Key facts
- A = P(1 + r/n)nt — interest earns its own interest, so growth accelerates.
- More frequent compounding grows money slightly faster: daily beats monthly beats yearly.
- Time matters more than rate: starting early beats investing more later.
- $10,000 at 7% compounded monthly for 10 years → about $20,097.
Compound interest is interest earning interest — the engine behind long-term wealth. This free compound interest calculator projects the future value of your savings from the starting amount, annual rate, number of years and compounding frequency.
Enter your principal, rate, years and how often interest compounds. The future value and the interest earned appear instantly — try monthly vs yearly compounding to see what frequency is worth.
Key facts
- A = P(1 + r/n)^(nt) — interest earns interest on earlier interest.
- Compounding frequency matters: monthly beats annual at the same nominal rate.
- Starting early beats investing more later — time is the biggest lever.
- The projected rate is an assumption; test a range (e.g. 8–12%) to see outcomes.
How to use
- Enter your initial investment and the annual interest rate.
- Choose how many years the money grows and the compounding frequency.
- The future value and interest earned appear instantly.
- Compare monthly vs yearly compounding to see the difference frequency makes.
Quick answer: Compound interest grows money on both the principal and previously earned interest: final amount equals principal times one plus the periodic rate, raised to the number of periods. More frequent compounding earns slightly more. The Rule of 72 gives a quick estimate: divide 72 by the annual rate to get years to double.
Last reviewed: October 2026
Compound interest formula
| Term | Meaning |
|---|---|
| A | Future value — what you end up with. |
| P | Principal — the starting amount. |
| r | Annual interest rate as a decimal. |
| n | Compounds per year (12 = monthly, 4 = quarterly, 1 = yearly). |
| t | Number of years. |
Each compounding period adds interest to the balance, and the next period pays interest on that larger balance — the snowball effect.
Worked examples
Ten years of growth
$10,000 at 7% compounded monthly for 10 years:
A = 10000 × (1 + 0.07/12)120 ≈ $20,096.61 — of which $10,096.61 is pure interest. The money roughly doubles.
Frequency comparison
$5,000 at 6% for 5 years, compounded quarterly:
A = 5000 × (1 + 0.06/4)20 ≈ $6,734.28 (interest $1,734.28).
Compounded yearly instead it would be $6,691.13 — quarterly compounding earns about $43 extra for free.
What compounding frequency is worth
| Frequency | n | $5,000 at 6%, 5 yrs |
|---|---|---|
| Yearly | 1 | $6,691.13 |
| Half-yearly | 2 | $6,719.58 |
| Quarterly | 4 | $6,734.28 |
| Monthly | 12 | $6,744.25 |
| Daily | 365 | $6,749.13 |
More frequent compounding always wins, but the gaps shrink: daily beats yearly by only about $58 here. Rate and time dominate.
Frequently asked questions
What is compounding frequency?
How often interest is added to the balance — yearly, quarterly, monthly or daily. More frequent compounding earns slightly more.
Why does compound interest matter for investing?
Because growth accelerates: each year’s gains earn their own gains. Starting early beats investing more later — the classic argument for starting retirement saving young.
Simple vs compound interest?
Simple interest pays only on the original principal. Compound interest pays on principal plus past interest, so it grows faster — compare with the Simple Interest Calculator.
What is the Rule of 72?
A shortcut: years to double ≈ 72 ÷ rate%. At 7%, money doubles in about 10.3 years.
Does inflation reduce the real return?
Yes. Subtract expected inflation from the nominal rate for the “real” growth in purchasing power.
How does compound interest work on credit card debt?
Credit card interest compounds daily on your average balance, so unpaid balances snowball fast: at 24 percent APR, a $1,000 balance left untouched grows to about $1,269 in a year. Paying more than the minimum, early in the billing cycle, shrinks the balance that interest is charged on.
How do monthly deposits change compound interest growth?
Each deposit starts earning interest immediately, so regular contributions dramatically raise the final balance. With monthly deposits, use the future value of an annuity formula alongside the lump-sum growth formula. Starting early matters more than the rate: time is the biggest lever.
Does compounding daily really beat compounding yearly?
Yes, but only slightly: $10,000 at 7% for 20 years grows to about $38,697 compounded yearly, $40,387 monthly and $40,547 daily. Moving from yearly to monthly matters far more than monthly to daily — time in the market beats frequency.
Why does starting early beat investing more later?
Because compounding multiplies time, not just money. Saving $200 a month at 7% for 40 years reaches about $524,963; starting 10 years later reaches about $243,994; starting 20 years later only about $104,185 — even though the late starter still invests $48,000 of their own money.
Related: 401K Calculator with Employer Match
Related: Retirement Calculator: How Much to Retire
Sources
- NerdWallet — Compound interest explained with examples
- Federal Reserve — U.S. central bank; interest-rate background
Key takeaways
- A = P(1 + r/n)nt — interest on interest is what makes savings snowball.
- Time beats rate: an early start matters more than a slightly higher return.
- Use the Rule of 72 (72 ÷ rate% ≈ years to double) for quick estimates.
Comments
Leave a comment