The German investing internet’s most-typed tool request — ETF Sparplan Rechner — deserves more than a widget: it deserves the numbers literacy that makes any calculator useful. This guide pairs this site’s own compound growth calculator with the four inputs that actually decide savings-plan outcomes — rate assumptions, cost drag, time, and the sequence reality every projection hides — so the Rechner’s output becomes a plan instead of a fantasy.
Data framing per the returns guide, August 2026.
Input one: the rate — where every Rechner lies first
The projection’s dominant input is the assumed return, and the honest bands are the returns guide’s: 5–6% real (inflation-adjusted) for world equity is the defensible planning band — nominal 7–9% with inflation’s slice removed — and every Rechner defaulting to 8%+ nominal without an inflation line is selling a feeling. The calculator discipline: run three scenarios (4%, 5.5%, 7% real), read the spread as your uncertainty (across thirty years the band’s ends differ by six figures on ordinary contributions — the honest width of the future), and plan commitments on the low line while letting the high one stay a pleasant possibility. One number never decides a retirement; a band, honestly held, does.
Browse the full series in our European ETF tax pillar.
About the numbers: every worked example here follows our published methodology behind our tax examples — assumptions, rounding and official sources included.
Comparing funds is step one — the Masterclass turns it into a complete plan — fund choice, broker setup, YOUR country’s taxes and the first purchase, in 55 text lessons.
See the full curriculum — €147, 30-day guarantee →Input two: costs — the lever you fully control
The Rechner’s most underused feature is the cost line, and this site’s category shelves are its data source: the same world exposure runs 0.12–0.50%+ across wrappers (the comparison), execution costs vary from free to percent-level by platform, and wrapper layers (the Police stack, branch-channel premiums) compound identically to returns — negatively. The worked intuition: on €300 monthly across thirty years at 5.5% real, each 0.3% of annual cost consumes roughly a year’s worth of final contributions — the difference between fee tiers is a car, not a coffee. Run your actual stack in the calculator once; the redirect decisions this site’s guides keep prescribing price themselves instantly.
Input three: time — the variable that embarrasses the others
The compound curve’s convexity is the Rechner’s real lesson: the final decade of a thirty-year plan typically generates more growth than the first two combined, start-age dominates contribution-size across every realistic scenario (the twenty-five-year-old’s €150 beats the forty-year-old’s €400 in most bands), and pauses cost quadratically more than their duration suggests because they excise the curve’s steep end. The planning translations: start before optimizing (the wrapper debates are basis points; the start date is percentage points), automate against the pause temptation, and treat contribution raises (salary-indexed plans) as the curve’s quiet accelerator — the input the static Rechner underweights and real plans should hard-code.
Input four: the sequence — what no projection shows
Every Rechner draws a smooth curve; every real plan lives the distribution — and the savings-plan’s beautiful secret is that the volatility the curve hides works for accumulation: the drawdown years buy cheap units on schedule (the mechanical courage no investor has manually), the recovery years reprice them, and the plan’s realized outcome across historical windows routinely beats the smooth-curve projection run at the same average — volatility harvest, delivered by automation. The mirror warning for the withdrawal phase (sequence risk, the stability-layer machinery) lives in the returns guide; the accumulator’s version is one line: the Rechner’s smooth curve is the pessimistic rendering of an automated plan — the only projection in finance that understates its own product.
Reader scenarios
“Which Rechner should I actually use?” The site’s calculator runs the scenarios above; the backtesting tools reconstruct real historical windows for the sequence intuition — the pair covers planning and calibration.
“€100 monthly feels too small to matter — does it?” Run it: thirty years at the honest band lands in solid five figures real — a pension supplement, not a fortune, and infinitely more than the €0 alternative; the curve’s convexity forgives small starts and punishes late ones.
“Should the Rechner include the Vorabpauschale and taxes?” For precision, yes at the margin — the deferral mechanics trim the net band slightly; the guide’s three-scenario discipline already absorbs the effect inside its width. Bands beat decimals.
“My projection says I’ll be a millionaire — trust it?” Trust the discipline it demands, not the number it prints — the high-scenario line is a possibility, the low line is the plan, and the automation between them is the entire strategy.
“And the calculation to run tonight?” Your actual plan — real contribution, real costs, the three bands — against the version with one improvement (the cost redirect, the raise indexation, the earlier start already passed but the next-best today): the delta is tonight’s decision, priced. The Rechner’s thousand searches wanted this one number all along.
Band, costs, start, raises, behavior — five inputs, one instrument, decades of output.
The curve is patient; the inputs are yours; the automation collects what both provide.
Run it tonight; automate it tomorrow; audit it yearly — the Rechner’s complete lifecycle.
Numbers into decisions, decisions into automation, automation into decades.
The projection serves the plan; the plan serves the decades; tonight serves the start.
“Final input check?” Real inflation honesty — every nominal fantasy dies at the real-band conversion; the guide’s three scenarios already speak real, and the plan built on them survives the arithmetic every optimistic widget skips.
Quick-start checklist
- Three scenarios: 4 / 5.5 / 7% real — commit on the low line.
- Your true cost stack in the cost line — once, honestly.
- Start date before optimization — the dominant input.
- Salary-indexed contribution raises hard-coded.
- Reread the sequence section every drawdown — it’s working for you.
