Find out how much you'll have if you invest today and contribute every month — and how much of that total is interest that time earned for you.
Compound interest is why starting early beats contributing a lot. Each period, the interest you earned is added to your principal, and the next round of interest is calculated on a larger base. This calculator shows your final balance, how much you put in, and how much grew on its own, with monthly contributions and the compounding frequency you choose.
Financial disclaimerIndicative result — not professional financial advice. Consult a specialist before making investment or credit decisions.
The final balance of a savings or investment plan from an initial principal, a monthly contribution, an annual rate, and a horizon in years. It separates what you contributed from the interest earned, computes how many times your money multiplied, and charts the growth year by year so you can see the moment interest starts to outweigh your contributions.
Who it's for
For anyone saving or investing over several years: index funds, retirement plans, a renewing time deposit, or just a monthly savings habit. Also for comparing scenarios: what changes if I raise the contribution, hold five more years, or earn one extra point of return.
When to use it
When planning a long-term goal (retirement, a down payment, an education fund), when deciding how much to contribute monthly, or to understand visually why time matters so much. It's ideal for weighing the opportunity cost of starting now versus starting a few years from now.
When NOT to use it
It's not a promise of returns: real investments fluctuate and the rate you enter is an assumption, not a guarantee. It doesn't subtract inflation or taxes on gains, and it doesn't model partial withdrawals or irregular contributions. For a loan you pay off (not save), use the loan or mortgage calculator.
What data it needs
Initial principal
The amount you invest today as a lump sum. It can be 0 if you start with monthly contributions only.
Monthly contribution
What you add every month, consistently. Periodic contributions are what turn a modest savings habit into meaningful wealth.
Annual interest rate
The expected annual return as a percentage. As a historical reference, a global equity index fund has run around 6–8% per year over the long run, before inflation.
Time horizon (years)
The number of years you let the money grow untouched. It's the most powerful lever: doubling the horizon more than doubles the result.
Compounding frequency
How often interest is credited to the principal (annual, quarterly, monthly, or daily). The more often, the slightly higher the return for the same nominal rate.
Formula
The initial principal grows with the classic compound-interest formula, Balance = Principal × (1 + i)^n, where i is the rate per period and n the number of periods. Monthly contributions accumulate as an annuity (ordinary annuity): each contribution earns interest from the month it enters until the end. The calculator simulates month by month with a monthly rate equivalent to your chosen compounding frequency, so the initial-principal result is exact for any frequency.
How to read the result
Look beyond the final balance at two things: how much of the total is interest (not your money) and how many times your contributions multiplied. Over long horizons, interest usually overtakes contributions — that crossover is the 'magic' of compounding. If interest is only a small fraction, you're short on time or rate: try extending the years before forcing an unrealistic return.
How this calculator was reviewed
What you'll see, what it prevents, and where you shouldn't trust it
Every flagship calculator ships with the same editorial structure: two hypothetical worked examples with numbers, the errors it helps you avoid, the model's declared limitations, and a visible financial disclaimer. The review is signed and dated.
Hypothetical case·Case A
Two siblings, the same contribution, a different starting point
Ana starts at 25, contributing 150 a month to a fund returning 7% a year, and keeps it for 35 years. Luis does exactly the same but starts at 35 and contributes for 25 years. Ana puts in 63,000 of her own money and ends with about 270,000. Luis puts in 45,000 and ends with about 122,000. Ana contributed only 18,000 more than Luis, yet ends with more than double: the ten extra years of compounding were worth more than any additional contribution. It's the clearest argument for starting today, even with little.
Illustrative figures. This example does not represent a real company or a financial recommendation.
Hypothetical case·Case B
Raising the contribution versus holding longer
Someone with 5,000 upfront and 200 a month at 6.5% over 20 years reaches about 105,000. They're torn between two moves: raise the contribution to 300 a month, or keep 200 but for five more years. Going to 300 takes them to ~138,000; holding 200 for five more years takes them to ~152,000. Here time beats effort: extending the horizon pays off more than contributing 50% more each month, because those final years carry the most interest-on-interest. The calculator makes that crossover visible at once.
