About the Dollar Cost Averaging Calculator
Most calculators on this subject show what averaging in produces and stop there, which quietly implies it beats the alternative.
Over the periods anyone actually invests for, it usually does not — and saying so is more useful than not saying so.
This dollar cost averaging calculator computes both strategies and reports which won, along with the genuine mathematical effect of averaging, which is real and almost never demonstrated.
How to Use the Dollar Cost Averaging Calculator
Enter the total to invest and how many periods to spread it over.
Starting price can be anything — the comparison depends on the shape of the path, not its level.
Market drift is the underlying annual trend, and the path is one of three shapes: steadily rising, falling then recovering, or rising then falling back.
The paths are deterministic rather than random. A calculator that returns a different answer each time cannot be checked, and the point does not need simulation — but each shape is one case, not a distribution, which matters when reading the result.
Step-by-Step Example
£12,000, spread over 12 periods, 7% annual drift, steadily rising.
Averaging in: 1,000.00 a period → 116.36 units → 12,450.30
All at once: 12,000.00 at 100.00 → 120.00 units → 12,840.00
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The lump sum wins by: 389.70Why the Lump Sum Usually Wins
The reason is simple and it is not about strategy: markets rise in more periods than they fall.
Money invested sooner is therefore invested at lower prices more often than at higher ones. Money held back to be drip-fed spends most of its time sitting out of a market that is, on average, going up.
Historically, a lump sum has beaten averaging the same money in roughly two times in three.
That is a statement about frequencies, and this calculator shows single paths rather than a distribution — so treat the two-in-three as context for the result rather than something the calculator has demonstrated.
When Averaging In Wins
Switch the path to falls, then recovers:
Averaging in: 146.72 units → 15,699.36
All at once: 120.00 units → 12,840.00
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Averaging in wins by: 2,859.36A gain of over £2,800 on the same £12,000, because the money went in while prices were low.
This is the minority case, and it is exactly the case where the lump sum was a mistake. Which is the real argument for averaging in: it makes the bad paths less bad. The gap between the two strategies is widest precisely when the lump sum turns out badly.
Paying a little expected return for a narrower range of outcomes is insurance. Insurance costs something, and that is not an objection to it.
The Actual Arithmetic of "Averaging"
Here is the part that is real and almost never shown.
Buying a fixed amount each period, rather than a fixed number of units, means the same money buys more units when they are cheap and fewer when they are dear. Your average cost therefore lands below the average price:
Rising path: average cost 103.13 against average price 103.17
Dipping path: average cost 81.79 against average price 83.44Mathematically, your average cost is the harmonic mean of the prices and the average price is their arithmetic mean. The harmonic mean is always at or below the arithmetic mean, with equality only when every price is identical.
So the effect is genuine and always in your favour — but notice its size. On a steadily rising path it is four pence on a hundred. On a bumpy one it is substantial. The effect scales with volatility, not with the strategy.
And note what it does not say: buying below the average price is not the same as ending up with more money, because the lump sum bought everything at the lowest price in a rising series. Both things are true at once.
The Comparison Most People Do Not Need
Here is the honest caveat to this entire page:
Most people never have a lump sum.
Saving monthly out of income is not a strategy choice between two options. It is the only option on offer. Comparing it against investing a lump sum nobody has is a false comparison, and it produces needless anxiety about a decision that was never available.
The comparison genuinely applies when you have received a windfall — an inheritance, a bonus, a house sale, a pension lump sum — and are deciding what to do with it. Then it is a real choice, and then the two-in-three finding matters.
Understanding Your Result
Which won is the direct comparison on the path you chose.
Averaging in and all at once show the units each strategy bought and what they were worth.
Average cost against average price demonstrates the harmonic mean effect.
Worth knowing puts the result in context — whether this was the usual outcome or the minority one, and why that matters.
When Should You Use This Calculator?
When you have a windfall to invest. This is the case the comparison is for.
To understand why averaging in feels safer. It is, in the sense that matters — a narrower range of outcomes.
To stop worrying about monthly saving. If you are investing out of income, this comparison does not apply to you.
To see what volatility does to the averaging effect. Compare the rising and dipping paths.
Common Mistakes
Assuming averaging in is safer in every sense. It has a lower expected return and a narrower spread. Those are different things.
Reading one path as evidence about frequencies. A single path is an illustration.
Treating a lower average cost as a better outcome. It is neither necessary nor sufficient for finishing ahead.
Averaging in over years rather than months. The longer the money sits out, the more expected return is given up, and the insurance stops being worth the premium.
Comparing against a lump sum you do not have. Most monthly investing is not a strategy choice.
Forgetting dealing costs. Twelve purchases cost more in fees than one, which narrows the gap further against averaging in.
Every figure here is an estimate for planning. Markets do not follow smooth paths, and this is not financial advice.