Most UK investors assume that buying or selling a fund simply happens at its published net asset value, with no further cost beyond any platform dealing charge. In reality, many open-ended funds use mechanisms called swing pricing or dilution levies, which can adjust the price paid or received specifically to protect the interests of continuing investors when large flows of money move in or out of the fund. These mechanisms are rarely explained clearly to retail investors, yet they can have a real, if usually modest, effect on the price at which units are bought or sold.
The problem these mechanisms are designed to solve
When investors buy units in an open-ended fund such as a unit trust or OEIC, the fund manager typically needs to buy additional underlying assets — shares or bonds — to invest that new money. Conversely, when investors sell (redeem) units, the manager may need to sell underlying assets to raise the cash to pay them. Buying and selling underlying assets incurs real transaction costs, including broker commissions, market spreads, and sometimes stamp duty or other taxes.
Without any adjustment, these transaction costs would be paid out of the fund's overall assets, meaning they are effectively shared by all investors in the fund — including those who did nothing that day and simply continued holding their existing units. This creates what is sometimes called a "dilution" effect: the value of existing investors' holdings is subtly reduced by transaction costs generated by other investors' buying and selling activity, particularly during periods of large net inflows or outflows.
How dilution levies work
A dilution levy is a direct charge applied to a specific investor's transaction — typically a large individual purchase or redemption — intended to make that investor bear the estimated transaction cost their trade generates, rather than spreading it across all fund holders. This is most commonly applied to significant one-off transactions rather than the more modest, regular purchases typical of many retail investors' ongoing contributions.
When a dilution levy might apply
- A large lump-sum investment or withdrawal relative to the fund's overall size.
- Periods when the fund experiences unusually high net inflows or outflows across all investors combined, sometimes leading to a levy applied more broadly rather than only to the largest individual trades.
- Funds investing in less liquid underlying assets, such as certain corporate bonds, smaller companies, or property, where transaction costs to buy or sell the underlying assets are inherently higher.
How swing pricing works
Swing pricing takes a somewhat different, more automated approach. Rather than applying a separate charge to specific large transactions, the fund's overall published price is adjusted, or "swung", up or down on days when net flows into or out of the fund exceed a pre-set threshold.
Full swing versus partial swing
Under "full swing pricing", the fund's price is adjusted on any day the net flow threshold is breached, in either direction. Under the more commonly used "partial swing pricing", the price is adjusted only when net flows exceed the threshold, and the size of the adjustment is calibrated to reflect the estimated transaction costs associated with that day's net flow.
If a fund experiences heavy net buying on a given day, the price may be "swung" upward, meaning new investors buying that day pay a slightly higher price, reflecting the estimated cost of the manager having to buy additional underlying assets. If the fund experiences heavy net selling, the price may be swung downward, meaning investors selling that day receive a slightly lower price, reflecting the estimated cost of the manager having to sell underlying assets to meet redemptions. In both cases, the mechanism aims to ensure the investors actually causing the fund's trading activity bear its associated cost, rather than continuing investors absorbing it through a diluted overall fund value.
Comparing the two mechanisms
| Feature | Dilution levy | Swing pricing |
|---|---|---|
| Applied to | Typically specific large individual transactions | The fund's overall published price on days meeting a net flow threshold |
| Visibility to the investor | Usually disclosed as a separate charge line at the point of a qualifying large transaction | Embedded within the published price itself, generally not separately itemised per transaction |
| Common in | A range of open-ended fund types, particularly for large trades | Increasingly common among larger retail-focused fund ranges, particularly for less liquid asset classes |
| Investor experience | Most retail investors making modest regular trades are unlikely to trigger it | Can occur without an individual investor's own transaction being unusually large, if overall fund-wide flows are heavy that day |
A worked example
Suppose an open-ended UK equity income fund has a net asset value of £1.00 per unit on a given dealing day. If the fund experiences unusually heavy net inflows that day — perhaps due to a wave of investors buying at once — and its swing pricing threshold is breached, the manager might apply a small adjustment, say 0.3%, resulting in an effective dealing price of £1.003 for investors buying that day. An investor placing a £5,000 lump-sum purchase on that particular day would, in this hypothetical scenario, receive marginally fewer units than they would have on a day when no swing adjustment applied, reflecting the estimated extra transaction cost their purchase (alongside everyone else buying that day) generated for the fund. This example uses illustrative, simplified figures purely to demonstrate the mechanism, not to represent any specific real fund's actual swing factor or pricing history.
