What your compound interest results mean
The balance at the top is what the account holds on the last day of the term. The split beneath it matters more: deposits came out of your pocket, interest is what the bank paid you for leaving it alone. Early on the deposits bar dwarfs the interest bar, and the year they swap places is the year compounding starts doing the heavy lifting.
Effective APY is what your stated rate is worth once compounding is applied, so you can compare an account paying 4.5% monthly against one paying 4.45% daily. Doubling time is how long the starting balance needs to become twice itself at this rate.
How to use this calculator
- Starting amount — the balance you have now, not what you plan to have.
- Regular deposit and frequency — match your real habit. Payday deposits beat monthly ones by a small but real margin.
- Rate — use the APY on your statement. A rate you hope for produces a number you cannot bank on.
- Years — push it to 20 or 30 once you have the shape. The curve is near-flat early and then bends hard, so short horizons hide the whole point.
- Compounding frequency — leave it on daily unless your account documents otherwise. It moves the answer far less than the rate does.
Every figure updates as you type, and the arithmetic runs in your browser — there is no email gate on the result.
What is compound interest?
Compound interest is interest paid on a balance that already includes previously earned interest. Simple interest pays only on your original deposit, so it grows in a straight line. Compound interest folds each payment back into the balance, so the next payment is calculated on a slightly larger number, and growth accelerates on its own.
The acceleration is invisible over months and decisive over decades. It also runs in reverse: credit-card balances compound against you, which is why a card at 24% is a far more urgent problem than a savings account at 4% is an opportunity.
The compound interest formula
A = P (1 + r/n)nt
- A — the balance at the end
- P — the amount you start with
- r — the annual rate as a decimal (4.5% is 0.045)
- n — compounding periods per year (365 for daily)
- t — years
That formula covers a lump sum left alone. Regular deposits need a second term — the future value of a series of payments — and the two are added together. Where the deposit schedule and the compounding schedule disagree, which is the normal case for a monthly deposit into a daily-compounding account, this calculator converts the compounding rate into a rate for one deposit period before applying it, so the answer is exact instead of rounded to the nearest convenient period.
A worked example
Put $10,000 into an account paying 4.50% APY, compounded daily, and add nothing. After one year you hold $10,460.25 — the extra $10.25 over a flat 4.5% is compounding at work. Leave it ten years and it is $15,683.12, of which $5,683.12 is interest.
Now add $250 a month to the same account. Ten years in, the balance is $53,415 on $40,000 of deposits: interest has grown from $5,683 to $13,415 because every deposit starts earning the day it lands. The deposits are doing most of the work at this horizon — which is the honest reading of most savings plans. Stretch the same numbers to 30 years and interest passes deposits, ending as the larger share of the balance.
What this calculator assumes
- The rate holds for the whole term. Savings and CD rates move with the Fed; no account pays one rate for 30 years. Treat long horizons as a shape, not a forecast.
- No tax is deducted. Interest in a taxable account is ordinary income in the year you earn it. In a Roth IRA or 401(k) it is not, which is why the same deposits land differently depending on the wrapper.
- No inflation adjustment. The balance is in future dollars, worth less than today’s.
- Deposits never miss. The series assumes every deposit arrives on schedule.
These are estimates for planning, not an offer, a product projection, or personalised advice.
Questions people ask
How often should interest compound to earn the most?
As often as possible — daily beats monthly beats annually — but the gap is small. On $10,000 at 4.5% for a year, daily compounding earns roughly $10 more than annual. The rate, and whether you keep adding, matter far more.
Is APY the same as the interest rate?
No. The rate is the headline number; APY is what it is worth after a year of compounding, so APY is always equal to or higher. Compare accounts on APY — it is the only figure that already accounts for frequency.
Does this calculator work for investments as well as savings?
It works for anything with a fixed rate. Market returns are not fixed: an average of 7% arrives as good and bad years, and their order changes the outcome. Use the investment calculator for that.