Formulas and 2026 figures checked & updated: July 2026
HYSAs: ~4–5% APY. S&P 500 avg: ~10%. Traditional banks: <0.5%.
Growth Chart
Rate Comparison
| APY | Balance | Interest Earned |
|---|---|---|
| 0.5% | $66,768 | $1,768 |
| 2.0% | $72,466 | $7,466 |
| 4.5% ◀ | $83,434 | $18,434 |
| 5.0% | $85,876 | $20,876 |
| 7.0% | $96,591 | $31,591 |
How to Use the Compound Interest Calculator
- Enter your starting balance — the amount you already have saved, if any.
- Enter your planned monthly contribution — how much you'll add on a regular basis.
- Set the annual interest rate (APY) you expect to earn, such as 4-5% for a high-yield savings account.
- Choose the number of years you plan to save before you'll need the money.
- Review the growth chart to see your balance build over time, split between your own deposits and interest earned.
- Compare the rate table to see how a higher or lower APY would change your total balance and total interest.
- Check your final balance, total contributions, and total interest earned. Example: $5,000 plus $200/month at 4.5% APY for 10 years grows to about $38,000 — roughly $9,000 of it interest.
What This Calculator Does
The Compound Interest Calculator projects how a savings balance grows over time when interest is earned not just on your initial deposit, but on all the interest that's accumulated along the way. Enter a starting balance, a monthly contribution, an annual interest rate (APY), and a time horizon to see your total balance, total contributions, and total interest earned.
The four inputs that drive your result are your starting balance, how much you add each month, the annual rate you're earning, and how many years you let the money compound — and each one matters more than most people expect. A handy shortcut is the 'Rule of 72': divide 72 by your interest rate to estimate how many years it takes your money to double, so a balance earning 6% doubles roughly every 12 years, while one earning 2% takes about 36 years. As of 2026, top high-yield savings accounts (HYSAs) pay roughly 4-5% APY, CDs offer similar or slightly higher locked-in rates, and the average traditional bank savings account still pays well under 0.5% — a gap that can be worth tens of thousands of dollars over a couple of decades on otherwise identical contributions.
Because interest compounds monthly in this calculator, even small differences in rate or in how early you start make a large difference over long periods. People typically reach for this tool when deciding whether to move idle cash out of a low-rate account and into a higher-yield one, when building a savings habit toward a medium-term goal like a home down payment or a wedding, or simply to see what a consistent monthly deposit can grow into decades from now. It's the same underlying math that applies to high-yield savings accounts, CDs, and retirement accounts, so it's equally useful for comparing 'what if I started 5 years earlier' or 'what if I found a 1% higher rate' scenarios side by side. Running a few different scenarios before you open an account, automate a transfer, or lock money into a CD can make the tradeoffs — rate versus liquidity, a bigger monthly deposit versus a longer timeline — much easier to see than trying to do the math in your head.
Formula
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]The first term grows your initial deposit (P) at the compounding rate. The second term adds the future value of your regular monthly contributions (PMT), each of which compounds for the remaining time after it's deposited. With monthly compounding, n = 12, meaning interest is calculated and added to your balance twelve times a year rather than once — which is why the effective annual yield (APY) is slightly higher than the stated nominal rate. Note that this figure is a nominal projection; if you want to know what your future balance is worth in today's dollars, you'd subtract expected inflation to get the real return.
- PInitial deposit (principal) — the lump sum you start with, if any
- rAnnual interest rate (as a decimal), e.g. 4.5% = 0.045
- nCompounding periods per year (12 for monthly)
- tNumber of years you let the balance grow
- PMTRegular monthly contribution amount added on top of the principal
Examples
Example 1: Starting from scratch with monthly deposits
$0 initial deposit, $500/month contribution, 4.5% APY, 10 years.
Balance grows to about $74,500 — roughly $60,000 in contributions plus about $14,500 in interest earned.
Example 2: A $5,000 head start
$5,000 initial deposit, $500/month, 4.5% APY, 10 years (this calculator's defaults).
Balance grows to about $82,300 — about $7,800 more than starting from zero, just from that one-time $5,000 deposit compounding for 10 years.
Example 3: The cost of a lower rate
Same $5,000 + $500/month, but in a traditional savings account earning 0.5% instead of 4.5%, over 10 years.
Balance reaches only about $66,500 — over $15,800 less than the high-yield account, even though contributions are identical.
Key Terms Explained
- Compound Interest
- Interest earned on both your original balance and previously earned interest, so growth accelerates the longer money stays invested.
