If you buy the same ETF month after month — the sensible European default — you soon own units bought at half a dozen different prices, and “what did I actually pay?” stops having an obvious answer. This average cost calculator answers it: enter each purchase (units and price, up to eight rows) and it returns your total units, total invested and average cost per unit. Add today’s market price and it shows your unrealized gain or loss in euros and percent — the honest baseline for every hold-or-rebalance decision.
By Kestutis Balciunas · Last reviewed: 17 July 2026 · Figures and rules are 2026-vintage estimates — verify current data before acting. Educational tool, not advice.
How average cost is calculated
The maths is deliberately simple: total invested = the sum of units × price across every purchase, and average cost per unit = total invested ÷ total units. Worked example: you buy 10 units at €80, 10 at €95 and 5 at €110. Total units 25, total invested €2,300, average cost €92.00 — note that this is not the simple average of the three prices (€95), because the sizes of the purchases differ. If the ETF trades at €105 today, your position is worth €2,625 for an unrealized gain of €325, or +14.1%. This weighted average is exactly what a steady monthly savings-plan approach produces over time: more units when prices are low, fewer when they are high.
Why your cost basis matters for tax
Your cost basis determines the taxable gain when you eventually sell, so keeping a clean record of every purchase — including small savings-plan executions and reinvested distributions — is one of the highest-value admin habits in investing. Average cost is what most EU brokers show on statements, but be aware that some countries mandate FIFO (first-in-first-out) for the tax calculation: Germany is the prominent example, where a partial sale is deemed to sell your oldest units first, which usually means a larger taxable gain in a rising market than the average method would suggest. Verify your country’s required method before filing — our European capital gains tax comparator shows how differently the resulting gain is taxed across 30 countries, and German investors will also want the Vorabpauschale calculator, since prepaid amounts raise your cost basis too.
Reading the result
The average cost line is your break-even price before costs: above it you are in unrealized profit, below it in unrealized loss. Resist the urge to “average down” purely to move this number — the market does not know your entry price, and each new purchase should stand on its own merits. Where the per-lot detail genuinely pays off is tax-loss harvesting: knowing which individual lots are underwater lets you realise losses deliberately where your country’s rules make that worthwhile.
Average cost vs FIFO — the same sale, two different gains
To see why the method matters, extend the worked example. Suppose you sell 10 of your 25 units at €105. Under the average method the taxable gain is 10 × (105 − 92) = €130. Under FIFO you are deemed to have sold the ten oldest units — the ones bought at €80 — so the gain is 10 × (105 − 80) = €250, nearly double, from the identical sale. Over the full life of the position the totals converge (the cheap units get taxed eventually either way), but FIFO pulls tax forward in rising markets, which matters for cash-flow planning and for any strategy that relies on partial sales. The defence is the same in every regime: keep a dated record of every purchase — broker, date, units, price, fees — so that whichever method your tax office requires, you can compute it in minutes rather than reconstructing years of statements. Figures here are illustrative, exclude broker fees and taxes, and nothing on this page is financial or tax advice.
These numbers make more sense with the guide
The free 12-page Starter Kit turns them into decisions: your first fund, a safe broker, and the tax basics for your country.
Frequently asked questions
Q: Is average cost the same as what my broker reports?
A: Usually yes for display purposes — most EU brokers show a weighted average purchase price on your statement. But the tax calculation on a sale may legally follow a different method in your country, so treat the broker’s average as informational and confirm the required method before filing.
Q: What is the difference between average cost and FIFO?
A: Average cost treats all your units as bought at one blended price. FIFO (first-in-first-out) assumes a partial sale disposes of your oldest units first. In a rising market your oldest units carry the biggest gain, so FIFO typically produces a larger taxable gain on a partial sale. Germany mandates FIFO; other countries differ — verify your own rule.
Q: Do reinvested dividends change my average cost?
A: Yes. Units bought with reinvested distributions are purchases like any other: add them as a row with the reinvestment price. Forgetting them overstates your gain — and can mean paying tax twice on the same money in some regimes.
Q: Should I buy more when the price drops to lower my average?
A: Lowering your average is a side effect, not a reason. The market does not know or care what you paid, so judge each purchase on whether you would buy the ETF today at today’s price. A disciplined fixed monthly plan achieves the averaging automatically without the temptation to time it.
Q: Why does the calculator cap at eight purchases?
A: It is built for quick checks — a year or two of savings-plan buys, or grouped lots. For longer histories, group purchases made at similar prices into one row (total units and the weighted price); the average comes out identical.