Saving & Investing
Compound Interest Calculator
Project what regular investing could grow to over decades — and see the same figure in today's money, which is the number that tells you what it would actually buy.
What it could grow to
Balance after 30 years
$691,150.47
In the dollars of that year.
Worth today
$284,744.84
What $691,150 would buy today, deflated at 3% a year for 30 years.
Where the balance came from
- Starting amount
- $10,000.00
- Contributions
- $180,000.00
- Total you put in
- $190,000.00
- Growth 72.5% of the end balance was earned rather than deposited.
- $501,150.47
Return across the whole plan
- Money-weighted return Return across the whole contribution stream, not an APY. It sits below the quoted rate because later deposits compound for fewer years, which is arithmetic rather than a cost.
- 4.4%
Starting with $10,000.00 and adding $500.00 a month, $190,000.00 of your own money grows to $691,150.47 over 30 years at 7%. Of that, $501,150.47 — 72.5% of the final balance — is growth rather than money you put in. In today's money that balance is $284,744.84, because 3% annual inflation erodes what a dollar buys over 30 years. The projection assumes a steady return, which no real investment delivers — actual results depend on the order returns arrive in as well as their average.
Contributions against growth
The solid line is the balance, rising from $10,000 to $691,150 over 30 years. The dashed line is what you have put in, reaching $190,000. The widening gap between them is growth, $501,150.47 in total. Growth overtakes total deposits in year 17, after which the balance owes more to compounding than to you.
The solid line is the balance. The dashed line is every dollar you have put in, starting amount included. The gap between them is growth — narrow for the first decade, because compounding has little to work with, and then widening as the earlier money has had longer to compound. The point where the gap exceeds the dashed line itself is where the balance owes more to growth than to you.
Year by year
Contributions and growth on each row sum to the balance, so any row can be checked by hand. The last column restates the balance in today's money, which is the column that shows what the projection is really worth.
| Year | Balance | You put in | Growth | Worth today |
|---|---|---|---|---|
| 1 | $16,919 | $16,000 | $919 | $16,426 |
| 2 | $24,339 | $22,000 | $2,339 | $22,941 |
| 3 | $32,294 | $28,000 | $4,294 | $29,554 |
| 4 | $40,825 | $34,000 | $6,825 | $36,273 |
| 5 | $49,973 | $40,000 | $9,973 | $43,107 |
| 6 | $59,782 | $46,000 | $13,782 | $50,066 |
| 7 | $70,299 | $52,000 | $18,299 | $57,160 |
| 8 | $81,578 | $58,000 | $23,578 | $64,398 |
| 9 | $93,671 | $64,000 | $29,671 | $71,791 |
| 10 | $106,639 | $70,000 | $36,639 | $79,349 |
| 11 | $120,544 | $76,000 | $44,544 | $87,084 |
| 12 | $135,455 | $82,000 | $53,455 | $95,005 |
| 13 | $151,443 | $88,000 | $63,443 | $103,125 |
| 14 | $168,587 | $94,000 | $74,587 | $111,456 |
| 15 | $186,971 | $100,000 | $86,971 | $120,009 |
| 16 | $206,683 | $106,000 | $100,683 | $128,798 |
| 17 | $227,820 | $112,000 | $115,820 | $137,835 |
| 18 | $250,486 | $118,000 | $132,486 | $147,134 |
| 19 | $274,790 | $124,000 | $150,790 | $156,709 |
| 20 | $300,851 | $130,000 | $170,851 | $166,574 |
| 21 | $328,796 | $136,000 | $192,796 | $176,744 |
| 22 | $358,760 | $142,000 | $216,760 | $187,234 |
| 23 | $390,892 | $148,000 | $242,892 | $198,062 |
| 24 | $425,345 | $154,000 | $271,345 | $209,242 |
| 25 | $462,290 | $160,000 | $302,290 | $220,792 |
| 26 | $501,905 | $166,000 | $335,905 | $232,731 |
| 27 | $544,384 | $172,000 | $372,384 | $245,076 |
| 28 | $589,934 | $178,000 | $411,934 | $257,847 |
| 29 | $638,777 | $184,000 | $454,777 | $271,063 |
| 30 | $691,150 | $190,000 | $501,150 | $284,745 |
What this calculates
This grows a starting balance and a stream of regular contributions at a rate you choose, stepping through the schedule period by period rather than applying a single closed-form annuity formula. It reports the nominal future value, what you put in, what the growth added, and — the figure this page is built around — the same balance restated in today's purchasing power. A year-by-year timeline shows how the balance divides between contributions and growth over time, which is where the shape of compounding becomes visible: for the first decade or so the money you add dominates, and only later does the growth outrun it.
