Monster Compound Interest Calculator
Watch your money grow. Enter a starting deposit and monthly contribution to see your future balance, how much you contributed, and how much interest did the work — year by year.
| Total contributions | $0 |
| Interest earned | $0 |
| Year | Contributions | Interest | Balance |
How it works
Each month interest accrues on your balance at the monthly rate (annual rate ÷ 12), then your contribution is added — and it earns interest the very next month. That's compounding. The year-by-year table shows exactly how much of your balance came from your deposits versus growth.
Formula & worked example
Formula: Each month the balance grows by the annual rate ÷ 12, then the monthly contribution is added: balance × (1 + rate ÷ 12) + contribution, repeated every month.
Worked example: $10,000 plus $200/month at 7% for 10 years grows to $54,714. You deposited $34,000 of that, so $20,714 is pure compound growth.
When to use this
- You are opening a savings or investment account and want the honest projection, contributions included.
- You are comparing two rates or two contribution levels over the same horizon.
- You want to show a teenager (or yourself) why starting ten years earlier beats saving more later.
- You are checking a 'guaranteed return' claim against arithmetic.
How to use it, step by step
- Enter the starting deposit.
- Add the monthly contribution you will actually keep up.
- Set the expected annual rate — conservative for savings, long-run average for investments.
- Choose the years and compounding frequency.
- Re-run with the start delayed 5 years to price procrastination.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Initial deposit | Money in on day one | $0 is fine — contributions can do the work |
| Monthly contribution | Added every month | $25–$500 typical |
| Annual rate | Expected yearly growth | Savings 3–5%; stock-market long-run ~7–10% nominal |
| Years | Time invested | 10–40 for retirement horizons |
| Compounding | How often interest is credited | Monthly is the common default |
Another worked example
Start with $5,000, add $200 a month at 7% for 20 years, compounding monthly: the projection reaches $124,379.03. Your own money in is $53,000 — growth supplies the other $71,379.03, which is the entire argument for starting before the contribution feels big.
Frequently asked questions
What is the Rule of 72?
Divide 72 by your annual rate to estimate how many years it takes money to double. At 7%, money doubles roughly every 10 years (72 ÷ 7 ≈ 10.3). Try 7% above and watch the balance nearly double the contributions over a decade.
Why does starting early matter so much?
Compounding is exponential: the last years do the heaviest lifting. Starting 10 years earlier at the same contribution can more than double your ending balance, because early money compounds the longest.
APY vs. APR — what's the difference?
APR is the simple annual rate before compounding; APY includes compounding, so it's slightly higher for the same nominal rate. This calculator compounds monthly, so the rate you enter behaves like an APY-style growth rate.
How much does the compounding frequency really matter?
Less than the rate and far less than time. At 7% over 20 years, daily vs annual compounding moves the result by well under 1%. If a product markets its compounding frequency as the headline, it is hiding a mediocre rate.
Is this a prediction?
No — it is a smooth-line projection of a jagged process. Real investments swing year to year; the line is the average the swings happen around, useful for planning and useless for timing.
💵 Turn knowledge into a plan
Pair long-term growth with smart borrowing using the business loan calculator.