Compound Interest Calculator

The compound interest calculator projects month-by-month account balances when interest compounds monthly and fixed contributions are added at the end of each month.

Inputs: Amount — starting balance (your currency), Annual rate (%), Years (1–60), Amount — monthly contribution (your currency), and optional what-if drop (% and year K). Example values in the form are not forecasts.

Illustrative 7% example ($10,000.00 start, 7% rate, 10 years, $500.00/mo): Final balance = $106,639.02, Total contributed = $70,000.00, Interest earned = $36,639.02, Simple interest comparison (starting amount only) = $17,000.00.

Year	Total contributed	Interest earned	End balance
1	$16,000.00	$919.19	$16,919.19
2	$22,000.00	$2,338.58	$24,338.58
3	$28,000.00	$4,294.31	$32,294.31
4	$34,000.00	$6,825.16	$40,825.16
5	$40,000.00	$9,972.70	$49,972.70
6	$46,000.00	$13,781.53	$59,781.53
7	$52,000.00	$18,299.43	$70,299.43
8	$58,000.00	$23,577.68	$81,577.68
9	$64,000.00	$29,671.22	$93,671.22
10	$70,000.00	$36,639.02	$106,639.02

Educational tool. Results are simple calculations based only on the numbers you enter. They are not predictions and not financial advice.

How it works

Compounding is monthly, and contributions are added at the end of each month. Starting from your initial amount, each monthly step multiplies the running balance by (1 + annual rate / 100 / 12) and then adds your monthly contribution. Total contributed equals the starting amount plus all monthly contributions across 12 * years months, and interest earned is the difference between the final balance and total contributed. For comparison, simple interest is calculated on the starting amount only without monthly compounding or contributions. If you turn on the optional what-if drop, the calculator reduces the balance once at the end of year K after that month's contribution and runs two paths forward: staying invested at the same monthly rate, or holding the reduced balance at 0% growth while continuing to add contributions in cash.

For i = 1 .. 12 * Y: balance = balance * (1 + R / 100 / 12) + C

Total contributed = P + C * 12 * Y

Interest earned = Final balance - Total contributed

Simple interest comparison (starting amount only) = P * (1 + (R / 100) * Y)

What-if balance after drop = Balance at end of year K * (1 - D / 100)

Cash path after drop = Balance after drop + C * 12 * (Y - K)

Worked example

Using illustrative inputs of a $10,000.00 starting amount, a 7.00% annual rate compounded monthly, 10 years (120 months), and a $500.00 end-of-month contribution (example values, not forecasts): total contributed is $70,000.00, the ending balance is $106,639.02, and interest earned is $36,639.02. Simple interest on the $10,000.00 starting amount alone over 10 years at 7% equals $17,000.00.

How to use the result

Compare the final compound balance with total contributed to see how much of the ending total comes from deposits versus compounded interest at your chosen rate. Check the simple interest comparison on the starting amount alone to isolate the effect of compounding and recurring contributions. If you enable the what-if drop panel, compare staying invested against holding cash solely under the stated assumption that the same constant rate applies.

Frequently asked questions

How often does this calculator compound interest?

Compounding is monthly, and contributions are added at the end of each month. In each month i from 1 to 12 * Y, the existing balance earns R / 100 / 12 and then C is added.

What does the simple interest comparison measure?

Simple interest comparison applies P * (1 + (R / 100) * Y) to the starting amount P only, excluding monthly contributions and excluding interest earned on prior interest.

What happens when the annual rate is 0%?

When R equals 0%, the monthly multiplier (1 + 0 / 100 / 12) equals 1, so final balance equals P + C * 12 * Y and interest earned equals 0 in your currency.

How does the optional what-if drop work?

At the end of year K (after month 12 * K's contribution), the balance is multiplied by (1 - D / 100). From month 12 * K + 1 through 12 * Y, staying invested continues compounding at rate R with contribution C, while selling to cash applies a 0% rate to that reduced balance plus remaining contributions C.

Why must years be at least 2 to use the what-if drop?

The drop year K must satisfy 1 <= K < Y so the drop occurs after year K with at least one full year remaining afterward. When Y equals 1, no valid integer K exists.

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