How does compound interest work?
Compound interest pays interest on your interest, so the balance grows faster each year. The formula is A = P(1 + r/n)ⁿᵗ, where P is the principal, r is the annual rate, n is how many times a year it compounds, and t is the number of years. Put $10,000 at 7% compounded monthly and in 30 years it grows to about $81,000 — roughly $71,000 of it pure interest, without adding a cent.
How is compound interest different from simple interest?
Simple interest only ever pays on your original deposit; compound interest pays on the deposit plus all the interest already earned. On $10,000 at 6% for 20 years, simple interest adds $12,000 for a $22,000 balance. Compounded monthly, the same account reaches about $33,100 — over $11,000 more from compounding alone. The longer the time horizon, the wider that gap grows.
How much does compounding frequency matter?
More frequent compounding helps, but less than most people expect. $10,000 at 6% for 10 years grows to $17,908 compounded annually versus $18,220 compounded daily — a difference of about $312, or under 2%. The rate and the time you stay invested matter far more than daily-versus-monthly. A useful shortcut is the Rule of 72: divide 72 by the rate to estimate the years to double. At 8%, money doubles in roughly 9 years.
Our method and assumptions
This calculator applies the standard compound interest formula and adds optional monthly contributions as an end-of-month annuity that earns the same effective annual yield. It assumes a fixed rate for the whole period and no taxes or fees — real returns vary year to year, especially for investments. Treat the result as a planning estimate, not a guarantee or financial advice. For the underlying concept and a regulator’s own calculator, see the U.S. Securities and Exchange Commission’s Investor.gov compound interest guide.
How to calculate compound interest
- 1
Enter your starting amount. Type the money you have invested today — the principal, such as $10,000.
- 2
Set the rate and the years. Enter the annual interest rate and how long it grows. Use ~4% for savings, ~10% for the long-run stock-market average.
- 3
Choose the compounding frequency. Pick how often interest is added: daily, monthly, quarterly, or annually. Most accounts compound daily or monthly.
- 4
Add a monthly contribution (optional). Enter a recurring deposit to see how steady investing multiplies the result over time.
Compound interest reference tables
How $10,000 grows by rate and time, what monthly investing becomes at 8%, and the effective yield by compounding frequency.
How $10,000 grows by rate and years (monthly compounding)
Future balance of a one-time $10,000 investment, with no extra contributions. Scale linearly for other amounts — double it for $20,000.
| Years | 4% | 6% | 8% | 10% |
|---|---|---|---|---|
| 10 years | $14,908 | $18,194 | $22,196 | $27,070 |
| 20 years | $22,226 | $33,102 | $49,268 | $73,281 |
| 30 years | $33,135 | $60,226 | $109,357 | $198,374 |
| 40 years | $49,399 | $109,575 | $242,734 | $537,007 |
Source: Socko calculation: A = P(1 + r/n)^(nt), n = 12.
What investing every month becomes at 8%
Future value of a fixed monthly contribution at an 8% annual return, compounded monthly, starting from $0.
| Monthly | 10 years | 20 years | 30 years |
|---|---|---|---|
| $100/mo | $18,295 | $58,902 | $149,036 |
| $250/mo | $45,737 | $147,255 | $372,590 |
| $500/mo | $91,473 | $294,510 | $745,180 |
| $1,000/mo | $182,946 | $589,020 | $1,490,359 |
Source: Socko calculation: future value of an ordinary monthly annuity at 8%/yr.
Effective annual yield by compounding frequency
A nominal rate yields slightly more the more often it compounds. The jump from annual to monthly is real; from monthly to daily, tiny.
| Nominal rate | Annually | Monthly | Daily |
|---|---|---|---|
| 4% | 4.00% | 4.07% | 4.08% |
| 6% | 6.00% | 6.17% | 6.18% |
| 8% | 8.00% | 8.30% | 8.33% |
| 10% | 10.00% | 10.47% | 10.52% |
Source: Socko calculation: APY = (1 + r/n)^n − 1.
Frequently asked questions
How much will $10,000 grow with compound interest?
At 7% compounded monthly, $10,000 becomes about $20,100 in 10 years, $40,400 in 20, and $81,200 in 30. Rate and time drive the result.
What is the compound interest formula?
A = P(1 + r/n)ⁿᵗ — P is the principal, r the annual rate, n the compounds per year, and t the years. The calculator also adds any monthly contributions.
What is the difference between compound and simple interest?
Simple interest pays only on your original deposit; compound interest pays on the deposit plus the interest already earned, so it grows faster every year.
Does compounding daily instead of monthly make a big difference?
Barely. On $10,000 at 6% for 10 years, daily beats annual by about $312 (under 2%). Rate and time matter far more than frequency.
What is the Rule of 72?
Divide 72 by the rate to estimate years to double. At 8% that’s about 9 years; at 6%, about 12. It’s an approximation, not exact.
What interest rate should I use?
Match it to the account: about 4% for a high-yield savings account, and a long-run ~10% nominal for the broad stock market. Use a lower rate to be conservative.
How do monthly contributions change the total?
Hugely. $500/month at 8% for 30 years grows to about $745,000 from a $0 start, because every deposit keeps compounding.
This tool is for estimation and education, not financial advice. See our methodology for how these figures are calculated and sourced.