Building an investment from nothing but monthly contributions
Not everyone has a lump sum to start with, and that's fine — a large share of long-term investors build their entire portfolio purely through regular monthly contributions from income, with no initial deposit at all. This calculator projects exactly that scenario: a fixed monthly amount, invested consistently at an assumed average annual return, over however many years you plan to keep contributing.
Why consistency tends to matter more than timing
Trying to time when to invest — waiting for a dip, or for markets to feel "safer" — is notoriously difficult to do reliably, even for professional investors, and delaying contributions while waiting for the right moment often costs more in lost time than it saves in avoided bad timing. Investing the same amount automatically each month, regardless of what markets are doing, is a well-established approach known as pound-cost averaging: when prices are lower, your fixed contribution buys more units; when prices are higher, it buys fewer. Over time this smooths out the impact of short-term volatility on your entry price, without requiring you to predict anything.
What "assumed annual return" is really representing here
The rate you enter should reflect the underlying investments your monthly contributions are actually going into — a diversified index fund, an actively managed fund, or a mix of assets adjusted for your own risk tolerance. It's worth testing a range of assumptions either side of your best estimate, since the projected value is highly sensitive to the rate over long periods, and no rate can be guaranteed in advance. Our index funds and ETFs guide explains how low-cost diversified funds are commonly used for exactly this kind of regular contribution investing.
Increasing your contribution over time
This calculator assumes a flat monthly amount for simplicity, but in practice many people increase their contribution over time as income rises — even a modest annual increase, in line with a pay rise, can meaningfully boost the final total compared with a contribution that never changes. If you want to see the effect of a step change (for example, doubling your contribution partway through), you can run the calculator twice for each period and add the results together as a rough approximation.
Comparing this against starting with a lump sum
If you're weighing up whether to wait and save a lump sum first versus starting to invest smaller amounts monthly right away, it's worth knowing that starting sooner, even with modest amounts, generally gives your money more time to benefit from compounding than waiting to accumulate a larger sum first. Our lump sum versus regular investment calculator compares the two approaches directly, and our investment growth calculator combines both an initial amount and ongoing contributions if you have some of each.
A worked example
Take the calculator's defaults — £300 invested every month for 15 years, at an assumed 6% annual return. Total contributions over the period add up to £54,000 (£300 × 180 months), yet the projected value comes to roughly £87,500, meaning growth accounts for around £33,500 of the final total, more than a third of it, despite no lump sum ever being involved.
Push the same £300 a month out to 25 years instead of 15, and contributions rise to £90,000 — but the projected value climbs to approximately £207,300, well over double the 15-year result. The extra ten years lets far more of the early contributions benefit from a long compounding runway, which is the clearest possible illustration of why starting a regular contribution habit early tends to matter more than the exact size of the monthly amount.
Setting up contributions so they actually happen
The single most reliable way to keep a regular investing plan on track is to automate it — a standing order or direct debit set up straight after payday, before the money has a chance to be spent on anything else, tends to succeed far more consistently than a manual "invest what's left" approach at the end of the month. Most investment platforms let you set up a recurring contribution once and largely forget about it, which removes the need to make a fresh decision to invest every single month, a decision that's surprisingly easy to defer indefinitely once it isn't automatic.
Frequently asked questions
Is pound-cost averaging always better than investing a lump sum?
Not necessarily — if markets rise steadily over your investing period, a lump sum invested immediately will typically outperform the same amount drip-fed in gradually, simply because more of it has more time in the market. Pound-cost averaging's main benefit is smoothing out the emotional and practical difficulty of investing a large sum all at once, particularly during volatile periods.
What if I miss a month's contribution?
This calculator assumes an unbroken series of contributions for simplicity — missing occasional months in reality will modestly reduce your actual final total compared with the projection, roughly in proportion to how much was missed.
Should I invest via a workplace pension, an ISA, or both?
Many people do both — a workplace pension is hard to beat where an employer contribution and tax relief are on offer, while an ISA offers more flexible access. Our pension growth calculator models the pension side of regular contribution investing specifically.
Does it matter which day of the month I invest on?
Not meaningfully over a long horizon — the small differences in exact timing within a month tend to average out over many years of regular contributions, so consistency matters far more than picking a particular day.
What if I want to stop contributing at some point but leave the money invested?
That's a perfectly normal approach — you can run this calculator with contributions only for the years you plan to actually invest, then separately project the remaining years using our compound interest calculator with the accumulated value as the starting amount and no further monthly contribution.
Is there a minimum amount worth investing monthly?
Many platforms accept regular contributions from as little as £25 to £50 a month, and starting small while building the habit is generally far better than waiting until you can commit a larger amount, given how much starting earlier matters to the eventual result.