Free calculator

Compound Interest Calculator

Run a starting balance and a monthly deposit forward through years of compounding, or name the target and solve for the deposit that gets you there.

Estimated. Every figure here is a projection built from the assumptions you entered. It is not a forecast, not a guarantee, and not financial advice. Real returns arrive unevenly and inflation rarely matches the assumption. OMM is a tracking and coaching app, not a broker or advisor.

Future value in 20yr
$301K
ESTIMATEDbefore inflation
In today's dollars
$167K
ESTIMATEDat 3.0% inflation
Total contributed
$130K
start + 20yr of deposits
Interest earned
$171K
ESTIMATED56.8% of the final balance
Growth over 20 years
Year-by-year breakdownESTIMATED
YearDepositedInterestEnd balanceIn today's $
0$0$0$10,000$10,000
1$6,000$919.19$16,919.19$16,426.40
2$12,000$2,338.58$24,338.58$22,941.44
3$18,000$4,294.31$32,294.31$29,553.86
4$24,000$6,825.16$40,825.16$36,272.62
5$30,000$9,972.70$49,972.70$43,106.89
6$36,000$13,781.53$59,781.53$50,066.09
7$42,000$18,299.43$70,299.43$57,159.87
8$48,000$23,577.68$81,577.68$64,398.17
9$54,000$29,671.22$93,671.22$71,791.19
10$60,000$36,639.02$106,639.02$79,349.44
11$66,000$44,544.25$120,544.25$87,083.73
12$72,000$53,454.70$135,454.70$95,005.20
13$78,000$63,443.02$151,443.02$103,125.33
14$84,000$74,587.14$168,587.14$111,455.96
15$90,000$86,970.62$186,970.62$120,009.32
16$96,000$100,683.03$206,683.03$128,798.03
17$102,000$115,820.45$227,820.45$137,835.12
18$108,000$132,485.91$250,485.91$147,134.07
19$114,000$150,789.85$274,789.85$156,708.81
20$120,000$170,850.72$300,850.72$166,573.75

That curve is arithmetic doing exactly what you asked. Your portfolio won't be that obedient. OMM tracks the version with your name on it, logging every dividend by ex-date and options premium into the same ledger, one combined income number beside one honest total return. Start free. No card, no brokerage login.

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How it works

How to read this

The chart splits your balance into three bands. The bottom is the principal you started with, the middle is every deposit you added, and the top is the interest all of it earned. Watch the top band. Put in $500 a month at 7% and you reach about $405,000 by year 25, though only $150,000 of that ever came out of your pocket. Around year 19 the interest you’ve earned overtakes everything you’ve deposited, and from there the account does most of the saving for you.

The second number under the result is the one to take seriously. At 3% inflation, that $405,000 buys what about $193,000 buys today, so the calculator shows both and never blends them. And the inputs are guesses. Markets pay lumpy years that only average to 7% if you stay through the ugly ones, so run a lower return before you trust the happy case. Try the step-up too. A deposit that rises the way a salary does finishes far ahead of one that never moves. When the plan holds up on paper, OMM measures the real thing on your actual portfolio: every dividend by ex-date, options premium in the same ledger, one combined income number beside one honest total return. Project the plan here. Then go watch the real one compound.

The math

The formula behind the curve

A=P(1+rn)ntA = P\left(1 + \frac{r}{n}\right)^{nt}

One line of algebra draws that whole chart. P is your principal, the money you start with. r is the annual rate written as a decimal, so 7% becomes 0.07. n is how many times a year interest gets credited, 12 if monthly. t is the years you leave it alone, and A is what you walk away with. Read it from the inside out. Divide the rate across the year’s compounding periods, add 1, then raise it to the power of every period that passes. The exponent is the entire story. Each period earns interest on everything every earlier period earned.

A=10,000(1+0.0712)12×10=$20,097A = 10{,}000\left(1 + \tfrac{0.07}{12}\right)^{12\times10} = \$20{,}097

Take the $10,000 starting balance at 7% compounded monthly, switch the deposits off, and give it ten years. That comes to $20,097. The money doubled and you never added a cent. This page runs that exact formula for your starting balance, and the deposits stack on top. Each monthly deposit then gets the same treatment from the month you make it, and the calculator adds up the piles. That stacking is the part no single formula covers, and the reason the deposits need a calculator while the lump sum only needs algebra.

