Early bird vs last-minute ISA investing – which is best for your portfolio?

The new tax year has begun, and with it comes a fresh £20,000 annual ISA allowance. But should you use this allowance right at the start of the year, or leave it later?

As well as knowing where to invest, and making sure you select the best funds and stocks for your portfolio, the timing of your ISA contributions will have a significant impact on your returns over time.

Analysis shows that early bird ISA investors who make use of their ISA allowance from the start of the tax year tend to do better than those who invest at the last minute.

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Analysis from asset manager Vanguard shows that a hypothetical investor who invested their entire £20,000 allowance on 6 April 2025, and did the same at the start of each subsequent tax year, would see their pot grow to £1,079,320 by the end of the 25th year (assuming a 5.5% annual return after fees).

Waiting until the end of each tax year to invest the £20,000, though, would leave the same investor with £1,023,052 – around £56,000 less, just by virtue of waiting until the end of the tax year.

“Time in the market really matters,” said James Norton, head of retirement & investments at Vanguard. “We see that many people rush to max out their ISA allowance at the end of a tax year, rather than at the beginning, missing out on almost a year of tax-efficient returns.

“The key is to make your money work for you as early as you can, in a way that fits your circumstances,” Norton added.

Over the entire history of ISAs, since their introduction in 1999, early bird ISA investors could be £83,000 better off than last-minute investors, according to analysis from investment platform InvestEngine.

Putting the £20,000 annual ISA allowance into the MSCI ACWI Net Total Return (GBP) index at the start of every financial year since April 1999 would have built a pot worth £1,277,963, compared to the £1,195,127 that the same contributions would have grown to if made at the end of each year.

“With the new lower cash ISA limit set to come in next year, those considering a stocks and shares ISA for the first time could benefit by starting early with their investments. Even small amounts can make a big difference over time.”

How do early bird ISA investors perform compared to regular monthly investors?

Meanwhile, asset manager Fidelity International compared three approaches to ISA investing: using the entire ISA allowance at the start of the tax year (Early), using it all at the end of the year (Late) or drip-feeding the allowance through monthly across the course of the year (Regular monthly).

Again, the best approach was to invest the entire allowance at the start of the year, based on historic returns of the FTSE All Share Index:

^{Source: Datastream, Fidelity International, 05/04/2001-06/04/2026 Total return in GBP of FTSE All Share}

^{Source: Datastream, Fidelity International, 05/04/2016-06/04/2026. Total return in GBP of FTSE All Share}

“For many people, investing regularly can make the process feel more manageable,” said Marianna Hunt, personal finance expert at Fidelity International. “It helps reduce the pressure of trying to time the market and can take some of the emotion out of investment decisions.

“What matters most is making use of your ISA allowance and maintaining a long-term focus,” Hunt added.