Drip Feed vs Lump Sum Calculator

You inherited some money. Sold a house. Got a bonus that finally cleared into the current account. Now you have a chunk of cash sitting in a savings account, and you need to decide whether to invest the lot on Monday or feed it in over the next twelve months.

The maths answer is lump-sum. The psychology answer is drip-feed. The drip-feed vs lump-sum calculator simulates both side by side so you can see what the gap actually looks like for your numbers, your time horizon, and your tolerance for being wrong at the worst possible moment.

Contents

• What drip-feed and lump-sum investing actually mean
• How the calculator works
• The Vanguard 2012 study and what it really says
• Why drip-feed still has a case
• Fitting either strategy into UK ISAs and SIPPs

What drip-feed and lump-sum investing actually mean

Lump-sum investing is exactly what it sounds like. You take the whole windfall and buy your chosen funds on day one. The full amount starts compounding immediately, and your time in the market is maximised from the first contract note.

Drip-feed investing, also called pound-cost averaging or DCA (the dollar-cost-averaging name imported from the US), splits the windfall into equal chunks and invests one chunk per period over a chosen window. The classic version is twelve monthly tranches: £24,000 becomes twelve £2,000 buys, one per month. While the cash is waiting to be deployed it usually sits in a money market fund or cash account inside the same wrapper, earning a near-base-rate yield.

The mechanical difference is simple. The strategic difference is what each one is actually solving for. Lump-sum optimises for expected return: markets trend up, so being fully invested earlier wins on average. Drip-feed optimises for regret: by spreading the entry across many prices, no single timing decision can dominate the outcome. If the market crashes the week after you lump-summed, you live with the full hit. If it crashes during a twelve-month drip-feed, only the chunks you have already deployed take damage, and the rest buys in cheaper.

Both descriptions are correct. The argument between them is really an argument about which risk you care about more: the risk of being wrong on the day, or the risk of leaving expected return on the table to feel better.

How the calculator works

The calculator takes six inputs. Total amount to invest is your windfall. Drip-feed frequency lets you pick daily, weekly, fortnightly, monthly, or quarterly tranches. Monthly is the standard choice and the one most behavioural research supports. Drip-feed over sets how many of those periods you want to spread across. Twelve months is the conventional window. Estimated annual return is your nominal expected return; 7-8% is a defensible assumption for a globally diversified equity portfolio, 4-5% if you want to model real (post-inflation) terms. Market volatility controls how bumpy the simulated path is, mapped from 0 (smooth deterministic growth) through 5-6 (typical equity market) up to 10 (turbulent).

The engine runs a Monte Carlo simulation under the hood. For any volatility above zero, it generates thousands of randomised return paths using the annual return as the mean and volatility as the standard deviation, then runs both strategies through each path. The summary cards show the median (50th percentile) outcome for each, the chart shows the median line with shaded 10th-to-90th percentile bands, and an amber callout reports the win rate ("lump sum comes out ahead 67% of the time" or whatever your settings produce).

The year-by-year table breaks the gap down annually and exports to CSV with full percentile data. Set volatility to zero if you want the clean deterministic case: that strips out the randomness and just compounds both strategies at the headline rate, which lets you see the pure expected-value gap before psychology enters the picture.

The Vanguard 2012 study and what it really says

The reference point for almost every modern argument about lump-sum vs drip-feed is a 2012 Vanguard research paper called Dollar-cost averaging just means taking risk later. The team backtested both strategies across rolling historical windows in the US, UK, and Australian markets going back to 1926. The headline finding reported by Vanguard: lump-sum investing beat a 12-month DCA strategy in roughly two thirds of 10-year periods across all three markets. On average, according to the paper, lump-sum left US investors with a small mid-single-digit percentage more wealth at the end of the 10-year window, with broadly similar directional results in the UK and Australia.

Subsequent replications by Vanguard and independent researchers have broadly confirmed the directional result. The mechanism is not mysterious. Historically, equity markets have been positive in roughly two thirds of years on a rolling basis, so any strategy that keeps cash on the sidelines longer than necessary tends to lose to one that does not, on average. Past performance is not a guide to future results.

