How To Dollar-Cost Average

Dollar-cost averaging means investing the same amount of money at regular intervals, regardless of price. The schedule is fixed, the amount is fixed, and no judgement about the market enters the decision at all.

The mechanism that makes it work is mechanical. A fixed dollar amount buys fewer shares when prices are high and more when they are low, so the average cost per share ends up below the average price per share over the same period.

That effect is not a free lunch. It is the same arithmetic that makes a fixed dollar amount buy less when prices rise, and it produces no gain at all if the price rises steadily from the very first purchase.

The real benefit is behavioural rather than mathematical. Averaging removes the decision of when to start, which is the decision most investors get worst, and it stops a painful drop from feeling like a reason to stop investing.

Averaging into a falling market lowers the average cost with every contribution, which is why a long decline is less alarming for someone investing monthly than for someone holding a lump sum.

Timing within the month matters less than most people expect, but it is not nothing. Contributing at the start of the month gives the money one more month of compounding than contributing at the end, worth about half a percentage point a year at a 6% return.

The maths is an ordinary annuity. A stream of equal payments at a constant return has a closed-form future value, which is why a ten-year projection can be computed exactly rather than simulated.

The closed form assumes a constant return. Real markets deliver a sequence of returns that is lumpy and unpredictable, so the formula gives an expected value rather than a forecast.

Sequence risk is the other side of the same coin. A £250 monthly investor sees far less variation in the outcome than a lump-sum investor, because later contributions are not exposed to the early-year losses.

Lump-sum investing beats averaging most of the time on average. If markets rise over long periods, investing everything immediately captures more of that rise, but it does so with far more volatility and a much worse experience in a bad start.

Costs matter more than the timing. A high ongoing charge compounds against the portfolio exactly as the return compounds for it, and the drag from fees is far larger and far more certain than the benefit from moving a payment to the start of the month.

Averaging works best when it is automatic. A standing order on the day after payday turns the intention into an event that happens without a decision, which is the whole point of the approach.

The contribution needs to grow, or inflation quietly erodes it. £250 a month for twenty years is a fixed commitment in a world where prices double, so increasing the amount annually is closer to the spirit of the strategy.

The calculator projects a monthly contribution over a chosen horizon at a chosen annual return, adds any initial lump sum, and reports how much of the final total is money invested and how much is growth.