Figures are illustrative only, ignore charges and tax, and are not a forecast or personal recommendation.
If you come into a lump sum — an inheritance, a bonus, or savings built up over time — one of the most common questions is whether to invest it all at once or drip it in gradually via monthly contributions instead. This calculator runs both scenarios side by side using the same assumed rate of return, so you can see the projected difference for your own numbers.
The two scenarios compared
The "lump sum" column assumes the full amount is invested immediately and left to compound for the whole period. The "regular investment" column assumes the money is instead paid in as equal monthly contributions spread across the same number of years, with each contribution starting to grow only from the point it's invested. Because the lump sum spends more total time invested and exposed to growth, it will almost always produce a higher projected figure at any positive assumed rate of return — this is a mathematical certainty of compounding, not a market prediction.
Why people still choose to drip-feed anyway
Despite the maths favouring lump-sum investing on average, many investors choose to phase a large sum in gradually — commonly over 6 to 12 months — because it reduces the risk of investing everything right before a market downturn, and because doing so is psychologically easier to sit with. This is a legitimate risk-management choice rather than a mistake, and it comes down to your own risk tolerance versus risk capacity: the maths favours investing sooner, but the emotional and behavioural case for phasing in is genuine, particularly for larger sums relative to your overall wealth. Our guide on monthly investing versus a lump sum in an ISA explores the historical evidence in more depth.
Worked example
£12,000 invested immediately at 6% a year for 10 years produces a noticeably higher projected total than £100 a month invested over the same 10 years (which totals the same £12,000 paid in), simply because the lump sum has, on average, roughly twice as much "time in the market" as a drip-fed contribution schedule reaching the same total.
A middle path
Many investors combine both: investing a portion immediately and phasing in the rest over a set number of months, which is easy to approximate using this calculator by splitting your total between the two fields, or by using our Investment Growth Calculator, which accepts a lump sum and a monthly contribution in the same projection.
Frequently asked questions
Is lump-sum investing always better?
On average, across most historical periods, yes, in pure return terms — because markets rise more often than they fall over any given period, having money invested sooner tends to produce a better outcome. But "on average" hides real variation: in any specific period, phasing in could turn out better if markets fall shortly after a lump-sum investment.
How long should I phase a lump sum in over?
There's no fixed rule, but 6-12 months is a common compromise between reducing timing risk and not leaving money in cash (earning less, and losing value to inflation) for too long.
Does this apply inside an ISA?
Yes, though bear in mind the annual ISA allowance may limit how quickly you can move a very large lump sum into the tax wrapper — see our ISA Calculator for a contribution-based projection.
What if I'm nervous about investing a lump sum at all?
That's a reasonable, common feeling, and phasing the money in gradually is a legitimate way to address it — the "right" answer depends as much on how you'll behave and feel during a downturn as on the pure expected-value maths.
Common mistakes in this comparison
A frequent mistake is treating this as a purely mathematical decision and ignoring the behavioural side entirely. The calculator will almost always show lump-sum investing producing a higher projected figure, but if investing a large lump sum immediately would cause enough anxiety that you'd be tempted to sell out of the market at the first downturn, the "optimal" mathematical strategy can become the worse real-world choice. The best strategy is one you can actually stick with through a market fall, not just the one with the highest expected value on paper.
Another mistake is assuming "regular investment" and "phasing in a lump sum" are the same thing — they're related but distinct. This calculator compares investing a lump sum immediately against building up the same total via ongoing new monthly contributions (i.e., money that wasn't available as a lump sum in the first place). If you already have the lump sum in hand and are deciding whether to phase that specific sum in over, say, 6-12 months rather than investing it all on day one, the practical comparison is similar in spirit but technically involves investing a shrinking cash balance over a short period, not a fresh monthly contribution over the calculator's full term.
What if markets fall right after I invest a lump sum?
This is the specific risk phasing in aims to reduce — a lump sum invested immediately before a fall will show a paper loss sooner than a phased approach would, though history shows this is impossible to predict in advance and markets rise more often than they fall over any given period.
Does this comparison change much at different assumed return rates?
The gap between the two approaches widens at higher assumed return rates and over longer time horizons, since more of the difference comes from how much time each contribution has had to compound — try adjusting the rate in the calculator to see this effect directly.
Is there a "correct" number of months to phase a lump sum in over?
There's no universally correct answer — shorter phasing periods (a few months) capture more of the statistical advantage of lump-sum investing, while longer periods (closer to a year) provide more protection against a poorly timed single entry point, so the choice comes down to your own comfort with the trade-off.
Can I change my mind partway through phasing in a lump sum?
Yes — there's nothing binding about a phasing schedule; many investors accelerate or pause based on how they're feeling about markets, though sticking to a predetermined plan generally removes emotional decision-making from the process, which is often the point of setting one in the first place.