The Complete Guide to Compound Interest
By Arshi Khan, Founder of Just4Marketing · Last reviewed October 2026
Compound interest is interest earned on your interest. Instead of growing by the same amount each year, money that compounds grows by a little more every period, and over long stretches of time that difference becomes enormous. This guide covers the formula, how compounding frequency and regular contributions change the result, and the rule of 72 for quick estimates.
How compound interest works
With simple interest, you earn the same amount every year because interest is only ever calculated on your original deposit. With compound interest, each period's interest is added to the balance, and the next period's interest is calculated on that larger balance. The growth curve bends upward over time.
The classic formula for a single deposit is A = P × (1 + r/n)^(n×t). A $10,000 deposit at 7% compounded monthly for 10 years grows to about $20,097 — more than double — without adding another cent.
Why time matters most
The same $10,000 at 7% compounded monthly, left for different lengths of time, shows how the growth accelerates. The last decade adds far more than the first.
| Years | Balance | Growth in that decade |
|---|---|---|
| 10 | ≈ $20,100 | ≈ $10,100 |
| 20 | ≈ $40,400 | ≈ $20,300 |
| 30 | ≈ $81,200 | ≈ $40,800 |
Getting the most from compounding
Start early
Extra years at the end of the timeline are the most valuable ones, because that's when the balance is largest.
Contribute regularly
Monthly contributions each get their own time to compound. Adding $200 a month to the example above turns $20,100 into roughly $54,700 after 10 years.
Compare APY, not just rate
APY already includes the effect of compounding frequency, so it's the fairest way to compare savings accounts.
Keep fees low
A 1% annual fee compounds against you just as returns compound for you, and can consume a large share of long-term growth.
Frequently asked questions
What is the compound interest formula?
For a single deposit, A = P × (1 + r/n)^(n×t), where P is the starting amount, r is the annual rate as a decimal, n is how many times a year interest compounds, and t is the number of years. Regular contributions add a second term, which is why the calculator works it out month by month.
Does compounding daily make a big difference?
Less than most people expect. At 7% a year, annual compounding gives an effective yield of 7.00%, monthly gives about 7.23%, and daily about 7.25%. The rate itself and the length of time matter far more than the compounding schedule.
What is the rule of 72?
It's a shortcut for how long money takes to double: divide 72 by the annual rate. At 6%, money doubles in about 12 years; at 9%, in about 8 years. It's an approximation that works best for rates between roughly 4% and 12%.
Is the result adjusted for inflation or taxes?
No. The calculator shows nominal growth before taxes and inflation. To estimate real growth, you can subtract an expected inflation rate (for example 2–3%) from your interest rate before calculating.