glunty Compound Interest Calculator
Grow a principal with compound interest and optional monthly contributions. Estimate only, not financial advice.
- Runs in your browser
- No sign-up
- No watermark
- No installs
This tool produces an arithmetic estimate based on the numbers you enter. It is NOT financial, investment, or tax advice. Real returns are not guaranteed and depend on the actual rate, contribution timing, fees, taxes, and market conditions not modeled here. Consult a qualified financial professional before making decisions.
What this tool does
Projects how a lump sum grows under compound interest, with the option to add a fixed
monthly contribution. The principal grows by the standard formula
A = P * (1 + r/n)^(n*t), where P is the principal,
r is the annual rate as a decimal, n is the number of
compounding periods per year (1, 2, 4, 12, or 365), and t is the number
of years. If you enter a monthly contribution, its growth is added as the future value
of a series. The tool reports the final balance, the total you contributed, and the
interest earned on top. Everything runs in your browser; nothing you type is sent
anywhere. To verify, open DevTools and watch the Network tab: zero requests.
How to use it
Enter your starting principal, the annual interest rate as a percent, and how long the money stays invested in years. Pick how often interest compounds (monthly is common for savings accounts; annually is common for simple projections). Optionally add a recurring monthly contribution to model regular deposits. The results update live as you type. A quick check: $1,000 at 5% compounded monthly for 10 years with no contributions gives a final balance of about $1,647.01, of which $647.01 is interest.
Common use cases
- Estimating how a savings account or certificate of deposit grows over several years.
- Projecting a long-term investment balance with steady monthly deposits.
- Comparing the effect of compounding frequency (daily vs. monthly vs. annually) at the same rate.
- Seeing how much of a future balance is your own money versus earned interest.
- Sanity-checking a rate-of-return assumption before a bigger financial planning exercise.
Common pitfalls
- The contribution math is an approximation for non-monthly frequencies. The principal uses the compounding frequency you pick, but a monthly contribution is always compounded monthly (i = annual rate / 12, over t times 12 months). When you choose annually, quarterly, or daily compounding, the contribution portion will be slightly off from a fully consistent model. For monthly compounding the two parts agree exactly.
- Nominal rate, not APY. The rate you enter is a nominal annual rate that then compounds n times per year. The effective yield (APY) ends up a little higher than the number you typed. If a bank quotes you an APY, that already bakes in compounding, so entering it here as the nominal rate slightly overstates growth.
- Real returns are not guaranteed. This is arithmetic on a fixed rate. Actual investments fluctuate, and fees, taxes, and inflation all reduce what you keep. Treat the output as a rough projection, not a promise, and never as financial advice.
- Contributions here are end-of-month (ordinary annuity). The series formula assumes each deposit lands at the end of its month. Depositing at the start of each month (an annuity due) would earn one extra period of interest, so real-world results can differ slightly depending on timing.
Frequently asked questions
- How does compound interest work?
- Compound interest earns interest on both your original principal and the interest already added to the balance. The core formula is A = P times (1 + r/n) raised to the power of (n times t), where P is the principal, r is the annual rate as a decimal, n is how many times per year interest compounds, and t is the number of years. Because each period adds interest to a slightly larger balance, growth accelerates the longer you leave the money invested.
- What does compounding frequency change?
- Compounding frequency is how often interest is calculated and added to the balance. Annually compounds once a year (n = 1), semiannually twice (n = 2), quarterly four times (n = 4), monthly twelve times (n = 12), and daily 365 times (n = 365). A higher frequency means slightly more interest, because interest starts earning interest sooner. At the same annual rate daily compounding beats annual compounding, but the gap narrows quickly and is often just a few dollars on a modest balance.
- How are monthly contributions handled?
- When you add a recurring monthly contribution, the tool computes its growth as the future value of a series (an ordinary annuity): FV = PMT times (((1 + i) raised to m) minus 1) divided by i, where i is the monthly rate (annual rate divided by 12) and m is the number of months (years times 12). This treats contributions as compounding monthly regardless of the frequency you picked for the principal, so the contribution figure is an approximation when you select a non-monthly frequency. For monthly compounding the two parts line up exactly.
- What is the difference between total contributions and interest earned?
- Total contributions is the money you put in: your starting principal plus every monthly deposit. Interest earned is everything the balance grew beyond what you contributed, that is, final balance minus total contributions. Added together, contributions plus interest equal the final balance. Watching the interest portion grow relative to contributions is the clearest way to see compounding at work over a long horizon.
- Does a 0 percent interest rate work?
- Yes. At 0 percent the principal does not grow, so the final balance equals your principal plus any contributions, and interest earned is zero. The tool handles this case without dividing by zero: when the rate is zero, the contribution series is simply the monthly amount times the number of months. This is useful as a sanity check, or for modeling a plain savings plan with no yield.
- Is this the same as APY or APR?
- Not exactly. The annual rate you enter here is a nominal rate that gets compounded n times per year. Annual percentage yield (APY) is the effective rate after compounding, so a 5 percent nominal rate compounded monthly has an APY slightly above 5 percent. APR on loans may bundle in fees and is quoted differently. Enter the nominal annual rate for the math on this page to match the standard compound interest formula.
Cite this tool
For academic, journalistic, or technical references. Pick a format:
Citations use 2026 as the publication year. Access date is left as a fillable placeholder where the citation style expects one.