Compound Interest and Savings Growth Calculator

Compound Interest Calculator

Compound interest is what happens when your interest starts earning interest of its own. Early on it looks unremarkable. Given enough years it quietly becomes the largest part of your balance.

This free compound interest calculator shows you exactly that. Enter a starting amount, what you add each month, a yearly rate, and a number of years. You get the projected balance, a chart tracking your money against what you actually deposited, and a clear split showing how much came from your own pocket and how much came from growth.

No sign up and no email. Everything runs in your browser, so none of your figures are stored or sent anywhere.

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Watch Your Savings Grow

See how a little saved each month can snowball over the years. Enter your numbers and watch compound interest do the heavy lifting.

Your savings plan

How Compound Interest Actually Works

Simple interest pays you only on the money you originally put in. Compound interest pays you on your deposit plus every bit of interest you have already earned. That second part is the whole story.

Put 1,000 dollars away at 7 percent. The first year earns 70 dollars. The second year earns interest on 1,070 rather than 1,000, so it earns a little more. The difference in year two is trivial. The difference in year twenty five is enormous, because by then most of your balance is interest earning more interest.

This is why the growth curve bends upward instead of running in a straight line. The chart in the calculator makes it visible. The dashed line is your own deposits, rising steadily. The solid line is your balance, pulling further and further away from it as the years pass.

The Formula

The calculator runs two pieces and adds them together. The first grows your starting amount. The second handles your monthly deposits, each of which grows for however many months it has left.

Balance = P × (1 + i)n  +  M × [ ((1 + i)n − 1) ÷ i ]
SymbolWhat it is
PYour starting amount
MYour monthly deposit
iThe monthly rate, meaning your yearly rate divided by 12
nThe total number of months, meaning years times 12

One technical note that explains small differences between calculators. This tool compounds monthly and treats your yearly rate as a nominal rate. Enter 7 percent and the effective annual growth works out to about 7.23 percent, because the monthly compounding stacks up slightly. Tools that compound annually will show a marginally smaller figure from the same inputs.

Why Starting Early Beats Saving More

This comparison is the single most useful thing on this page, so it is worth walking through slowly.

Two people both save 200 dollars a month at 7 percent.

The first starts at 25 and saves for ten years, then stops completely at 35 and never adds another dollar. She contributed 24,000 dollars in total. That balance then sits untouched for thirty more years.

The second waits until 35, then saves diligently every month until 65. That is thirty years of contributions totalling 72,000 dollars, three times as much.

Saver ASaver B
Saving yearsAge 25 to 35Age 35 to 65
Total deposited$24,000$72,000
Balance at 65About $281,000About $244,000

Saver A put in one third as much money and finished ahead. Her ten years of deposits simply had more time to compound, and time is the input that does the heavy lifting.

What this means if you are starting late

The honest reading is not that late savers are doomed. It is that the balance shifts. With fewer years available, the amount you contribute has to carry more of the load, because compounding has less room to work. Starting at 45 is far better than not starting, and the same graph that rewards early savers still bends upward for you. It just bends later.

The Rule of 72

A quick mental shortcut for how long money takes to double. Divide 72 by your yearly rate and you get roughly the number of years.

Yearly rateYears to double
2%About 36 years
4%About 18 years
6%About 12 years
8%About 9 years
10%About 7 years

It is an approximation rather than exact maths, and it drifts at very high rates, but for anything in the normal range it is close enough to do in your head. It also shows why a couple of percentage points matter so much. The gap between 4 percent and 8 percent is not twice the money. Over forty years it is dramatically more, because you get twice as many doublings.

Choosing a Realistic Rate

The rate box is where this calculator can either inform you or mislead you, so it is worth thinking about what number belongs there.

Seven percent is a common default because it sits near the long run average of a broad stock market portfolio after allowing for inflation. It is not what a savings account pays, and it is not guaranteed anywhere.

