Compound Interest Calculator
Project growth period by period, with optional regular contributions, a deposits-versus-interest split, and the effective annual rate beside every result.
Results are projections from the standard compound-interest formula under the assumptions stated on the page — a constant rate, untaxed interest, no fees, contributions credited at the end of each period. A planning aid, not advice. See the Editorial Policy.
79.7% of the closing balance is money you put in, 20.3% is interest the balance earned.
12% nominal compounded annually reads 12% effective — the figure for comparing accounts. Contributions land at the end of each period and first earn interest in the next one.
Growth year by year
The full ledger posts 2 periods — one for each time interest is credited. This table rolls them up to one row per year.
| Year | Deposited | Interest | Balance |
|---|---|---|---|
| 1 | $0.00 | $12.00 | $112.00 |
| 2 | $0.00 | $13.44 | $125.44 |
The account
How often the account credits interest — this sets both each period's interest and the effective annual rate.
Results update as you type. Every posting is rounded half-up to the cent, amounts are plain numbers in whatever currency the account uses, and everything runs in your browser.
How it works
- 1
Enter the starting position
The principal is what the account holds before any interest is credited — zero is fine if you are saving from scratch, $10,000.00 if you are rolling an existing balance forward. The annual rate is the nominal figure from the account's terms, entered as a percentage like 12 or 4.5.
- 2
Choose how often interest is credited
Annually, semiannually, quarterly, monthly or daily. The frequency sets both the size of each period's interest — the annual rate divided by the number of periods — and how many postings the ledger makes per year. It also sets the effective annual rate shown with the results.
- 3
Add a contribution, if the plan has one
The monthly contribution is optional; leave it empty for a plain deposit. Whatever you enter is spread across the periods — each one credits 12/n of the monthly amount, so monthly compounding credits a month's worth every month and quarterly compounding credits three months' worth every quarter. Each contribution lands at the end of its period and first earns interest in the next one.
- 4
Read the ledger, not just the total
The summary cards give the closing balance, what you put in, what the interest added and the effective annual rate. Below them, the growth table rolls the period-by-period ledger up to one row per year — deposits, interest and closing balance — so the year where interest overtakes contributions can be pointed at, not just inferred from the totals.
Two years at 12%, credited once a year
Take the two figures this page opens on: $100.00 at a 12% nominal rate, compounded annually, for two years. The first year credits 12% of $100.00 — $12.00 — and the balance stands at $112.00. The second year takes 12% of $112.00, not of the original hundred: $13.44. The closing balance is $125.44, of which $25.44 is interest.
| Period | Deposit | Interest credited | Closing balance |
|---|---|---|---|
| Year 1 | $0.00 | $12.00 | $112.00 |
| Year 2 | $0.00 | $13.44 | $125.44 |
The $1.44 gap between the two years' interest is the whole concept in one number: it is 12% of the $12.00 the first year produced. Under simple interest the same two years pay $24.00 and stop at $124.00; compounding pays $25.44. Nothing about the rate changed between the years — only the base the percentage was taken on. Type 100.00, 12, 2 and Annually into the calculator and the cards should read exactly the figures in this table.
Starting from zero, $100.00 a month
The second worked example adds the contribution thread: no principal, $100.00 every month, 12% compounded monthly, one year. The first month earns nothing, because the balance stood at zero while the period ran and the contribution lands only at its end. From the second month on, interest is taken on a balance that includes the months before it — 1% of $100.00 in month 2, 1% of $201.00 in month 3.
| Month | Deposit | Interest credited | Closing balance |
|---|---|---|---|
| 1 | $100.00 | $0.00 | $100.00 |
| 2 | $100.00 | $1.00 | $201.00 |
| 3 | $100.00 | $2.01 | $303.01 |
| 12 | $100.00 | $11.57 | $1,268.25 |
Over the twelve months the deposits add to $1,200.00 and the interest to $68.25, closing at $1,268.25. The interest column grows every single month — $1.00, then $2.01, then $3.03, and by the last month $11.57 — because every new balance is a slightly larger base for the next 1%. That is also why the first row's zero is honest rather than a bug: a contribution made at the end of a period has not been in the account long enough to earn anything yet.
What the frequency is worth
Frequency changes the answer without changing the rate. The same $10,000.00 at the same 12% for the same ten years lands on five different closing balances depending only on how often the interest is credited, because each mid-year posting starts earning its own interest that much sooner. The effective annual rate column is the fair-comparison figure: it folds the frequency into one number.
| Credited | Effective annual rate | Interest over 10 years | Closing balance |
|---|---|---|---|
| Annually | 12% | $21,058.48 | $31,058.48 |
| Semiannually | 12.36% | $22,071.39 | $32,071.39 |
| Quarterly | 12.5509% | $22,620.36 | $32,620.36 |
| Monthly | 12.6825% | $23,003.87 | $33,003.87 |
| Daily | 12.7475% | $23,194.68 | $33,194.68 |
Read the steps between the rows and the effect is modest: monthly crediting beats quarterly by about $383 over the decade, and daily beats monthly by about $191. Both are real money, but both are smaller than what a single half-point of rate does over the same horizon. The gains also converge — the daily closing balance sits within about seven dollars of what interest compounded continuously, every instant, would produce — so an account advertising hourly crediting is selling rounding, not growth.
The daily row carries one more caveat, and it is specific to this tool. At a $100.00 balance, 12% a year works out to 3.29 cents a day, which the ledger posts as a round 3 cents — and on small balances that rounding costs more than the extra frequency gains: a year of daily postings on $100.00 earns $12.44 of interest, less than the $12.67 monthly crediting earns on the same money. In exact arithmetic more frequent crediting never loses; this ledger rounds every posting to a whole cent, and the cent is the limit. Banks that track fractional cents show slightly larger daily figures.
What this tool leaves out
The ledger is exact for the model it runs, so the model is worth stating. The rate is assumed constant for the whole horizon — real accounts reprice when central banks or issuers move. Interest is assumed untaxed — most jurisdictions take a share of it, which lowers the effective figure. No fees are modeled — monthly account fees come straight off the balance and compound against you. Contributions are assumed to arrive regularly and at the end of each period — a plan that skips months, or deposits on the first of the month, will not match the ledger.
| Assumption in the model | What real accounts do |
|---|---|
| Constant nominal rate | Rates move with the market and the bank's terms |
| Interest untaxed | Most jurisdictions tax interest income |
| No fees | Account and management fees come off the balance |
| Contributions at period end, every period | Deposits are irregular and sometimes skipped |
None of that makes the tool useless — it makes it a planning aid. Comparing two shapes of a decision (this rate against that one, ten years against twenty, $100.00 a month against $150.00) survives the missing detail; reading the closing balance as a prediction of a statement does not. Treat the output as the start of a decision, and the account's own terms as the authority on what it will actually pay.
Frequently asked questions
What does compounding actually change?
What is the difference between the nominal rate and the effective annual rate?
When does a contribution start earning interest?
Does it matter whether interest compounds annually or daily?
Is this financial advice?
Can I use any currency?
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Last updated: October 10, 2026