Lump Sum or Drip-Feed: How to Invest a Windfall

An inheritance, a bonus, a house sale, a pension lump sum: suddenly there is real money sitting in cash, and the question is whether to invest it all at once or feed it in gradually. This is one of the few investing questions with a clear statistical answer — and a good reason many people should ignore it.

What the evidence says

Markets rise more often than they fall — historically roughly two years in three. So money invested immediately has, on average, beaten the same money drip-fed over 6–12 months in the clear majority of historical periods studied (the well-known research puts it around two-thirds of the time, across markets). Waiting in cash is, statistically, betting on a fall that usually does not come. If you are a spreadsheet, invest the lot today.

Why drip-feeding survives anyway

You are not a spreadsheet. The scenario that destroys investors is not "slightly lower average returns" — it is investing £100,000 on Monday, watching it become £80,000 by Christmas, selling in despair, and never returning. Drip-feeding (pound-cost averaging) is insurance against that behavioural catastrophe: by entering gradually, a crash mid-way means your remaining cash buys cheaper units, converting a terrifying headline into a mild consolation. You pay a modest expected-return premium for a dramatically higher chance of staying the course — often a trade worth making, because compounding only pays people who remain invested.

A decision rule that works

• Small relative to your existing portfolio (a bonus equal to a few months of normal contributions): just invest it. Ceremony is unnecessary.
• Large and you are experienced (you have held through at least one real downturn without flinching): lump sum, aligned with your existing asset allocation.
• Large and you are new, or you already feel nervous: split it over a fixed schedule — commonly 3, 6 or 12 equal monthly instalments, automated so the plan cannot be renegotiated with yourself each month. Longer than 12 months stops being caution and becomes market timing.

The mistakes on either side

The lump-sum mistake: investing before the boring foundations — emergency fund intact, expensive debt cleared, and the money's actual time horizon confirmed as five-plus years. A house deposit needed in two years does not belong in shares at all. The drip-feed mistakes: schedules that quietly stall ("I'll do the next instalment when things look calmer" — that is timing, not averaging), and cash languishing at zero interest during the schedule — park it in the best easy-access rate or your ISA's cash facility while it waits. And on wrappers: a large sum will exceed the £20,000 annual ISA allowance — the standard play is filling this year's ISA immediately, holding the remainder in a taxable account, and "bed-and-ISA-ing" chunks across future Aprils, with pension contributions absorbing more where appropriate (ISA vs pension).

The honest summary

Lump sum wins on average; drip-feeding wins the psychology; both beat the actual worst option, which is cash indefinitely while you wait for certainty that never arrives. Choose by temperament, automate the choice, and judge yourself in ten years, not ten weeks.

This is general education, not personalised financial advice. Investing involves risk, including the risk of losing money, and past performance is not a guide to future returns. Nothing here recommends any specific investment — for anything genuinely complex or high-stakes, speak to a regulated financial adviser.