Where the money sitsWhat to expect
Regular savings accountLow and variable, moving with central bank rates. Safe and liquid.
High yield savings or money marketHigher than a standard account, still variable and still modest over long periods.
Certificates of depositFixed and known in advance, in exchange for locking the money up.
BondsHistorically between cash and stocks, with less swing than stocks.
Broad stock market fundsHighest long run average historically, with real losses along the way and no guarantee.

Deposit rates change constantly, so any specific figure printed on a page goes stale quickly. Check current rates rather than trusting a number in an article, including this one.

The other thing worth understanding is that an average is not an experience. A portfolio averaging 7 percent does not deliver 7 percent each year. It delivers 22 percent, then negative 9, then 14, and so on. The calculator draws a smooth curve because smooth curves are easier to read, not because that is what actually happens.

What the Number Is Really Worth

The calculator gives you future dollars, and future dollars buy less than today's dollars.

Take the default settings: 1,000 to start, 200 a month, 7 percent, twenty years. That produces roughly 108,000 dollars. At 3 percent inflation over the same twenty years, that balance would buy about what 60,000 dollars buys today. Still a substantial result from 49,000 dollars of deposits, but a very different number from the one on the screen.

How to see today's money instead

Enter a real rate rather than a nominal one. Subtract your inflation assumption from your expected return, so 7 percent growth against 3 percent inflation becomes roughly 4 percent. The projection shrinks, and what you are left with is the version you can actually compare against the life you want.

What the Calculator Leaves Out

  • Taxes. Interest and investment gains are often taxable depending on the account type and where you live. Tax sheltered accounts behave differently from ordinary ones.
  • Fees. Fund and platform charges come straight off your return. One percent a year sounds small and is not, across decades.
  • Inflation. Not applied automatically, as covered above.
  • Market ups and downs. The model assumes identical steady growth every single month.
  • Changing deposits. It assumes the same amount every month forever, where most people's saving rises with income and pauses during difficult periods.

Compound Interest Runs Backward Too

Everything on this page works identically when you owe the money, which is the part people notice much later than they should.

A credit card charging around 20 percent compounds against you on the same curve. Paying only the minimum means a large share of each payment goes to interest, the balance barely moves, and the debt can persist for years on a purchase you have long forgotten.

That is also why clearing expensive debt is often the highest return move available. Paying off a balance charging 20 percent is a guaranteed 20 percent return, which no savings account or portfolio can promise you.

This calculator is an educational illustration, not financial advice, and Number Know is not a financial advisor. It cannot account for your tax situation, your account types, fees, or your risk tolerance, and past investment returns never guarantee future ones. Before making decisions about saving, investing, or paying down debt, speak with a qualified financial professional who can look at your full picture.

Frequently Asked Questions

What is compound interest?

Interest calculated on your original deposit plus all the interest already added to it. Because each round of interest joins the balance, later interest is bigger than earlier interest, and growth accelerates over time.

What is the difference between simple and compound interest?

Simple interest is paid only on the original amount, so it grows in a straight line. Compound interest is paid on the growing balance, so it curves upward. Over one year the difference is small. Over thirty it is the whole game.

How often does interest compound?

It varies by product. Daily, monthly, quarterly, and yearly are all common. More frequent compounding produces slightly more. This calculator compounds monthly.

What is a good interest rate to use?

It depends entirely on where the money sits. Cash savings pay far less than the long run average of a diversified stock portfolio, and that gap is the price of certainty. Seven percent is a common planning figure for long term investing, not for a savings account.

What is the rule of 72?

Divide 72 by the yearly rate for a rough number of years until your money doubles. At 6 percent that is about twelve years. It is an approximation, not exact.

Is it too late to start saving?

No. Starting late means contributions matter more and compounding matters less, because there is less time. That shifts what your plan should emphasize, but the curve still bends upward.

Does this account for inflation?

Not automatically. Results are in future dollars. To see today's money, subtract your inflation assumption from the rate you enter.

Is the calculator free?

Yes. No sign up, no email, and no data stored. Everything runs in your browser.

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