How it works
Periodic rate i = annual rate ÷ compounds per year
Steps per year s = max(compounds per year, contributions per year)
Then, for each step from 1 to years × s:
if a contribution falls due and timing is 'begin':
balance = balance + contribution
if interest accrues this step:
balance = balance × (1 + i)
if a contribution falls due and timing is 'end':
balance = balance + contribution
Totals:
total invested = principal + contributions
total growth = future value − total invested
growth multiple = total growth ÷ total invested
Real terms:
real value = future value ÷ (1 + inflation)^years
The loop steps on the finer of the two schedules, so a
monthly contribution into a daily-compounding account is
modelled as it happens rather than approximated by assuming
the two intervals match.Compounding is the habit of interest earning interest. A balance grows, the growth joins the balance, and next period's growth is calculated on the larger figure — so the curve steepens rather than rising in a straight line. That is the whole mechanism, and its effect over a long horizon is large enough that the arithmetic tends to be described in language better suited to conjuring tricks. It is worth resisting that, because the plain version is more useful.
Time does more work here than anything else. On the default scenario the balance passes $16,919.19 after one year and $106,639.02 after ten, of which $60,000 was contributed and $36,639.02 was growth. Growth has not yet overtaken contributions at the ten-year mark. Over the following twenty years the contributions add another $120,000 while growth adds a further $464,511.45 — the same monthly deposit, the same assumed rate, and a completely different division of the result. Nothing changed except how long the earlier money had been compounding.
The rate matters nearly as much and is the input you control least. A quoted return is a number you are assuming, not one anyone has agreed to pay, and a smooth rate is a modelling device rather than a description of markets. Real returns arrive unevenly: long flat stretches, sharp falls, recoveries of uncertain length. Two portfolios that average the same return over thirty years can end very far apart if one suffers its worst years while the balance is large and the other while it is small. This calculator cannot show that, and a single-rate projection should be read as one arithmetic scenario rather than a forecast.
Compounding frequency, by contrast, matters far less than its prominence suggests. Moving $10,000 at 7% from annual to monthly compounding changes the thirty-year result by a few percent; moving the rate from 7% to 8%, or the term from thirty years to thirty-five, changes it by far more. If you are choosing between accounts, the rate and the fees decide it, and the compounding interval is close to a tiebreaker.
Which leaves the figure this page treats as the headline. A projection in nominal dollars answers the question "how many dollars will there be", and almost nobody is actually asking that. The question is what those dollars will buy. Deflating the balance by the inflation rate over the same period answers that instead, and on the default scenario it cuts $691,150.47 to $284,744.84. Both numbers are correct. Only one of them is about your life.
A worked example
Starting with $10,000, contributing $500 a month, assuming a 7% annual return compounded monthly over 30 years, with inflation assumed at 3%.
Over thirty years the contributions come to $180,000, which with the $10,000 starting balance means $190,000 left your account. The projection ends at $691,150.47, so growth accounted for $501,150.47 — a growth multiple of 2.638, meaning the money earned about two and a half times again what was put in.
The path there is less even than the total suggests. After one year the balance is $16,919.19: $6,000 contributed and $919.19 of growth. After ten years it is $106,639.02, of which $60,000 is contributed and $36,639.02 is growth — so a decade in, contributions are still comfortably ahead. The crossover arrives later than most people expect, and everything dramatic about compounding happens on the far side of it.
Now the figure that changes what the projection means. At 3% inflation, $691,150.47 in thirty years' time has the purchasing power of $284,744.84 today. The nominal total overstates what the money would buy by roughly 2.4 times. That is not a defect in the arithmetic and it is not pessimism; it is the difference between counting dollars and counting what dollars do. A plan built on the $691,000 figure is a plan built on a number that will not buy what it appears to.
Timing is a smaller lever, and worth measuring rather than guessing at. Moving the same contributions to the start of each period instead of the end gives $694,708.72 rather than $691,150.47 — a difference of $3,558.25 over thirty years. Real money, and about half of one percent of the total. Useful if the choice is free, not worth restructuring anything for.
