Tax-loss harvesting sounds like the sort of cleverness that belongs in a hedge fund, but it is a straightforward idea any investor can use. When one of your holdings has fallen in value, you sell it, “bank” the loss against any capital gains you have made elsewhere, and immediately put the money into a similar investment so that your exposure to the market does not actually change. You have swapped real losses on paper for a genuine tax saving, while remaining invested as if nothing had happened.
The strategy is one of the few in personal finance that is close to risk-free — but only for the right investors in the right accounts under the right rules. It is specific to stocks and ETFs; crypto has its own treatment, covered separately in our crypto tax-loss harvesting guide. This guide works through how the strategy operates in the UK and the US, where the rules meaningfully differ.
How the mechanism works
The logic is simple. Selling an investment crystallises a loss; holding it does not. When you realise losses, you can set them against realised capital gains, and you are taxed only on the net figure. The four steps are: sell positions that sit below your cost basis, use the losses to offset gains from other sales, pay tax only on the net gain that remains, and reinvest the proceeds in a similar — not identical — holding so you stay invested.
It is worth being precise about what you are and are not buying. You are not escaping tax debt entirely; you are deferring it and reducing the taxable total. You are also not changing your portfolio in any meaningful financial sense if you replace the holding with a close substitute — the benefit shows up in your tax position rather than in your returns.
A worked example in a taxable account
Imagine a £100,000 portfolio containing a global equity ETF that has fallen from a £40,000 cost basis to £36,000, and a US tech stock down from £20,000 to £16,500. Elsewhere this tax year you have sold a UK small-cap stock for a £6,000 realised gain.
| Position | Cost basis | Current value | Gain / (loss) |
|---|---|---|---|
| Global equity ETF | £40,000 | £36,000 | (£4,000) |
| US tech stock | £20,000 | £16,500 | (£3,500) |
| UK small-cap stock (sold) | £15,000 | £21,000 | £6,000 |
Without harvesting, you face capital gains tax on £6,000 of gains. Sell the two losers and you have £7,500 of realised losses, leaving a net position of £6,000 − £7,500, or a £1,500 loss. At a 20% tax rate, you have removed £1,500 of taxable gains from the year. In the UK, the unused portion of that loss carries forward indefinitely to offset future gains.
The crucial point is that you sold the two losing holdings and reinvested in replacements, so your exposure to the market is essentially unchanged. Only the tax position has improved — which is exactly the point of the exercise.
The US wash-sale rule in detail
The main thing standing between a US investor and unlimited harvesting is the wash-sale rule. It exists to stop people selling a position, booking the tax loss, and buying it straight back a day later — which would be harvesting with no real change of position. The rule disallows a loss if you buy a “substantially identical” security within 30 days before or after the sale. Note the before: the window covers purchases made up to 30 days prior to the sale as well as afterwards, so the total window is 61 days, not 60 as some people assume.
| Element | Rule |
|---|---|
| Window | 30 days before and 30 days after the sale |
| Trigger | Buying a “substantially identical” security within that window |
| Penalty | The loss is disallowed (added to the replacement’s cost basis) |
| Workaround | Buy a different but similar fund — S&P 500 to total market, one sector ETF to another |
The practical workaround is to switch to a different but similar fund. Replace an S&P 500 ETF with a total US market ETF, or a sector ETF with an adjacent one. Your exposure stays broadly the same while the position is no longer “substantially identical” in the eyes of the rule. The penalty when you breach it is not a fine — the loss is disallowed and simply added to the cost basis of the replacement. December harvesting is the classic trap: if you re-bought the same fund in late November and sell it for a loss in December, you may still be caught by the backward-looking 30-day window.
The UK rules
The UK has its own cluster of matching rules. The annual capital gains tax exemption for the 2026/27 tax year is £3,000 — gains within that figure are tax-free, so there is nothing to harvest against until you exceed it. When you sell and rebuy, two matching rules apply: same-day purchases are matched to sales first, and if you repurchase the same shares within 30 days, the loss is not allowed. The latter is very close in effect to the US wash-sale rule.
| Rule | Details |
|---|---|
| Annual exemption | £3,000 of gains tax-free per year (2026/27) |
| Same-day rule | Same-day purchases are matched to sales first |
| 30-day rule | Repurchasing within 30 days means the loss is not allowed |
| Carry forward | Losses carry forward indefinitely against future gains |
The practical UK workaround is the same as the US one: if you want to remain invested, switch to a different fund family or a different index rather than rebuying the same ETF within 30 days. For UK investors, one further simplification is heavily recommended: the vast majority of regular savers should be investing inside an ISA in the first place, where gains are wholly tax-free and the strategy becomes unnecessary.
Combining harvesting with rebalancing
Harvesting works naturally alongside the rebalancing most investors should be doing anyway, because both involve selling. When a taxable account needs rebalancing, prefer to sell the losers (harvesting the loss) instead of the winners (which would trigger a tax bill), and when a holding has grown above its target weight, harvest in the replacement fund instead. When a broad index is well below its basis, you can swap index funds — FTSE All-World to MSCI World, say — while keeping your allocation intact.
Good replacement pairs that keep your portfolio essentially unchanged include swapping an S&P 500 ETF for a total US market ETF, a FTSE All-World ETF for an MSCI World plus emerging-markets combination, a growth index for a value index, or a global bonds fund for a global bonds ex-credit fund. These are close cousins financially, but distinct enough for tax purposes.
When harvesting is not worth it
For all its appeal, the strategy has real limits, and knowing when to skip it is part of using it well. A loss of under roughly £500 in the UK or $1,000 in the US is usually not worth the dealing fees, spreads, and administrative effort — the tax saved is smaller than the cost of realising it. If you know you will want to re-enter the exact same fund within the 30-day window, the rules will disallow the loss anyway. If the position sits inside an ISA, SIPP, or 401(k), there is no capital gains tax to offset and harvesting is pure waste. And in a low tax year — when your gains fall inside the annual allowance — the harvest produces no benefit.
| Situation | Why to skip |
|---|---|
| Loss under ~£500 / ~$1,000 | Fees and effort exceed the tax saved |
| You’ll rebuy the same fund within 30 days | Wash-sale / 30-day rule disallows it |
| Inside an ISA / SIPP / 401(k) | No capital gains tax to offset |
| A low tax year | Gains may fall inside your annual allowance anyway |
| You’ll sell the position again soon | Avoid a double sale — sell, rebuy, sell |
Keeping track pragmatically
None of this requires expensive software. Your broker’s statements provide cost basis and realised-gain reports, tax software such as TurboTax, TaxScouts, or FreeAgent can import and compute harvesting around £30-£100, and portfolio trackers like Sharesight or Portfolio Performance give continuous gain/loss tracking on free or paid tiers. In the UK, HMRC records are free but you must track your own gains and losses manually to report them accurately. However you do it, keep the records — a loss claimed incorrectly, or a three-year-old record lost, is a tax problem waiting to happen.
Bottom line
For stock and ETF investors, tax-loss harvesting is a genuine, repeatable saving — but only in taxable accounts, only when the losses are meaningful, and only when you respect the wash-sale and 30-day rules. Pair it with rebalancing so you sell losers instead of winners, use replacement funds to stay invested, and skip it when the numbers are too small to matter or the position lives in a tax wrapper. Handled sensibly, the savings compound year after year; handled badly, it is fees and effort for a disallowed loss. The figures above describe the UK and US specifically — check the rules for your own jurisdiction. When in doubt, a modest taxable portfolio is often better served by an ISA and a simple indexed portfolio than by cleverness. This article is educational and not financial advice.