Free Compound Interest Calculator

Your Compound Interest Results

The result card above leads with the ending balance — the future value of your money at the end of the time horizon you chose. Underneath, three rows show exactly where that figure came from.

Those three always add up to the ending balance, which makes the split easy to sanity-check. The Total You Put In box combines the first two: it is the money that came out of your pocket. Everything above that figure is growth.

Open View Growth Breakdown for the same numbers in table form, plus the compounding frequency in use and a growth multiple — the ending balance divided by what you put in. A multiple of 1.50× means every dollar you contributed became a dollar and fifty cents.

That interest share is the figure worth watching as you change the time horizon. Over short periods it is a small slice. Stretch the horizon and it grows disproportionately, because interest starts earning interest of its own. Try moving the time horizon from 5 years to 30 with everything else unchanged and watch which row grows fastest.

What Is Compound Interest?

Compound interest is interest earned on your original balance and on the interest already added to it. Each round of interest joins the balance, so the next round is calculated on a larger figure — and the one after that on a larger figure still.

The distinction sounds small and compounds into something large. Suppose an account credits interest once a year. In year one, interest is calculated on your deposit alone. In year two, it is calculated on the deposit plus year one's interest. By year twenty, a meaningful portion of each year's interest is being earned by previous years' interest rather than by anything you deposited.

This is also why starting early is advice you hear so often. Two people contributing identical amounts can finish with very different balances purely because one began a decade sooner. The early contributions are not larger; they simply have more years to compound.

The mechanism is neutral, though. It works exactly the same way when you are the borrower — see the FAQ below on debt.

How Is Compound Interest Calculated?

Three things determine the result: the formula itself, how often interest is credited, and whether you keep adding money.

The compound interest formula

The standard compound interest formula is:

Read it from the inside out. r/n is the interest rate for a single compounding period — an annual rate divided into twelve monthly slices, say. nt is how many of those periods occur in total. Raising (1 + r/n) to that power applies the period rate again and again, each time to the balance the previous period produced. That repeated application is compounding.

The formula on its own describes a lump sum left untouched. It does not account for money you add along the way, which is where most real savings plans differ — and why this calculator extends it.

Compounding frequency

The n in the formula is the compounding frequency: how often the interest is actually credited to your balance. Daily is n = 365, monthly is n = 12, quarterly is n = 4, and annually is n = 1.

For the same stated rate, more frequent compounding produces a slightly higher ending balance, because interest starts earning interest sooner. The effect is real but modest — far smaller than the effect of the rate itself or of the time horizon. There is also a ceiling: as compounding becomes infinitely frequent the result converges on a fixed limit rather than running away.

This is the difference between a quoted rate and an effective annual yield. Two accounts advertising the same rate are not identical if one compounds daily and the other annually, which is precisely what APY exists to express. Our guide to understanding interest rates covers APR versus APY in detail.

Adding regular contributions

Most people do not deposit once and walk away; they add money every month. Each contribution begins compounding from the moment it arrives, which means an early contribution earns for longer than a late one — the same dollar is worth more to your final balance the sooner it goes in.

That makes a plain application of the formula insufficient. This calculator therefore walks the balance month by month: it credits interest whenever a compounding period completes, then adds your contribution at the end of each month. That approach handles any combination of contribution timing and compounding frequency exactly, including awkward pairings like monthly deposits into a quarterly-compounding account.

Set the monthly contribution to zero and the tool reduces to the pure formula above — useful if you want to isolate what a single lump sum would do.

Compound Interest vs Simple Interest

Simple interest is calculated only on the original principal. It never earns interest on interest, so it grows in a straight line:

Over a single year at the same rate the two are nearly identical. Over a decade the gap is noticeable. Over thirty or forty years it is dramatic, because the compound curve keeps steepening while the simple line never changes slope.

Which one applies is a matter of the product, not your choice. Most savings accounts, certificates of deposit and investment accounts compound. Many installment loans — including typical US auto loans — are described as simple interest loans, meaning interest is charged on the outstanding principal and does not capitalize, so paying early genuinely reduces the interest you owe.

The practical takeaway: when you are saving, compounding is the thing you want working for you, and the way to maximize it is to leave the money alone for longer. When you are borrowing, you want the opposite — the shortest sensible term, so there is less time for interest to accumulate.

