Compound Interest Calculator
See how a starting balance and regular monthly deposits grow once interest compounds. Enter your initial amount, monthly contribution, annual interest rate and the number of years to get the future value, how much of it is your own money, and how much is interest earned. The figures are currency-neutral.
Last updated: June 2026
Enter a starting amount or monthly contribution, the interest rate and the number of years above.
Future value = start × (1 + r)^n + monthly × ((1 + r)^n - 1) ÷ r · interest = future value - what you paid in
How compound interest builds a balance
Compound interest pays you interest on your interest. Each month the account earns a return on the whole balance, including the returns already added, so the growth is not a straight line but a curve that steepens over time. Add regular monthly deposits and two engines run at once: your contributions raise the balance directly, and the interest compounds on an ever larger sum. The result after a few decades is often startling, because most of the final figure is interest the account earned rather than money you put in.
Why time beats the interest rate
The single most powerful input is the number of years, not the rate. Money left to compound for 30 years spends most of that time growing on interest that earlier interest created, which is why the last decade adds far more than the first. Saving 100 a month at 6 percent reaches about 6,977 after 5 years but around 100,451 after 30 years. Over that span you paid in 36,000 and the account earned roughly 64,000 in interest, nearly double your contributions. Starting a few years earlier usually beats chasing a slightly higher rate.
The two levers you control
You set two things: how much you add and how long you leave it. Raising the monthly contribution lifts the whole curve, while extending the term lets the compounding do more of the work. The starting amount matters most when it is large relative to the deposits, because it compounds for the full term from day one. Use the fields above to test combinations: a smaller monthly amount over a longer horizon frequently ends up ahead of a larger amount started late.
Worked example
Begin with 1,000, add 200 a month, and earn 6 percent a year for 20 years. The monthly rate is 0.5 percent applied over 240 months. The starting 1,000 grows to 3,310.20, and the stream of 200 monthly deposits grows to 92,408.18, for a future value of 95,718.38. Of that, 49,000 is money you contributed (the 1,000 start plus 48,000 in deposits) and 46,718.38 is interest earned, almost as much as you paid in. Cut the term to 10 years and the interest shrinks dramatically; that gap is the reward for leaving money invested.
Saving 100 a month at 6 percent, no starting balance
| Years | Total contributed | Interest earned | Final balance |
|---|---|---|---|
| 5 years | 6,000.00 | 977.00 | 6,977.00 |
| 10 years | 12,000.00 | 4,387.93 | 16,387.93 |
| 20 years | 24,000.00 | 22,204.09 | 46,204.09 |
| 30 years | 36,000.00 | 64,451.50 | 100,451.50 |
Same deposit and rate, longer term: the interest earned grows far faster than the amount paid in. Figures are currency-neutral.
Frequently Asked Questions
How does compound interest grow my savings?
Compound interest pays you interest not only on your original money but also on the interest already added, so your balance grows faster and faster over time. In the early years the effect is small, but it accelerates, because each year's interest is calculated on a larger balance than the year before. Saving 100 a month at 6 percent builds to about 6,977 after 5 years but around 100,451 after 30 years, and by then the interest earned is nearly double everything you paid in. Time is the most powerful input, which is why starting early matters more than the exact rate.
How much should I save each month to reach a goal?
Work backwards from the target. Enter your starting balance, interest rate and the number of years you have, then adjust the monthly contribution until the future value matches your goal. Because interest does part of the work, the monthly amount you need is smaller than the goal divided by the months, and the gap widens the longer your horizon. Testing a few contribution figures here is faster than solving the formula, and it shows immediately how much a longer term or a higher rate reduces the monthly amount required.
Does the compounding frequency matter?
It matters a little, but less than people expect at ordinary savings rates. This calculator compounds monthly, which is the convention for most savings accounts. Compounding daily instead of monthly, at the same annual rate, adds only a fraction of a percent to the yearly growth, and annual compounding is slightly lower than monthly. The rate and the term dominate the result; the frequency is a minor adjustment. What matters far more is whether you are quoted a nominal rate or an effective annual rate, so compare accounts on the same basis.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it adds the same amount every year and the balance grows in a straight line. Compound interest is calculated on the principal plus all the interest already earned, so the balance grows on a curve that steepens over time. Over one year the two are identical, but over decades compound interest pulls far ahead. Savings accounts, investments and most debts use compound interest, which is why this calculator uses it; simple interest appears mainly in some short-term loans and bonds.
How does inflation affect my savings?
Inflation erodes what your future balance can actually buy, so the real return is roughly the interest rate minus the inflation rate. A savings pot growing at 5 percent while prices rise 3 percent gains only about 2 percent in real spending power. This calculator shows the nominal balance, the actual number of currency units you will have, not the inflation-adjusted value. To gauge real growth, subtract your expected inflation rate from the interest rate before entering it, and treat the result as a rough real-terms estimate.
Methodology and sources
This tool projects a savings balance forward from a starting amount and a regular monthly deposit, compounding monthly, and splits the result into what you paid in and what the interest added.
- Method: Future value = P × (1 + r)^n + PMT × ((1 + r)^n - 1) ÷ r, where P is the starting amount, PMT is the monthly contribution, r is the annual rate divided by 12, and n is the term in months. Total contributed = P + PMT × n. Interest earned = future value - total contributed. A zero percent rate gives future value = P + PMT × n.
- Standards and sources: The standard future-value formula for a lump sum plus an ordinary annuity (contributions at the end of each month), the same maths used by banks and financial planners for savings projections. It assumes monthly compounding, the convention for most deposit accounts.
- Assumptions and limits: Assumes a fixed interest rate, monthly compounding, and deposits made at the end of each month, with no tax on interest and no account fees. Real rates change over time, and interest may be taxed depending on the account and country. Figures are nominal and currency-neutral; they are not adjusted for inflation. This is a planning aid, not financial advice.
Reviewed and maintained by Rick Oosterling, a maker and developer who runs long-horizon numbers before committing savings and tools. Last reviewed: June 2026. This is a planning aid, not financial advice; confirm any account's real return and terms with the provider.