Compound Interest FAQ

What is compound interest?

Compound interest is interest earned on both your original money and on the interest it has already earned, so each period's growth builds on the last and the balance grows faster the longer you stay invested. Our guide to how compound interest works walks through it with examples.

How is compound interest different from simple interest?

Simple interest pays only on your original amount, while compound interest pays on the whole balance: your original money plus all the interest earned so far. The gap widens with time. $10,000 at 7% reaches $17,000 with simple interest after 10 years but about $19,672 with annual compounding, and after 30 years it is $31,000 versus about $76,123.

How much difference does starting early make?

A large one. At 7% with monthly compounding (the calculator's default), $500 a month starting at age 25 grows to roughly $1.31 million by 65, while the same contributions starting at 35 reach about $610,000. The extra decade of compounding roughly doubles the outcome even though you invest only $60,000 more.

Is compound interest guaranteed?

The mechanism is, the rate is not. In a savings account, a CD, or a bond, the rate is contractual for its term, so the compounding is arithmetic the institution owes you. An expected market return is not a contract, so any projection built on one is a scenario you chose to test rather than a forecast. This calculator applies whatever rate you type without judging whether it is realistic, which is why every example here is described as illustrative.

What is the difference between compounding and reinvesting?

Compounding happens by default: interest is credited to your balance and the larger balance earns more next period. Reinvesting is a decision you make about a payout that lands somewhere else, such as a dividend or a coupon, to put it back to work. The feedback loop is identical either way, but a payout that leaves the account stops compounding the moment it leaves.

Where can I look up a term I do not recognize?

The compound interest glossary defines the vocabulary these pages use, including principal, nominal rate, periodic rate, APY, compounding period, annuity, and real return, each with a short worked illustration.

What is the Rule of 72?

The Rule of 72 is a mental shortcut: divide 72 by your annual return to estimate how many years it takes money to double. At 8%, money doubles in about 9 years; at 6%, about 12 years. See the Rule of 72 explained for how accurate it is and where it breaks down.

Does compounding frequency matter much?

Not much. At the same rate, more frequent compounding earns slightly more, but the gap is tiny. $10,000 at 5% for 10 years grows to $16,470.09 compounded monthly and $16,486.65 compounded daily, a difference of about $17. Your rate of return and time horizon matter far more than the schedule, as the daily vs monthly compounding breakdown shows.

What is APY, and how is it different from the interest rate?

APY (annual percentage yield) is what you actually earn in a year once compounding is included, which makes it the honest number for comparing accounts. A 5% nominal rate compounded monthly is a 5.116% APY, and compounded daily it is a 5.127% APY. Because APY already accounts for the frequency, comparing APYs is the only comparison you need.

What return rate should I assume?

The return rate is an input you choose, not a figure this calculator predicts. Our examples use illustrative rates such as 5% to 8% to show how the math behaves, but past results never guarantee future returns, so no rate here is a recommendation. A sensible approach is to model a scenario at one rate and then try a lower rate to see the downside. This is a math tool, not investment advice.

How long does it take money to double?

Divide 72 by the annual rate for a fast estimate. At 7% that predicts about 10.3 years, and the engine agrees: $10,000 at 7% compounded monthly passes $20,000 during year 10, closing that year at $20,097. The shortcut applies to a lump sum with no contributions, and the Rule of 72 explained shows where its accuracy starts to slip.

Does one extra percentage point of return really matter?

More than almost anything else you can change. Contributing $500 a month for 30 years with monthly compounding gives $502,258 at 6%, $609,985 at 7%, and $745,180 at 8%. That single point between 6% and 7% is worth $107,728, about 21% more balance, on identical deposits of $180,000.

Should I enter the nominal rate or the APY?

Enter the nominal rate and select the compounding frequency that matches the account, and the calculator produces the compounding itself. If the only number you have is the APY, enter it with annual compounding, since APY already states a full year of growth. Mixing them, by entering an APY and then also selecting daily compounding, double-counts the effect. APY vs APR covers which number banks quote and why.