It is also worth seeing what the starting balance alone does. The $10,000, left entirely alone at 7% compounded monthly for thirty years, reaches $81,164.97 — $71,164.97 of growth from a single deposit. Monthly compounding on a 7% nominal rate produces a true annual percentage yield of 7.229%, which is the correct way to describe a lump sum with no contributions.
That last figure does not carry over to the scenario with contributions, and the difference is a trap worth naming. With a contribution stream the engine reports 4.3984%, which is a money-weighted figure across everything invested — final value divided by total invested, annualised over the term. It is lower than the 7% assumption not because the return was worse but because most of the money arrived late: a contribution made in year twenty-eight compounds for two years, not thirty, while the arithmetic spreads it across the full thirty. It is a description of the plan's overall efficiency, not a rate of return, and it should never be compared with a quoted rate.
What this assumes
- The return you enter is an assumption you are choosing, not a rate anyone has offered or guaranteed. The projection is arithmetic on that assumption and nothing more.
- The rate is applied smoothly and identically to every period. Real markets do not deliver a smooth rate, and this is the single largest simplification on the page.
- Interest compounds monthly by default. The engine allows the compounding and contribution intervals to differ and steps through the finer of the two, so neither is approximated.
- Contributions are the same amount for the whole term. They do not rise with inflation, with pay, or with anything else.
- Every contribution arrives exactly on schedule, and none are missed, paused or withdrawn.
- Contributions at the end of each period earn no growth in the period they are made; contributions at the start earn one extra period. That choice is yours and the difference is reported.
- Inflation is applied at a single constant rate across the whole term to restate the final balance in today's money. Set it to zero and the real and nominal figures coincide.
- No money is withdrawn during the term. This models accumulation only.
What it does not model
- Taxes are not modelled at all. Dividends, interest and realised gains may be taxable each year in a regular brokerage account and not in a tax-advantaged one, which can make a large difference to the same nominal projection. Your rate, your account type and your jurisdiction all matter and none of them appear here.
- Fees and expense ratios are ignored, and this omission is larger than it looks. Costs are levied on the whole balance every year, so they compound against you exactly as returns compound for you. On a thirty-year horizon a single percentage point of annual cost is not a detail — it is a structural change to the result. Enter a rate net of expected costs if you want the projection to reflect them.
- Volatility is not modelled. This grows the balance at one smooth rate, and a real portfolio delivering the same average return would arrive by an entirely different path. The order in which good and bad years fall — sequence of returns — can change the outcome substantially, and matters more the larger the balance has become.
- It cannot tell you what rate to assume. There is no default that is correct for a particular portfolio, and the 7% here is a round illustrative figure rather than a claim about any asset class.
- The inflation adjustment uses one rate for everything. Actual inflation varies by year and by category, and the things a specific person spends money on — housing, healthcare, education — have often not tracked the general index.
- Contributions are fixed in nominal terms, which means their real value falls across the term. Someone contributing $500 a month for thirty years is contributing progressively less in purchasing power each year.
- The effective annual rate reported alongside a contribution stream is a money-weighted figure, not a rate of return and not an APY. It is lower than the rate you entered because later contributions compound for less time, and comparing it with a quoted rate is not a meaningful comparison.
- This is not advice on whether to invest, what to invest in, or whether this plan suits your circumstances. It is one arithmetic scenario, and its usefulness depends entirely on assumptions you supplied.
Questions
Why is the inflation-adjusted figure so much lower than the projection?
Because thirty years of inflation is a long time and it compounds too. On the default scenario the nominal projection is $691,150.47 and the same balance in today's purchasing power is $284,744.84 — the nominal figure overstates what the money would buy by roughly 2.4 times. Neither number is wrong; they answer different questions. The nominal figure tells you how many dollars there will be, and the real figure tells you what those dollars would buy at today's prices. Since what you actually want from an investment is goods, services and time rather than dollars, the real figure is the one to plan against. It is also why most compound calculators feel more encouraging than this one: they report only the large number.
Does compounding frequency make much difference?
