Finance tool
Compound interest calculator
See how your savings grow at compound interest — configurable compounding frequency, with or without monthly contributions.
Start with the free manual tool. If you want the real document view after that, analyze a statement PDF.
Free tool
See how money compounds over years
Enter principal, monthly contribution, rate, time, and frequency. The calculator separately shows what the principal grows to and what the contributions grow to, plus the total interest earned.
Compound interest inputs
See how initial principal + regular contributions grow at compound interest.
Initial deposit or balance.
Recurring monthly amount added.
APY on the account. HYSA ~4-5%, index funds ~7% real.
How long you'll leave the money invested.
Daily compounding (most HYSAs, most savings accounts) slightly outperforms monthly over long periods.
After 20 years at 7% compounded monthly, starting from $10,000 plus $300/month.
Principal + sum of monthly deposits.
139.8% of contributions.
What the starting balance becomes alone.
What the monthly additions grow to.
FV of principal = P × (1 + r/n)^(n·t)
FV of contributions = C × [((1 + r/n)^(n·t) − 1) / (r/n)]
where r = annual rate, n = periods/year, t = years
Natural next step
Is your actual contribution the one you used?
Projections only matter if your actual monthly contribution matches the input. Upload your bank statement to see your real monthly saving rate over the last 3-6 months — then plug that number in here to see where it leads.
What it gives you
Fast enough for a first pass
Each tool is intentionally narrow. The job here is a clean estimate, not a fake replacement for a full statement analysis.
Any compounding frequency
Toggles daily, monthly, quarterly, or annual compounding — matches your actual account terms.
Principal vs contribution split
Separately shows growth from principal vs growth from monthly contributions so you see which half is dominant.
Formula shown
Shows formula used so you understand the math — no black box. Good for teaching compound interest to yourself or someone else.
When it's useful
A clean compound interest calculator with monthly contributions and configurable compounding frequency.
Savers comparing accounts
Plug in your balance, monthly deposit, and two different APYs — see the long-term gap. Useful for deciding between a 0.5% big-bank savings and a 4.5% high-yield online savings.
Investors projecting long-term growth
Run at a conservative 5%, moderate 7%, and aggressive 9% to see the range of possible outcomes. Compound interest is sensitive to rate assumptions — use scenarios instead of single numbers.
Parents saving for college or kids' futures
Use at 3-5% for a 529 or HYSA, 6-7% for moderate index funds. 18-year time horizons make even small monthly contributions significant.
Anyone curious about the Rule of 72
The calculator produces exact numbers; the Rule of 72 gives you a mental-math estimate for doubling time. Use both to build intuition around compounding.
Deeper context
Why starting early matters more than contributing more
Compound math favors time more than amount.
$200/month starting at 25 beats $400/month starting at 35
Over a 65-year old timeline with 7% return, the 25-year-old saver reaches ~$525k while the 35-year-old reaches ~$480k — even though the 35-year-old contributed twice as much per month. Time under compounding beats contribution size.
The first 10 years feel slow; the last 10 feel impossible
After 10 years at 7% with $500/month, you have ~$87k — mostly contributions. After 30 years, you have ~$612k — mostly interest. Compound returns accelerate dramatically in the back half of the timeline.
Missing 5 years early costs more than missing 5 years late
A 5-year pause at age 25 costs far more than a 5-year pause at age 55, because the early years have more compounding runway ahead of them. This is why IRA / 401(k) matching is valuable even when you're young and broke.
Deeper context
Common compound interest mistakes
A few quick wins if you're planning around compound returns.
Forgetting taxes and fees
A 7% gross return with 0.5% expense ratio and 20% tax on gains becomes roughly 5.2% net. For taxable brokerage accounts, model at the net rate or use tax-advantaged accounts (401k, Roth IRA) to protect the growth.
Using nominal returns instead of real
Historical stock returns of ~10% are nominal (not inflation-adjusted). Real (inflation-adjusted) returns are closer to 7%. For long-term planning, use real returns so your FI number and projections retain purchasing power.
Assuming constant returns every year
Real markets fluctuate — a 7% average means some years at -20% and some at +30%. Compound interest calculations use averages; your actual path will be bumpier. Don't panic in down years — the math assumes averages over decades, not every year.
FAQ