What is continuous compounding?

It is the mathematical limit of compounding more and more often, and in practice it changes almost nothing. $10,000 at 5% for 10 years reaches $16,486.65 compounded daily and $16,487.21 compounded continuously, a gap of 56 cents. The calculator stops at daily for that reason; see daily vs monthly compounding for the full ladder.

How do monthly contributions change growth?

Dramatically, because each contribution starts its own compounding clock. A one-time $10,000 at 7% with monthly compounding grows to about $81,000 in 30 years, but adding just $200 a month turns it into about $325,000 over the same period. Regular contributions keep feeding the curve, which is why consistency matters as much as the rate.

Can compound interest work against me?

Yes. The same math that builds savings also grows debt: a credit card at 24% APR doubles an unpaid balance in about 3 years, and credit cards typically compound daily. Inflation works the same way, since at 3% prices double in about 24 years, which halves the buying power of idle cash.

What happens if I stop contributing?

The balance keeps compounding, which is most of the work. Ten years of $10,000 plus $500 a month at 7% compounded monthly reaches $106,639. Leave that alone for another 20 years and it becomes $430,687 without another dollar added. Keeping the contributions going for all 30 years would give $691,150 instead, so the pause costs $260,463, yet the untouched balance still more than quadrupled. Monthly contributions and starting early vs starting late take that trade apart.

How do I model annual, weekly, or biweekly contributions?

Convert to a monthly equivalent, because the calculator contributes once a month. $6,000 a year is $500 a month, $100 a week is $5,200 a year or about $433.33 a month, and $250 every two weeks is 26 payments or about $541.67 a month. The timing difference between paying weekly and paying monthly is worth a rounding error over long horizons, far less than the amount itself.

How much do fees and taxes take out?

Enough to model deliberately, which is why the honest approach is to enter a rate already net of both. One percentage point of annual fee applied to $500 a month over 30 years turns a 7% result of $609,985 into a 6% result of $502,258, a difference of $107,728. Taxes depend on the account type and jurisdiction, so this tool ignores them entirely. How fees compound works through the fee side in detail.

Does this calculator adjust for inflation?

No, it reports future dollars, so a long projection buys less than its size suggests. To see today's buying power, enter your real rate instead of your nominal one: at a 7% return and 3% inflation the real rate is 3.88%, which turns $500 a month for 30 years from $609,985 into $339,908. Inflation and compound interest shows the conversion and both views side by side.

Is this calculator private?

Yes. Every calculation runs entirely in your browser, and nothing you enter is uploaded, stored, or shared. There is no sign-up and no account, and the tool works the same on any device.

What do "total invested" and "growth multiple" mean?

Total invested is your starting amount plus every contribution you made, so it is all the money that came from you. Growth multiple is the final balance divided by that number. In the reference case of $10,000 plus $500 a month at 7% for 20 years, $130,000 goes in, $300,851 comes out, and the multiple is 2.31 times. Anything above 1.00 is interest.

Why does the calculator work only in whole years?

Because it simulates twelve months per year and reports one row per completed year, which keeps the chart and the table readable. A fractional entry is rounded down, so 7.5 years is treated as 7. There are two other limits worth knowing: the rate is capped at 100% and the horizon at 100 years, both to keep the chart legible rather than because anything breaks.

Can I model withdrawals, a raise in my contribution, or a rate that changes?

Not in a single run, but chaining runs works well. Compute the first stretch, then start a second run using that ending balance as the new starting amount with the new rate or contribution. The stop-contributing example above was built exactly that way, and the same trick handles a mid-plan withdrawal by subtracting it before the second run.

Is there a version of this for certificates of deposit?

Yes. The CD calculator is the same engine preset for the classic CD case: a lump sum, no monthly contribution, and daily compounding, which is the schedule most banks use. For choosing between CDs, compare the advertised APY rather than the compounding schedule.