Much less than its prominence suggests. Moving from annual to monthly compounding at the same rate changes a thirty-year result by a few percent — real, but small beside the other inputs. To see the scale: $10,000 at 7% compounded monthly for thirty years reaches $81,164.97, and monthly compounding turns that nominal 7% into a true annual percentage yield of 7.229%. That gap between 7% and 7.229% is the entire frequency effect on a lump sum. A quarter of a point on the rate itself, or a couple of extra years in the market, moves the result further than any compounding interval available. If you are comparing accounts, compare the rate and the fees; the compounding schedule is close to a tiebreaker.
What return rate should I use?
There is no answer this calculator can give you honestly, and it is worth being direct about that rather than supplying a confident number. The 7% default is a round illustrative figure, not a forecast and not a claim about any particular asset class. What a portfolio returns depends on what is in it, what it costs to run, and a future nobody has. The practical approach is to run the projection at several rates rather than trusting one — try two points lower and see whether the plan still works, because that tells you something a single scenario cannot. Whatever rate you choose, subtract your expected annual costs from it, since this calculator has no field for fees and they come out of the same growth.
Does it matter whether I contribute at the start or the end of the month?
It matters a little, and the amount is worth knowing precisely. A contribution made at the start of a period earns one extra period of growth, so on the default scenario contributing at the start gives $694,708.72 against $691,150.47 at the end — a difference of $3,558.25 across thirty years, or about half of one percent. Worth taking if your payroll or transfer schedule makes it free. Not worth reorganising your finances around, and not worth worrying about if your employer determines the timing. The reason this page exposes the choice at all is that most calculators pick one silently, and a reader comparing two tools can find a difference of several thousand dollars with no visible cause.
Why does the calculator show an effective rate lower than the rate I entered?
Because with a contribution stream that figure is money-weighted rather than a rate of return. It divides the final value by everything invested and annualises the result over the term, which on the default scenario gives 4.3984% against a 7% assumption. Nothing underperformed. The reason it is lower is that most of the money arrived late — a contribution made in year twenty-eight compounds for two years, not thirty — while the calculation spreads the whole $190,000 across the full term as though it had all been there from the start. It is a summary of how efficiently the plan as a whole converted deposits into value, and it should not be compared with a quoted rate. Only when there are no contributions does that field report a true annual percentage yield, which for a lump sum at 7% compounded monthly is 7.229%.
How much do fees and taxes take out of this?
Enough that the projection above should be read as a ceiling rather than an expectation, and neither is modelled here. Fees are the more insidious of the two because they are charged on the whole balance every year, so they compound against you on exactly the same mechanism that compounds returns for you — which means a fee that sounds trivial as an annual percentage is substantial as a share of a thirty-year result. Taxes depend on the account: a tax-advantaged account may defer or eliminate them, while a regular brokerage account can generate a tax bill on dividends and realised gains along the way. The practical adjustment is to enter a return net of your expected costs, and to treat the result as pre-tax unless the account is one where tax does not apply.
Is compound growth really guaranteed if I stay invested long enough?
No, and the smooth curve this calculator draws is the reason that question gets asked. What is guaranteed is the arithmetic: if a balance grows at a constant positive rate, it grows exponentially. What is not guaranteed is the rate. Market returns arrive unevenly, can be negative for years at a stretch, and there is no term length that makes a positive outcome certain. Sequence matters too — two portfolios averaging identical returns over thirty years can finish far apart depending on whether the bad years struck while the balance was small or large, and this model, growing everything at one steady rate, cannot show that at all. Read the projection as one arithmetic scenario conditional on assumptions you chose, which is a genuinely useful thing, rather than as a description of what will happen.
Sources
- Investor.gov (SEC) — Compound Interest Calculator (opens in a new tab)
The SEC's own compound interest tool and its explanation of how principal, contributions, rate and compounding frequency combine over time. It establishes the mechanics of compounding and the convention of projecting a chosen rate; it does not establish any expected rate of return, and it does not provide the inflation adjustment this page treats as the headline figure.
- 12 CFR 1030.2 — Regulation DD definitions, including annual percentage yield (opens in a new tab)
The regulatory definition of annual percentage yield as a total amount of interest expressed as an annualised percentage reflecting the effect of compounding — the basis for this page distinguishing a true APY, which applies to the lump-sum case, from the money-weighted figure the engine returns when contributions are present. It governs deposit-account disclosure and says nothing about investment returns or projections.
- Formula reviewed
- Formula version
- 1
Version 1 means the formula has not changed since this page was published. If it changes, this number moves and the change is described here.