Why these mechanisms matter for fund selection
Swing pricing and dilution levies are disclosed in a fund's prospectus and are a normal, regulator-sanctioned feature of many open-ended funds, rather than a sign of a poorly run fund. They are, in fact, generally viewed as a protective mechanism for continuing investors, since they help ensure that investors triggering large flows in or out of a fund bear a fairer share of the associated trading costs rather than passing that cost on to everyone else.
For most retail investors making modest, regular contributions — for example, through a monthly ISA or pension contribution — these mechanisms typically have a negligible or no practical effect, since regular modest flows are far less likely to trigger fund-wide swing thresholds compared with occasional very large lump-sum transactions from institutional or large individual investors.
Where this is particularly relevant
- Property funds investing in physical buildings, where buying or selling underlying assets is slow and expensive, making swing pricing and dilution mechanisms especially relevant to protecting continuing investors.
- Corporate bond funds, where underlying bond markets can be less liquid than major equity markets, particularly for lower-rated or smaller issues.
- Smaller company funds, where underlying shares may trade with wider spreads and lower volumes than large-cap equities.
- Funds experiencing unusually rapid growth or a wave of redemptions, for example following a period of strong performance attracting large new inflows, or a period of market stress prompting large withdrawals.
Practical points for UK investors
- Check a fund's prospectus or Key Investor Information Document for whether it uses swing pricing, dilution levies, or neither, particularly for funds investing in less liquid assets.
- Understand that ETFs, which trade on an exchange with their own bid-offer spread mechanism, are structured differently and generally do not use swing pricing or dilution levies in the same way as traditional open-ended funds.
- Recognise that these mechanisms are generally a protective feature for continuing investors, not a hidden cost designed to disadvantage retail investors specifically.
- For very large lump-sum transactions, consider checking with the platform or fund provider whether a dilution levy might apply, particularly for less liquid fund types.
- For typical regular monthly contributions, these mechanisms are unlikely to have a meaningful practical effect in most cases.
How this differs from a fund's ongoing charges figure
It is worth being clear that swing pricing and dilution levies are entirely separate from a fund's ongoing charges figure (OCF), which covers the manager's recurring annual fee and administrative costs regardless of trading activity. Swing pricing and dilution levies relate specifically to the transactional cost of buying and selling underlying assets in response to investor flows, applied only when relevant thresholds are triggered, whereas the OCF is deducted continuously from fund assets regardless of whether any investor buys or sells on a given day. A fund with a very low OCF can still use swing pricing, and a fund with a relatively higher OCF might not use it at all — the two features are independent design choices made by the fund's operator, disclosed separately in the fund's official documentation, and neither substitutes for the other in a fund's overall cost profile.
What this means when comparing similar funds
When comparing two funds that appear similar on OCF and investment approach, it can be worth checking whether one uses swing pricing or dilution levies more aggressively than the other, particularly for funds investing in less liquid asset classes such as property or high-yield bonds. A fund with a well-calibrated swing pricing mechanism may, in principle, better protect long-term continuing investors from the dilution effects of other investors' large flows, though this is difficult for a retail investor to verify precisely in practice, since the specific swing factors applied are not always published transaction by transaction. This is one of the less visible aspects of fund selection, but it is a legitimate part of understanding how a fund is actually operated day to day, beyond the headline cost and performance figures most commonly compared.
Key takeaways
- Swing pricing and dilution levies exist to ensure investors whose transactions generate significant trading costs bear a fairer share of those costs, rather than diluting the value of continuing investors' holdings.
- Dilution levies are typically applied as a separate charge to specific large transactions, while swing pricing adjusts a fund's overall published price on days when net flows exceed a set threshold.
- These mechanisms are most relevant to funds investing in less liquid assets, such as property, certain corporate bonds, and smaller companies.
- Most retail investors making modest, regular contributions are unlikely to be materially affected in practice.
- ETFs use a different mechanism — the exchange-traded bid-offer spread — rather than swing pricing or dilution levies in the traditional open-ended fund sense.
- These features are disclosed in fund documentation and are a normal, regulator-recognised part of how many open-ended funds operate.