Understand before you calculate
The calculator gives you a number. These guides explain what sits behind it: the mechanism, your country's tax rules, and the decisions that actually change the outcome.
Cumulo is first and foremost a calculation tool. But a result without an explanation is useless: the same monthly contribution produces very different outcomes depending on the horizon, the country you live in and the type of fund you choose. These six guides cover the decisions that weigh most, with the figures to back them.
They all draw on the same data as the rest of the site: thirty-one years of index series, forty-six European ETFs tracked daily, and the tax regimes of ten countries as they stand in January 2026. The methodology page sets out the sources and the limits of each calculation.
Compound interest, demystified
The mechanism, the rule of 72, and why starting fifteen years earlier beats contributing twice as much.
Read the guide →How much should you invest each month?
What €50, €100, €300, €500 or €1,000 a month produce over ten, twenty and thirty years — tax and inflation included.
Read the guide →ETF taxation: ten European countries compared
Ten countries, three very different ways of taxing, and a comparison table as of January 2026.
Read the guide →Belgium's capital gains tax, explained
10 %, a €10,000 annual allowance, and only the gains made after 31 December 2025.
Read the guide →PEA or brokerage account: the honest maths
17.2 % versus 30 %: the gap in figures, the PEA's three constraints, and when the brokerage account wins.
Read the guide →Accumulating or distributing: the real difference
Acc or Dist: the difference is not about performance, but about when the tax lands.
Read the guide →Where to start
You are starting out. Read compound interest first, then how much to invest each month. Together they cover most of the reasoning.
You already invest and want to optimise. Begin with ETF taxation: that is where the widest gaps sit, and they are routinely overlooked. Then accumulating or distributing.
You live in Belgium. The capital gains tax introduced in 2026 affects everyone, including portfolios built before that date — even though earlier gains are protected.
You live in France. PEA or brokerage account is probably the single most profitable decision an ordinary saver makes.