Compound Interest Calculator

Project savings growth with compounding and monthly contributions.

This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.

Projects what a starting balance and a regular monthly contribution grow into over time, and separates how much of the final figure you paid in from how much the interest added.

How to use it

1Set the initial amount, the annual rate and the number of years.
2Choose a compounding frequency — annually, semi-annually, quarterly, monthly or daily.
3Add a monthly contribution and read the year-by-year table underneath the three totals.

Example

Input: $10,000 at 7% for 10 years, compounded monthly, plus $100 a month
Output: Future value $37,506.06 · total contributed $22,000.00 · interest earned $15,506.06

Contributions are credited at the start of each period and then interest is applied, so they earn a full period of growth. Choosing Annually drops the total to $37,411.83, because a whole year of contributions arrives as one lump instead of twelve.

What happens to your data

Nothing about you is detected: the dollar figures come from a fixed en-US Intl.NumberFormat baked into the component, not from your location, your IP or any exchange-rate lookup. Every keystroke rebuilds the entire year table from the current field values and discards the previous one, so no projection accumulates and none is transmitted.

Last updated August 2026

You have a number from somewhere — the 7% a fund quotes, the rate on a savings account, a figure a colleague swears by — and you want to know what it does to your money over twenty years. Guessing is hopeless, because the answer is not a straight line and intuition runs low on it every time.

Decide first which of two situations you are in. A lump sum left alone grows on itself and nothing else, a curve that looks flat for years before it lifts. Add money every month and it becomes a different animal: over a long term most of the closing balance is the contributions and the growth on them rather than the original deposit. Setting the monthly figure to zero shows the first case alone.

The second thing to get right is which rate you type. The annual rate is treated as nominal and divided by the frequency you choose, so entering an AER or APY — already the compounded annual figure — then picking monthly compounding counts the compounding twice and flatters the result. Take the nominal rate from the account terms, or leave the frequency on annually if the effective figure is all you have.

The common mistake is the last one: reading the future value as money you can spend. It is a nominal figure, with no inflation, tax or charges taken out, and no field asks for any of them.

How it works

Toolvore The projection is built by stepping through periods rather than evaluating a closed formula, which is what lets an odd term work. Each step adds the monthly contribution scaled to the length of that period — a quarterly step receives three months of it, a daily step a fraction of one — then applies growth of the annual rate divided by the number of periods in a year. A term ending mid-period, two and a half years compounded annually, takes that fraction of the period's contribution and that fractional power of its growth; anything past 100 years is clamped. The year table comes out of the same loop, a row per completed year plus a closing row on the term itself, so a 6.5-year run ends on a row labelled 6.5. What it cannot model is everything that makes a real account messier: no inflation, tax, fee or rising contribution has a field, the rate never varies, and every figure prints as US dollars.

Common use cases

Projecting a pension pot from a balance and a monthly payment
Comparing a monthly-compounded account with an annually-compounded one
Seeing how much of a twenty-year total is contributions rather than growth
Testing what an extra 50 a month adds across a decade
Checking a bank's projected balance against the arithmetic behind it
Showing someone the year-by-year path, not a single closing number
Sanity-checking a claim about doubling your money in so many years

Frequently asked questions

What is the difference between simple and compound interest?＋

Simple interest is paid on the original amount only, so 5% on 1,000 pays 50 every year and ten years pays 500. Compound interest is paid on the balance, which already includes the interest added earlier, so the second year earns 5% of 1,050 instead. Over a year or two the gap is small enough to dismiss; over twenty or thirty it is most of the answer. Nearly everything you meet compounds — savings accounts, funds, and debt, the same mechanism running against you. Set the years to one with annual compounding and the two are identical, which is where the divergence starts.

Does compounding daily instead of monthly actually make a difference?＋

Less than the marketing on a savings account implies. Going from annual to monthly compounding at 7% lifts the effective rate by roughly a fifth of a percentage point; monthly to daily adds a couple of hundredths on top. Each extra split shrinks the periods about as fast as it multiplies them, and the sequence converges on continuous compounding. Where frequency does bite is with regular contributions: a payment made monthly into an account that compounds annually can sit most of a year before earning anything.

What is the difference between the interest rate and the APY or AER?＋

The nominal rate is the headline figure before compounding is applied. APY in the United States, or AER in Britain, is what a year of that compounding actually produces, which is why it is the figure two accounts should be compared on. An account advertising 5% APY compounded monthly has a nominal rate a little under 4.89%, because the monthly compounding is what carries it to 5%. The distinction matters in any calculator asking for a frequency as well as a rate, since it divides whatever you give it. If the terms quote only the effective figure, set compounding to annually and enter it unchanged.

How long does it take to double your money?＋

Divide 72 by the rate written as a whole number and you get roughly the number of years: 72 over 6 is twelve years, 72 over 9 is eight. It approximates the logarithm sitting behind compounding, closest around 8% and drifting at very high or very low rates. The rule assumes nothing is added along the way, so it answers a question about a lump sum only — add a monthly contribution and the balance passes double far sooner. For the exact crossing, set the monthly figure to zero and read down the year table.

Does compound interest keep up with inflation?＋

Not automatically, and the projection here is nominal — the units you typed, with nothing adjusted for the falling value of money. A balance of 100,000 in thirty years buys noticeably less than 100,000 does now, and a rate below inflation loses purchasing power while the figure on screen still rises. The usual way to handle it is to subtract your inflation assumption from your rate and read the answer as today's money: 7% growth against 3% inflation becomes 4% typed in. That understates slightly, since the exact adjustment divides rather than subtracts, but the gap is small at ordinary rates.

Is it better to invest a lump sum or contribute monthly?＋

A lump sum invested at the start comes out ahead more often, because the money spends longer in the market and duration is the ingredient compounding needs. That is a statement about averages rather than a promise about any particular decade, and it carries the risk of committing everything just before a fall. Spreading contributions across months buys at a range of prices and is easier to keep up, which counts for more than the arithmetic when the alternative is not investing at all. Most people are not choosing anyway — the money arrives with the salary.

Do you pay tax on compound interest?＋

Usually, though the rules differ enough by country that only your local ones are worth relying on. Interest on ordinary savings is commonly taxed as income in the year it is credited, whether or not you take it out, which quietly lowers the rate that goes on to compound. Sheltered accounts are the exception — an ISA, a 401(k), an IRA and their equivalents — and over decades the difference is large, because untaxed interest keeps compounding on itself. Nothing is deducted here. For an after-tax view, reduce the rate you type by your marginal rate.

Is anything I type here saved or sent anywhere?＋

No. The whole projection is arithmetic done in the page as you type: the loop that builds the year table runs on the five field values and no request is made. Nothing is stored either — each keystroke rebuilds the table and discards the previous one, and reloading brings back the built-in starting values rather than your numbers. One practical consequence is worth knowing before you spend time on a scenario: there is no save, no export and no copy button, so a projection you want to keep needs a screenshot before you navigate away.