Skip to content
Log in
← Glossary

lump sum

Investing an available amount all at once instead of in parts over time.

A lump sum means investing money you already have all at once, instead of feeding it into the market in parts. The usual question is "lump sum or dollar-cost averaging?", for example with a bonus, an inheritance or savings that have been sitting in cash.

Because markets rise in more years than they fall, money that is invested earlier usually spends more time growing, so investing at once tends to end higher more often than spreading it out. The cost is regret risk: if a fall arrives right after you invest, the whole sum takes the hit, and that memory is what makes many people prefer to spread.

Neither choice is wrong if you understand the trade-off. Investing at once is the better bet on average; spreading over a few months buys a smaller worst-case regret at the price of a slightly lower typical result. What matters most is deciding the rule in advance and not reopening it every week.

Example

Zapas compared two ways to invest $12,000 on its synthetic Tessaro World history: all at once, or $1,000 at the start of each of the first 12 months. Across 31 ten-year windows, investing at once ended higher in 21 of them (67.7%).

The two extreme windows tell the story. A start in year 23, just after the crash bottom: $35,023 at once versus $29,280 spread. A start in year 22, just before the bottom: $23,649 at once versus $27,884 spread. Nobody knows in advance which year they are in, which is why Zapas shows both facts and lets the user choose.

Common mistakes

  • Treating "lump sum usually wins" as "lump sum always wins"; about one window in three went the other way here.
  • Spreading over so many months that the money mostly sits in cash for a year or more.
  • Investing a lump sum you may need within a few years, when a fall would force a sale at a loss.
  • Waiting for a dip before investing, which turns the decision into market timing.

Learn it in the course