Daily, Monthly or Annual Compounding: What Changes

Changing the compounding frequency while holding the rate and the horizon constant changes the ending balance — but by less than most people assume. The ordering is always the same: daily compound interest beats monthly compound interest, which beats quarterly, which beats annual, because interest joins the balance sooner and starts earning on itself sooner.

The gain, however, shrinks with each step. Moving from annual to monthly captures most of the available benefit; moving from monthly to daily adds very little on top. Mathematically there is a hard ceiling — the continuous-compounding limit — that no frequency can exceed, which is why the improvements taper off rather than continuing indefinitely.

To see the size of the effect for your own numbers rather than taking anyone's word for it, set your figures in the calculator above and switch the Compounding Frequency selector between Annually and Daily. Watch the ending balance move, and note how small the swing is compared with changing the rate by a single percentage point, or extending the horizon by five years.

One practical note: the compounding frequency should match what your account actually does, which will be stated in its terms. Guessing daily compound interest when the account in fact credits monthly compound interest will overstate your projection slightly.

Compound Interest by Time Horizon

Time is the input with the most leverage on savings and investment growth, and it is the one people most often underestimate. Rather than publish a table of figures built on a rate we have invented, the most honest demonstration is one you run yourself with your own assumptions.

Here is a method that takes about a minute and shows the effect clearly:

1. Enter your real starting amount and monthly contribution in the calculator above.
2. Enter a rate you have actually been quoted, or one you are willing to assume for the sake of comparison. We deliberately do not suggest one.
3. Set the time horizon to 5 years and note the interest earned row and the growth multiple.
4. Change it to 10, then 20, then 30 years, keeping everything else fixed, and write down the interest earned each time.

What you will see is that the interest figure does not merely double when the years double — it grows far faster than that, while the amount you contributed grows in a straight line. Somewhere along that progression the interest overtakes everything you put in, and from then on the majority of your balance is growth rather than deposits. Exactly where that crossover falls depends on your rate and contribution, which is the point of running it with your own numbers.

The same exercise explains why a delay is expensive. Compare a 30-year horizon with a 25-year one: the five years you lose are the five in which the balance was largest, so they would have generated the most interest of any five years in the whole projection. Starting late does not cost you the first five years of growth — it costs you the last five, which are worth considerably more.

How to Use This Compound Interest Calculator

Five inputs, and the results update as you type:

1. Enter your starting amount — the balance you have today. Enter 0 if you are beginning from nothing and relying on contributions alone.
2. Add your monthly contribution, or leave it at 0 to model a single lump sum left to grow untouched.
3. Enter an annual interest rate. This is your own assumption — use a rate your account or provider has quoted. We do not supply one.
4. Choose a time horizon, from 1 to 40 years. This is the input worth experimenting with most.
5. Set the compounding frequency to match your account: daily, monthly, quarterly, semi-annually or annually.

Read the ending balance first, then the three-row split beneath it, then open View Growth Breakdown for the full table and the growth multiple.

A useful habit is to run the same scenario at two or three different rates rather than relying on one. If the outcome changes dramatically between a cautious assumption and an optimistic one, that tells you the plan depends heavily on returns you cannot control — which is worth knowing before you build around it.

Nothing you type is stored or sent anywhere, so run as many scenarios as you like.

Our Methodology

Projections use the standard compound interest formula A = P(1 + r/n)^nt. Because contributions are monthly while compounding may be daily, quarterly or annual, the balance is walked month by month rather than solved in one step: interest is credited whenever a compounding period completes, and your contribution is added at the end of each month. Both methods were computed independently and agree to the cent.

The rate of return is entirely your input. We do not supply, recommend, average or forecast a rate, and no figure on this page represents a market return. Any example here uses either your own entries or figures explicitly labelled for illustration. For impartial information on investing, see Investor.gov from the SEC.

What the projection excludes: tax on interest or gains, account fees and fund charges, inflation (results are in today's dollars, not adjusted for purchasing power), and investment risk. Real returns are not a steady line — they vary year to year and can be negative — whereas this model applies one constant rate throughout. Treat the output as an illustration of how compounding behaves, not as a forecast.

Figures shown in the result card are rounded to the nearest cent for display, with no rounding applied during the calculation itself. If you find a figure that looks wrong, please tell us — we check every report.

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