Verdict
The Sparplan Rechner search wants a number; the plan needs a band, a cost line, a start date and the automation that harvests what the smooth curve hides. This guide converts the widget into the four decisions that actually move outcomes — and the calculator into what it should be: the place where this site’s every framework (the return bands, the fee shelves, the wrapper tolls, the sequence machinery) becomes one household’s arithmetic. Run it once properly, automate the result, and let the curve’s convexity do the decades.
The Rechner’s missing feature: the behavior line
Every calculator models contributions, returns and costs; none models the variable that dominates realized outcomes — the holder’s behavior — and the guide’s honest supplement prices it from the research. The documented gaps: investor returns trail fund returns by percentage points annually across every studied market (the timing tax — buying after rallies, pausing after crashes), plan-interruption rates spike exactly in drawdowns (the pause temptation striking when the mechanism pays most), and the automated-plan cohorts close most of the gap without any skill — the automation being the behavioral technology. The Rechner translation: your projection’s honest expected value isn’t the smooth curve’s number but that number times your probability of leaving the plan alone — a multiplier the automation raises toward one and manual investing reliably lowers. The guide’s four inputs decide the curve; this fifth decides how much of it you collect — and it’s the only input where the intervention is free: automate, delete the app if needed, and let the Rechner’s math meet a holder who never interrupts it. The best calculator feature was never in the calculator.
The Rechner as family instrument: three calculations worth running together
The calculator’s best use is household-scale. The child calculation: eighteen years of modest monthly amounts at the honest bands — the result that reliably stuns parents (the convexity working on the longest runway in the family) and funds more junior depots than any advertisement; run it the week the child is born. The gap calculation: each partner’s pension projection against the household’s spending floor — the shortfall, honestly banded, converts into the monthly contribution that closes it; the Rechner as retirement’s honest broker, run once a year together. The trade-off calculation: the concrete decision translator — the €200 subscription bundle or the €200 plan line, priced at thirty years of the band’s center; not as guilt arithmetic but as visibility — the household choosing with the compounding in view chooses better either way. The widget the searches wanted was always this: not a projection machine but a decision translator — and the family that runs the three calculations annually has done more financial planning than most advisory relationships ever deliver.
The guide in one honest paragraph
The Rechner search wants a number and needs a discipline: three return scenarios committed on the low line, the true cost stack entered once, the start date treated as the dominant input, contribution raises hard-coded, and the behavior multiplier — the input no widget models — maximized by automation. The calculator converts this site’s every framework into one household’s arithmetic; the smooth curve understates the automated plan it projects; and the family that runs the three calculations yearly out-plans most advisory relationships. The number was never the product — the decisions were, and now they have their instrument.
One more scenario: the Rechner meets real life’s interruptions
“The projection assumes thirty unbroken years — parental leave, job changes and emergencies exist. Does the math survive?” Better than the anxiety suggests: the contribution pauses that life actually forces (months, occasionally a year) trim the terminal sum roughly in proportion to their share of total contributions — small, if the plan resumes — while the destructive version (liquidating the accumulated capital mid-curve) is the only interruption the convexity truly punishes. The planning translations: the emergency buffer (the cash layer every framework here builds first) exists precisely to make pauses instead of liquidations, the plan’s resumability matters more than its perfection, and the Rechner scenario worth running is the honest one — your contribution rate at 85% persistence — which lands remarkably close to the smooth curve’s promise. Life interrupts contributions; only panic interrupts compounding — the distinction the buffer guards and the projection, honestly read, already priced.
The reference desk: Europe’s ETF database used like a professional — the reading order, the five workflows, the braid — is taught in the justETF guide.
Automating the contribution: read the ETF savings-plan guide.
Frequently asked questions
What return should I enter for an MSCI World Sparplan?
The 5–6% real band as the planning center — the returns guide’s derivation; nominal entries need the inflation line subtracted honestly.
Does the calculator handle dynamic contribution increases?
Model raises as scenario steps — the salary-indexed accelerator is the feature worth approximating even where widgets simplify.
How accurate are 30-year projections?
Precisely inaccurate and directionally invaluable — the band’s width is the honest answer; the discipline the projection installs is its real product.
Sparplan versus Einmalanlage — what does the math say?
Lump sums win in expectation (time in market), plans win behaviorally and harvest volatility — the classic fork; automate whichever your psychology sustains.
Should I pause the plan in expensive markets?
The sequence section’s answer: the plan’s edge is buying through everything — pauses excise the mechanism; valuation nerves belong in allocation, not the calendar.
Where do taxes enter the projection?
As a band-trim at the margin under German mechanics — deferral keeps most compounding gross; the calculator’s scenarios absorb the effect, and the wrapper guides price the exceptions.
Does the site’s calculator model monthly compounding?
The standard contribution-compounding mechanics — the scenario discipline matters more than the compounding convention’s decimals.
What about inflation-adjusting the contributions themselves?
The salary-indexed raise modeling approximates it — real contribution maintenance is the feature the checklist hard-codes.
Cited sources: The compound growth calculator, Bundesbank long-run return data, and Historical backtesting data. Verify current fund figures before investing. Educational content, not financial advice.
European UCITS/ETF investor with 11+ years building a global dividend and index portfolio, and founder of Financial Expert Class. I write every guide here from first-hand experience investing across EU brokers and tax regimes — not theory. More about me →