Common questions

Frequently asked

Is this compound interest calculator financial advice?
No. It's arithmetic on the numbers you typed in. It doesn't know your tax bracket, your timeline, or whether the 7% you entered is brave or sane. Neither does OMM. We're a tracking and coaching app, not a broker or an advisor: we show you what your money is doing, and what you do about it is your call.
What is the difference between simple and compound interest?
Take a $10,000 lump, left alone for 30 years at 7%. Simple interest pays on your principal alone, so you collect the same $700 every year, and 30 years hands you $21,000 in interest and $31,000 in total. Compound interest pays on the principal plus every dollar of interest already earned, so each year's $700 joins the base and starts earning its own. That same $10,000 at 7% compounded annually reaches about $76,000 in 30 years. The extra $45,000 is interest earned by interest. Most things you would actually hold compound, from a savings account to an index fund with dividends reinvested, which is why this calculator only does compound. But look at the size of that gap once. It is the whole argument for leaving money alone.
What compounding frequency should I use?
It moves the answer less than people expect. $10,000 at 7% for 30 years grows to about $76,000 compounded annually and about $81,000 compounded monthly. Now drop the return to 6% and the annual case falls to about $57,000. The frequency changed the outcome by $5,000; the return assumption changed it by $19,000. So match the frequency to the thing you're modeling. A savings account credits interest monthly. A stock portfolio is better modeled annually, since long-run market returns are quoted as annual numbers. Then put your worry where the leverage is, on the return.
Why show the value in today's dollars?
Because inflation is the quiet counterparty to every long projection. At 3% a year, prices double roughly every 24 years, which means $1,000,000 arriving in year 25 spends like about $478,000 does today. Both numbers are true. The nominal one is what your statement will say. The today's-dollars one is what the money will buy. This calculator keeps them side by side and never sums or blends them, the same discipline OMM applies when it refuses to mix options income into total return. If the today's-dollars figure still clears your goal, you built the plan on the right number.
Should I enter a nominal or a real return?
Nominal, then let the inflation field do the subtracting. Returns get quoted two ways. Nominal is the raw number a statement shows. Real is what's left after inflation. This calculator deflates the final balance by whatever inflation you set, so if you type a return you already trimmed for inflation, set inflation to 0 or the haircut lands twice. The trap is easy to see on the defaults. At 7% with 3% inflation, $10,000 plus $500 a month for 20 years shows about $301,000 on paper and about $167,000 in today's dollars. Enter 4% instead because you already took inflation out, leave the field at 3%, and today's dollars reads about $114,000. The missing $53,000 is the same inflation subtracted twice. For scale, US large-cap stocks have averaged around 10% a year nominal over the long run, which is why the 7% default here, with inflation switched on, is a sober assumption and not an optimistic one.
How long does it take for money to double?
Divide 72 by your annual return and you get the answer in years, close enough for planning. At 7% the rule says a little over ten years, and the calculator agrees: a $10,000 lump with no monthly deposit doubles to about $20,000 in year 10, and reads about $18,742 in year 9. The same shortcut runs on prices. Divide 72 by 3% inflation and the cost of living doubles about every 24 years, which is why this page keeps showing you today's dollars next to the headline number. The rule runs in reverse, too. A balance compounding against you doubles on the same 72-divided-by-rate clock, so the arithmetic that builds a portfolio also builds a debt.
How does goal-seek work?
You give it the destination and it solves the route backward. Type a target and a year, and the calculator finds the monthly deposit that lands there under the return, compounding, and timing assumptions you set. For scale, $1,000,000 in 25 years at 7% with monthly compounding, starting from zero, takes about $1,234 a month. Give it 30 years instead and the number falls to about $820. That gap is the whole argument for starting early. If the answer looks impossible, lengthen the horizon or trim the target and watch which one moves the number more.
Can OMM track my real returns, not just project them?
Yes. Tracking the real thing is the product. A calculator draws one clean line out of your assumptions, and a real portfolio never follows it. OMM logs every dividend you’re paid by ex-date and books covered-call and cash-secured-put premium into the same ledger, so your income shows up as one combined number. Total return sits beside it, benchmarked against the S&P 500, with nothing quietly blended in. The free plan covers your first 10 holdings and 5 options positions. No card, no brokerage login.

Each tool shows one income stream. OMM is the only place you see both dividends and options income in one honest return.

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