The important word is average. Vanguard's own paper is careful about this. The one third of periods where DCA wins are not random: they cluster around market peaks and the start of bear markets. If you happened to lump-sum into the FTSE in January 2000, January 2008, or February 2020, you spent the next year watching the spreadsheet answer look very bad indeed. The 67% win rate is real, and so is the 33% loss rate, and the loss rate concentrates exactly where it does the most psychological damage.

That is the honest summary. Lump-sum is the higher expected-value strategy. It is not the strategy with the lowest worst-case outcome, and worst-case outcomes are what cause investors to sell at the bottom and lock in permanent losses.

Why drip-feed still has a case

If you only care about expected return and you have the emotional discipline of a granite slab, the Vanguard data is conclusive: lump-sum wins. Most people do not have the emotional discipline of a granite slab, and that is where the case for drip-feeding actually lives.

The first argument is regret minimisation. If you put £80,000 into a global tracker on a Monday and the world drops 25% by Friday, you are sitting on a £20,000 paper loss with the decision still fresh. That kind of loss in a short window has a documented behavioural effect: investors panic-sell, swear off equities, or sit in cash for years waiting for "the right time" that never comes. Drip-feeding doesn't change the expected outcome of the market itself, but it does cap the worst-case version of your own decision. Six tranches deployed before a crash is a much easier mistake to live with than one big tranche the day before.

The second argument is sequence-of-returns risk at the entry point. The order in which returns arrive matters most when the pot is largest relative to your contributions; we cover the mechanics in detail in sequence-of-returns risk. A big initial loss on a freshly invested lump sum is mathematically the same as a big loss in early retirement: it permanently lowers the base that all future compounding works from. Drip-feeding spreads that exposure.

The third argument is psychology, full stop. A plan you can stick to beats a mathematically optimal plan you abandon when the market takes 30% off the headline number. If drip-feeding is the only way you actually get the money into the market, it wins your specific scenario regardless of what the average says. The cost of that psychological insurance is real and visible (the calculator will quote it back at you in pounds), but for an investor who would otherwise stay in cash for fear of a crash, it is excellent value.

Fitting either strategy into UK ISAs and SIPPs

UK wrapper limits force a hybrid on anyone with a meaningful windfall. The ISA allowance is £20,000 per tax year, so a £100,000 inheritance can only be sheltered fully over five tax years. That is not optional: you simply cannot pay £100,000 into a Stocks and Shares ISA in April and call it done. The practical pattern is to invest the surplus in a General Investment Account (GIA) in the same funds you would hold inside the ISA, then "Bed and ISA" each April: sell £20,000 in the GIA, immediately repurchase the equivalent inside the ISA, repeat the following tax year.

CGT friction matters here. The annual exempt amount dropped to £3,000 for 2024/25 onwards, and gains above it are taxed at 18% (basic rate band) or 24% (higher rate band) from 30 October 2024. Crystallising gains in tranches that use the £3,000 allowance each year is materially cheaper than letting five years of gains stack up and trigger one large taxable event. Married couples and civil partners can use inter-spouse transfers (no CGT on the transfer itself) to double the available wrapper capacity.

For pensions the wrapper itself is more accommodating. The standard annual allowance is £60,000 for most earners in 2026/27, with taper down to £10,000 for high earners with adjusted income above £260,000. That means a higher earner can lump-sum a sizeable windfall straight into a SIPP and claim immediate tax relief, with no equivalent of the ISA's bed-and-repeat dance. The trade-off is access: pension money is locked until age 55, rising to 57 from April 2028. For windfalls earmarked for retirement and an investor under 50, the SIPP is usually the better wrapper even if the lump-sum-vs-drip-feed decision is still psychologically uncomfortable.

Mechanically, drip-feeding is also easier than it used to be. Fractional shares on platforms like Trading 212, Vanguard Investor, and iWeb mean you can invest exactly £1,666.67 each month without leaving an awkward cash residual. Most platforms support recurring buys you can set up once and ignore for a year. The compound interest calculator and time-in-the-market calculator are the right tools to use alongside this one when you want to see how the entry-timing decision compounds across the whole holding period rather than just the deployment window.