- APY (Annual Percentage Yield)
- The real rate of return on savings over a year once compounding is included. A 5% rate compounded monthly produces slightly more than 5% in APY.
- Interest Rate
- The percentage a lender charges on the principal, before fees. Unlike APR, the interest rate alone doesn't include origination or other loan costs.
- Principal
- The original amount borrowed or invested, before interest. Each loan payment is split between paying down principal and paying interest.
- Real Return
- An investment's return after subtracting inflation — what your money actually gains in purchasing power.
Methodology
FV = PV·(1+r/12)^(12·t) + PMT·[(1+r/12)^(12·t) − 1]/(r/12). Monthly compounding. r = annual rate. The first term grows your starting deposit; the second term adds up the future value of every monthly contribution, each compounding for whatever time remains after it's made.
Frequently Asked Questions
What is compound interest?+
Compound interest means you earn returns on both your principal and previously accumulated interest. A $10,000 deposit at 5% earns $500 in year 1. In year 2, you earn 5% on $10,500 — $525. This snowball effect accelerates dramatically over decades, which is why starting early is so powerful.
What is the best savings interest rate in 2026?+
High-yield savings accounts (HYSA) from online banks offer 4–5% APY as of 2025. CDs (certificates of deposit) can offer similar or slightly higher rates for locked-up terms. Traditional bank accounts average under 0.5% — a significant opportunity cost for money sitting idle.
How much will $10,000 grow in 10 years?+
At 5% annual interest compounded monthly, $10,000 grows to about $16,470 in 10 years. At 7%, it grows to $20,097. At 10%, it reaches $27,070. Adding monthly contributions of $200 at 7% would yield about $54,000 — more than doubling the result of the lump sum alone.
What is the difference between APY and APR for savings?+
APR (Annual Percentage Rate) is the stated annual interest rate. APY (Annual Percentage Yield) accounts for compounding within the year and always equals or exceeds APR. A 5% APR compounded monthly yields an APY of 5.116%. Banks are required to advertise APY for savings accounts, so the APY is the true return you earn.
Does compounding frequency matter?+
Yes, but the effect is smaller than most people expect. At 5% APR on $10,000 for 10 years: annual compounding yields $16,289; monthly compounding yields $16,470; daily compounding yields $16,487. The difference between monthly and daily compounding is minimal — what matters most is the rate and how long you save.
Can I save my results?+
Yes. Use “Save results” to store a snapshot. Without an account it remains in this browser. If you log in, saved scenarios sync securely to your SmartRates account so they are available on your other devices.
How do I share my calculation?+
Click “Share” in the toolbar to copy a link (or open your device’s share sheet). The link encodes your exact inputs, so whoever opens it sees the calculator pre-filled with the same numbers and the same result.
Can I email my calculator results?+
Yes. Click “Email results” to open your default email application with the current inputs, results, and calculator link already included. Review the message and choose the recipient before sending.
Can I export or print my results as a PDF?+
Yes. Click “Export PDF” to open a clean, printable summary of your inputs and results that you can save as a PDF or print. It includes a timestamp and a link back to the calculator.
How accurate are the calculator results?+
The arithmetic follows the formula and assumptions documented on this page. The result is still an estimate because actual rates, fees, taxes, timing conventions, eligibility rules, and provider calculations can differ. Use figures from your official quote, statement, contract, or tax form before making a financial decision.
Which inputs have the biggest effect on the result?+
Rate, time, starting balance, recurring payments or contributions, and fees usually have the largest effects. Change one input at a time to create a conservative, expected, and optimistic scenario instead of relying on a single forecast.
Are taxes, fees, and inflation included?+
Only when they appear as an input or are explicitly described in the methodology. Do not assume an omitted cost is zero. Review the formula and methodology sections to see exactly what is included before comparing the result with an outside quote.
Can this calculator predict future rates or returns?+
No. A calculator projects the assumptions entered; it cannot predict market returns, inflation, variable interest rates, tax-law changes, or provider decisions. Rerun the calculation with several assumptions to understand the range of possible outcomes.
Why might my lender, bank, broker, or tax software show a different result?+
Professional systems may use daily timing, transaction dates, compounding conventions, rounding rules, account-specific fees, credits, eligibility details, or regulations that a general-purpose calculator cannot know. A small difference can be rounding; a large difference usually means an assumption or included cost is different.
Disclaimer: Calculations are for informational purposes only and do not constitute professional financial advice. Please consult with a certified professional before making financial decisions.