Building £1,000 a month in passive income is a goal many investors have, yet few understand how much they need in a Stocks and Shares ISA to achieve it.

An ISA is a useful tool because both capital gains and dividends are tax-free. Over time, those tax benefits can be truly powerful.

I thought I’d dig into the numbers to look at how big a nest egg needs to be to deliver a sizeable £1,000 passive income. 

Working out the numbers

Generating £1,000 a month in passive income with dividend shares works out to £12,000 a year.

Assuming a 3.5% average dividend yield, that works out to be about £342,000 worth of shares in an ISA. That’s a big chunk of change. However, there could be ways to reduce this further.

For example, NatWest (LSE: NWG) shares are yielding 5.5% as I write on 6 March. Applying that same maths, the required portfolio value with an average 5.5% yield would drop to around £218,000.

This is where picking the right stocks, being patient, and diversifying becomes critical. A good spread of high-quality dividend stocks delivering an above-average yield, combined with steady contributions, can quickly accelerate investors’ passive income goals.

How long will it take me to save?

Very few investors will have a lazy £200,000 or more just lying around. However, it’s unlikely that you’re not starting from zero on day one.

Let’s say you could save £500 a month and put it into high-yield dividend shares in an ISA with an average 5.5% yield. That £218,000 portfolio is achievable in around 20 years’ time.

Higher-yielding stocks, higher contributions, or some additional share price gains could help to get there even sooner. Markets are uncertain, however, so it’s unlikely to be a smooth and linear journey.

Why NatWest stands out

NatWest has rebuilt itself following the financial crisis and now operates with a strong balance sheet. It returned to full private ownership in 2024 after the UK government sold its final stake, and management has signalled an aggressive capital return programme for shareholders.

I also like that the price-to-earnings (P/E) ratio sits at a modest 8.7 times, well below the FTSE 100 average.

Then there’s the yield. Its dividend has been growing steadily. In February, the company announced a final dividend of 23p per share, up from 15.5p the previous year. It also committed to buying back a further £750m of shares in the latest return of capital to shareholders.

Risks to consider

What about the risks? No dividend is ever guaranteed, which is a risk with relying on dividend shares in general.

For NatWest, UK banks remain sensitive to economic cycles, and a sharp downturn in mortgage lending or rising loan defaults could force management to scale back payouts. NatWest’s profitability is also closely tied to interest rates, the outlook of which remains unclear.

My verdict

For investors targeting £1,000 a month in passive income, the maths are clear. Steady investment into a portfolio with a high average yield can fast track those second income goals.

In my mind, NatWest offers a combination of yield, valuation, and capital return potential right now. While this stock might not suit everyone, there are plenty of opportunities across the Footsie worth exploring for those building an income-focused ISA today.