as the base, TWD holdings ÷ the rate. Current weight = the holding’s value ÷ the value of all holdings.
- New cash only: total after T = current total + new cash, each holding’s goal = T × target weight, and its shortfall = goal − current value. The cash goes to the largest shortfall first, filling it down to the next shortfall, then filling those together, until the cash runs out. Every holding that gets bought ends up the same number of percentage points below target, which gives the smallest sum of squared drifts. Holdings with a negative shortfall (already overweight) are not bought.
- New cash needed for a full rebalance = the largest “current value ÷ target weight” across holdings − current total. The most overweight holding sets how much must go in to dilute it back to target. A holding with a 0% target that still has value can never get back to target by buying alone.
- Selling to rebalance: each trade = current total × target weight − current value, positive to buy and negative to sell; they add up to exactly 0. Total traded = total sells + total buys.
- Drift band: no trades while every holding’s drift (current weight − target) is within the band; once any holding is outside it, all holdings go back to target.
- With a price per share, buys round down and sells round up to whole shares (never more than you hold), so the trades never need more money than planned; the difference shows as leftover cash. Weights after trading use the rounded amounts, and leftover cash is not counted in the weights.
- If the targets add up to within 0.05 percentage points of 100% (for example 33.33 three times), they are scaled to exactly 100%; a larger gap asks you to fix them. Negative numbers count as 0.
What is not included
- Fees, securities transaction taxes, currency spreads and bid/ask spreads. Weights after real trades will differ slightly.
- Tax on gains from selling. Depending on your residency and the rules that apply to you, gains may need to be reported; check for yourself.
- Exchange rate moves: the tool uses the single rate you enter, and the rate when you actually convert may differ.
- Trading units: some markets and brokers do not trade in single shares; adjust the share count to your broker’s rules.
- Assets not in the table, such as cash and an emergency fund. To include cash in the allocation, add it as a holding.
FAQ
How often should I rebalance?
There are two common approaches. Calendar rebalancing: check on a fixed schedule, such as every quarter, every six months or once a year, and bring everything back to target then. Threshold rebalancing: leave the portfolio alone until a holding drifts from its target by more than a set amount (for example 5 percentage points); the drift band in this tool works this way. Some people combine them: check on a schedule and trade only when a holding is outside the band. Rebalancing more often keeps the weights closer to target but means more trades and more costs. The tool does not decide for you; pick the approach that fits your own situation and costs.
What should I watch for when selling to rebalance?
Selling can cost money: selling Taiwan-listed securities usually incurs a securities transaction tax, brokers charge fees, and converting between USD and TWD has a currency spread. Selling at a gain may also have to be reported under the tax rules that apply to you. The tool does not calculate these fees or taxes, so check your broker’s fee page and the relevant tax rules before you trade. If you prefer not to sell, use the new cash only mode.
What does it mean when the new cash is not enough for a full rebalance?
An overweight holding is too far above its target to be diluted back by buying the others. The tool puts the new cash into the holdings with the largest shortfalls first, keeping the drift after buying as small as possible, and shows how much new cash a full rebalance would take and how much more that is. You can close the remaining gap with your next contribution or switch to selling to rebalance.
How do I combine Taiwan and US holdings?
Pick each holding’s currency, enter its market value and share price in that currency, and enter one USD to TWD exchange rate. The tool converts everything to your base currency to work out weights and totals, then shows each trade back in the holding’s own currency, ready for orders at a Taiwan broker and an overseas broker. Use the rate you actually get when converting; the tool never fetches one.
Is my data sent to the website?
No. The math runs in your browser, and what you enter is stored only in this device’s browser so you can pick up where you left off. “Clear data” deletes it. In a private window, or with site data blocked, nothing is stored and the tool still works.
Further reading
- Overseas Income and Taiwan's Alternative Minimum Tax: How to Report U.S. Stocks, Crypto and Hong Kong Interest (NT$1 Million Threshold, NT$7.5 Million Deduction, Foreign Tax Credit)
- Sub-Brokerage vs an Overseas Broker: Break-Even Points by Annual Investment and Number of Trades (2026 Official Fees)
- The Real Cost of Wiring Money from Taiwan to a U.S. Broker: Bank Wires, Digital Banks and Wise to Firstrade, IBKR and Schwab, and How Much to Send at Once
- Stop Investing Blindly: 10 Principles of Index Investing for Steady Wealth. Market-Cap Index Funds vs Active Funds, Notes on a 清流君 Video with Supporting Data