Investing a fixed amount every month: DCA, your mix and rebalancing

Investing a fixed amount on a fixed day each month is the simplest habit in investing. You don't need to guess the right moment, and each payment buys more units when prices are low and fewer when they are high.

When you already hold a large sum and drip it in over months instead, that is called dollar-cost averaging (DCA). Research finds investing it all at once usually ends ahead, but DCA lowers regret if prices fall soon after. This guide shows both, with a real index record.

Two different situations

If you invest part of your pay every month, you are not really choosing DCA. You are investing money as soon as you have it, which is what the research favours anyway. The question only arises when you hold a lump sum.

How averaging works

These numbers are an example. You invest $100 a month for four months in a fund whose unit price moves.

Check: 100 ÷ 10 = 10, 100 ÷ 8 = 12.5, 100 ÷ 5 = 20. You paid $400 for 52.5 units, an average of about $7.62 a unit. The average price over the four months was $8.25. Because a fixed amount buys more when prices are low, your average cost lands below the average price.

At month 4 the price is back at $10, where it started. Your 52.5 units are worth $525: a 31% gain on a market that ended flat. This only works if you keep buying through the fall.

What the research says about lump sums

Vanguard compared investing a lump sum at once with splitting it into three monthly parts. Its February 2023 paper used MSCI World returns from 1976 to 2022, an all-share portfolio, and measured results after one year.

• Investing at once beat spreading it out 68% of the time.
• Spreading it out still beat holding cash 69% of the time.
• Vanguard found similar results in the US, UK, Australia, Canada and the EU.

The reason is simple: markets rise more often than they fall, so money waiting in cash usually misses some of the rise. Vanguard's view is that most investors should invest a lump sum at once. DCA may suit people who would be badly shaken by a fall right after investing.

Money waiting to be invested can earn interest in a savings account or money-market fund. That narrows the gap a little.

A real record: the S&P 500, 2021 to 2026 (USD)

The S&P 500 is a stock index of 500 large US companies. Here, $1,000 goes in at the last close of each October from 2021 to 2025. Everything is valued at the close on 22 October 2026, the same date Investment Growth uses, from FRED's daily data.

Value on 22 October 2026, at 7,663.43: about $7,934 for $5,000 paid in, a gain of about 59%. Putting all $5,000 in at once in October 2021 would have grown to about $8,320, a gain of about 66%.

Two lessons follow. The 2022 fall made the second payment the best buy of the five, which is averaging at work. But over the whole period the market rose, so the money invested earliest did best, as the research predicts.

These figures are the index's price only: no dividends, fees or taxes. Five snapshots show one path and don't predict the next five years.

Try it with your numbersInvestment GrowthSee how investing a fixed amount every month could grow your wealth over time.

Deciding your mix

Monthly investing works best with a written plan for where each dollar goes. Most people split between three kinds of holding.

How much goes into shares depends mainly on when you will need the money and how you would react to a 30% fall. These mixes are examples to show the idea, not recommendations.

Write your own mix down. A target you set in a calm month is easier to keep in a scary one.

Rebalancing once a year

Over time, the part that rose fastest grows bigger than you planned. Rebalancing means moving money back to your target mix, usually once a year.

These numbers are an example. Your target is 60% shares and 40% bonds. You start with $10,000: $6,000 and $4,000. A year later, shares have risen 25% and bonds returned 4%.

Check: 6,000 × 1.25 = 7,500, 4,000 × 1.04 = 4,160, and 11,660 × 0.6 = 6,996.

Rebalancing makes you trim what rose and add to what lagged. You can often do it without selling, by pointing the next few monthly payments at the part that is below target. That can also avoid tax on gains in a taxable account.

Doing it in practice

Many brokers offer an automatic monthly investment plan. Pick a fixed day, such as the day after payday, and set it up once. Our first order guide covers order types, fees and FX costs.

Your plan

1. Settle your base: no expensive debt, and an emergency fund in place.
2. Pick a monthly amount you can keep paying in a bad month, and a fixed day.
3. Write your target mix for shares, bonds and cash, based on when you need the money.
4. Choose how to invest: a broad fund for simplicity, or shares if you will put in the work.
5. Automate it with a standing transfer or a broker's monthly plan.
6. If you hold a lump sum, decide in writing: all at once, or spread over a fixed number of months.
7. Rebalance once a year, using new money first, and check your results in Investment Growth.

What to watch out for

• Stopping in a fall. Averaging only helps if you keep buying when prices drop.
• Spreading a lump sum forever. Pick an end date; cash left waiting usually misses gains.
• Fees on small orders. A $2 fee on a $100 purchase is 2% before you start.
• Nominal vs real gains. Compare results after inflation, not just in dollars.
• Past returns. The S&P 500's rise from 2021 to 2026 doesn't predict the next five years.