Investment Calculator
Compound interest and ROI calculator for long-term investment planning.
What is an investment calculator?
This tool projects how a starting balance plus regular contributions can grow when returns compound over time. You set currency, contribution cadence, expected return, compounding frequency, horizon in years, and optional knobs such as inflation, tax on gains, management fees, and annual contribution increases.
Use it to sanity-check retirement or savings goals, compare conservative vs optimistic return assumptions, or illustrate the effect of fees and taxes. It is a simplified cash-flow model, not a forecast of market prices, fund performance, or tax law in your country.
Everyday examples
Short scenarios people model before talking to a planner: retirement pace, a house deposit, education savings, and volatile assets.
🏖️ Retirement pace
Set horizon to your target retirement year, add employer + personal contributions, and stress-test return between 4% and 8% to see how sensitive the ending balance is.
🏡 House deposit
Use a shorter horizon (3-7 years), lower return (cash or short bonds), and monthly contributions to see if you reach a down-payment goal on time.
🎓 Education fund
Start with a small lump sum at birth, monthly family contributions, moderate return, and stop at age 18 to compare with expected tuition bands.
📈 Stocks or crypto “what-if”
Temporarily raise the return to illustrate upside, then drop it below inflation to see downside; keep tax and fee fields realistic so the net line stays honest.
💼 Side income invested
Model irregular boosts as a higher annual contribution increase instead of guessing monthly noise.
🧾 Fee comparison
Duplicate scenarios: one with 0.2% annual fee and one with 1.0% - the gap after 25 years shows why expense ratios matter.
How this calculator works
Each month the tool accrues return on the current balance, subtracts a monthly slice of any management fee, then adds your scheduled contribution (which can grow once per year). At the end it compares ending wealth to what you put in to show gains, optional tax on gains, and optional inflation-adjusted purchasing power.
When should you use this?
Ballpark savings targets
You want a single number for “if I keep investing X at Y% for Z years, what might I have?” without building a spreadsheet.
Sensitivity checks
You need to see how +1% return or +0.5% fees changes the outcome before committing to a fund or allocation.
Teaching compound growth
You are explaining why starting earlier or increasing contributions beats chasing an extra percent of return.
Comparing contribution cadence
You are deciding between investing a bonus annually vs smoothing the same cash monthly.
Inflation intuition
You want the ending balance in nominal euros or dollars plus a second line that shows what that might buy in today’s purchasing power.
Before professional advice
You prepare numbers to bring to a financial planner or tax adviser instead of arriving with no framework.
Common mistakes
Using last year’s stock return forever
Markets swing; long plans should use blended conservative assumptions, not one hot year.
Ignoring fees and taxes
Even small annual fees drag compounding; gains-only tax fields are simplified but remind you that net matters.
Mixing real and nominal rates
If you already lowered the return for inflation, keep the inflation field near zero to avoid double-counting.
Monthly contribution with annual frequency
Check Advanced: contribution frequency must match how you actually invest, or totals will be off.
Expecting tax precision
Real accounts have allowances, deferred wrappers, and different rates for dividends vs gains - this model is indicative only.
Horizon longer than the product fits
A 40-year equity plan is common, but the tool still cannot predict sequence-of-returns risk or job interruptions.
Core growth relationship (simplified)
The engine is a discrete monthly loop with periodic additions. The table below shows how different levers change the story you tell yourself - not a guarantee of future performance.
balance[0] = initial\nbalance[t] = balance[t-1] * (1 + periodicReturn) * (1 - periodicFee) + contribution[t]\nperiodicReturn from annual rate and compounding frequency\ncontribution[t] may step up once per year if you set an annual increase
Planning levers (illustrative)
| Lever you change | What you usually hold fixed | What moves in the output |
|---|---|---|
| Raise expected return by 1% | Same contributions, horizon, fees | Ending balance curves up; ROI rises, but risk assumption is higher. |
| Add 100 / month to contributions | Same return and horizon | Principal line steepens; gains grow because there is more capital at work. |
| Extend horizon by 5 years | Same return and contributions | Late-year compounding dominates; small changes early become large differences. |
| Turn on 2% inflation | Nominal return unchanged | Nominal ending balance unchanged, real purchasing-power line drops. |
| Increase management fee to 1% | Gross return unchanged | Net path drifts down; long horizons amplify the fee drag. |
| Tax on gains at 15% | Simple profit definition | Net after-tax wealth falls versus pre-tax ending balance; still a rough estimate. |
Glossary
Compound interest
Returns applied to prior gains so the balance grows faster than a straight line.
Nominal return
Headline percentage before adjusting for inflation or fees.
Real value
Ending balance expressed in today’s purchasing power using the inflation field.
Principal
Money you contributed (initial plus periodic deposits), excluding market gains.
Management fee
Annual charge modeled as a monthly drag on the balance, similar to an expense ratio.
ROI (here)
Gain divided by total contributions for the whole horizon - a simple summary, not an annualized IRR.
Frequently Asked Questions
Compound interest, realistic return assumptions, and limits of this projection tool.
What is compound interest?
It is interest (or returns) calculated on the latest balance, which already includes earlier gains. Over long horizons the curve bends upward compared with simple interest.
What is a realistic annual return to type in?
Broad equity indices have long-run averages near high single digits before inflation, but year-to-year volatility is large. For multi-decade planning many practitioners stress-test between roughly 4% and 8% nominal depending on the mix of stocks and bonds.
Does compounding frequency matter much?
At the same nominal annual rate, monthly compounding usually beats annual by a modest margin because gains are credited sooner and start earning their own return.
How are taxes modeled?
The “tax on gains” field applies one flat percentage to gains (ending balance minus total contributions). Real tax systems have brackets, wrappers like ISAs or 401(k)s, and timing rules - use the result as a directional check only.
About these results
Outputs are educational projections from your inputs, not investment advice or a market forecast. Fees, taxes, and inflation are simplified. Confirm any decision with a qualified professional and official product disclosures.