Illustrative figures. This example does not represent a real company or a financial recommendation.
Common mistakes it helps you avoid
Things a team or decision-maker might assume that this calculator forces you to verify before closing the math.
Using an unrealistic rate (10–12%) and treating the projected balance as a fact. Real investments fluctuate; use a conservative rate and treat it as a scenario, not a promise.
Ignoring inflation. A large balance 30 years out is worth less than it looks today. Subtract your expected inflation from the rate to reason in real terms.
Comparing the result with another calculator that assumes contributions at the start of the month instead of the end. Small convention differences change the total; what matters is being consistent.
Assuming that contributing more always beats waiting longer. Over long horizons, adding years usually pays more than raising the contribution, because interest-on-interest concentrates at the end.
Model limitations
What the calculator does not do, and where you need a professional or a specialized tool.
It projects nominal growth: it doesn't subtract inflation or taxes on gains, which depend on your country and instrument.
It assumes a constant rate and regular contributions. Reality has good and bad years and changing contributions; the result is an idealized average.
It doesn't model partial withdrawals, management fees, or one-off contributions. For fine planning, subtract fees from the rate.
Compounding frequency is applied uniformly; real instruments may compound or pay differently (coupons, reinvested dividends).
When NOT to use this calculator
Don't use this calculator for a loan you pay off (loan, mortgage, credit card): there compound interest works against you and the math is different — use the loan or mortgage calculator. And don't take it as investment advice: it's a projection tool with assumptions you set yourself.
Financial, tax, accounting and legal notice
The result is an informative estimate based on the data you enter. It does not constitute financial, tax, accounting, or legal advice. For decisions that affect taxes, financing, or wealth, validate the numbers with a certified professional in your jurisdiction.
Editorial review
Reviewed by the Simúlalo editorial team
This simulator was reviewed by the people listed below before being published. The review covers the declared formula, the model's assumptions, the explicit limitations, and the absence of unsupported financial claims.
They are part of the Simúlalo editorial team, focused on building financial tools that are clear, educational, and easy to interpret.
Last updated: ·We update this page when the methodology, sources used, or simulator structure change.
This tool uses standard financial formulas and user-supplied data. To explain concepts like rates, credit, risk, or cash flow we consult public and official sources (Banxico, SAT, CONDUSEF, CNBV, Banco de España, IFRS, BIS, among others). Simúlalo is not affiliated with, sponsored by, or endorsed by these institutions.
Frequently asked questions — compound interest
1What's the difference between simple and compound interest?
Simple interest is always calculated on the original principal; compound interest is calculated on the principal PLUS the interest already accumulated. With compounding, the base grows each period, so the money accelerates over time. Over long horizons the difference is enormous.
2How does compounding frequency affect the result?
The more often interest is credited (daily > monthly > quarterly > annual), the slightly more you earn for the same nominal rate, because interest starts earning interest sooner. The gap between monthly and daily is usually small; what really moves the needle is the rate and, above all, the time horizon.
3What interest rate should I use?
It depends on where you invest. A conservative time deposit might pay 3–5%; an equity index fund has run around 6–8% per year historically over the long run, with volatility. Use a realistic rate for your instrument and remember the result is a projection, not a guarantee.
4Does the calculator account for inflation and taxes?
No. It shows nominal growth. To estimate real purchasing power, subtract your expected inflation from the rate (for example, 7% return minus 4% inflation ≈ 3% real). Taxes on gains depend on your country and instrument.
5Why does starting early matter so much?
Because compound interest rewards time exponentially, not linearly. A contribution made at 25 has decades to multiply; the same amount at 45 has half the time and ends up worth far less. Each year of delay costs more than the one before.
6Is it useful for retirement or a savings goal?
Yes. Set the horizon to your years until the goal, enter a realistic monthly contribution and a conservative rate. It tells you whether you'll reach the target or how much you'd need to raise the contribution. To compare scenarios, change one variable at a time and watch the final balance.