How the compound interest calculator works
Compound interest is interest that earns interest, and this calculator shows where that leads. Enter a starting amount, a regular contribution and how often you add it, an annual interest rate, and a time frame. It projects your future balance, splits it into what you paid in versus what compound interest added, and charts the growth year by year. Switch to reach-a-goal mode and it works backward to the contribution a target balance requires.
How to use it
- Enter your starting amount (what you have saved today).
- Set your regular contribution and choose how often you add it.
- Enter the annual interest rate you expect and how often it compounds.
- Set the time period in years.
The result updates as you type. Open Advanced options for contribution timing, an annual step-up, and an inflation adjustment.
How compound interest is calculated
Interest is added to your balance at each compounding interval, and the next interval is calculated on that larger balance. That feedback loop is what makes growth accelerate, and it is the whole difference from a simple interest calculator, where interest is only ever charged on the original amount and the balance grows in a straight line.
A starting sum on its own grows by:
When you add money regularly, each contribution compounds for the time it stays invested, and their combined future value is added on top:
Rather than evaluate these once, the calculator simulates month by month, so contribution timing, a step-up, and any one-time deposits land on the right date.
Monthly, daily, or annual: does compounding frequency matter?
More frequent compounding pays slightly more because interest starts earning sooner. The difference is most noticeable at higher rates and longer time frames:
The jump from annual to monthly is worth about $6,000; monthly to daily adds only a few hundred. Set the frequency to match your account: most savings accounts compound daily, CDs and money markets monthly, and bonds semi-annually. Choosing a frequency that doesn't match your real account overstates or understates the projection.
Growing savings with monthly contributions
This is the default mode: you supply the inputs and read the projected balance. Use it when you know roughly what you can set aside each month and want to see where compound interest takes it.
Almost two-thirds of the ending balance here is interest, not contributions. The chart plots the contribution total and the interest total for every year, so you can watch the year the interest line overtakes the contribution line — the point where the account earns more each year than you add.
Choosing a realistic interest rate
The rate you enter should match the actual return of your savings or investment vehicle:
- High-yield savings accounts: 3–5% (varies with the rate environment)
- CDs and fixed deposits: 4–6% depending on term and issuer
- Bond funds: 4–6% historically
- Broad equity index funds: 8–10% long-term historical average
A common mistake is using an aggressive rate for conservative planning. If you are setting a savings goal, enter a rate 1–2% below your best estimate. The calculator is an estimate — real returns fluctuate year to year, and past performance does not guarantee future results.
Reverse (backwards) compound interest: reaching a goal
Switch to reach-a-goal mode when you know the destination but not the payment. You enter a target balance, a rate, and a time frame, and the calculator solves backward for the recurring contribution that gets you there. It still respects your starting balance, compounding frequency, step-up, and any one-time events.
Use this mode to plan against a number: a retirement target, a house down payment, or an education fund where the contribution is the unknown.
The annual step-up
Under Advanced options, an annual step-up raises your contribution by a set percentage each year to track income growth. Because the larger contributions compound for years, the effect is bigger than it looks:
A 5% annual step-up added about $134,000 here — a 46% larger balance — for raises you would likely take anyway. It is usually the highest-leverage setting in this calculator.
How inflation affects your target
The inflation adjustment (under Advanced) restates your ending balance in today's purchasing power. A $500,000 balance in 20 years buys significantly less than $500,000 today. At 3% inflation, you need roughly $900,000 in nominal terms to match $500,000 in today's dollars. Always check the inflation-adjusted figure when planning a goal, and consider setting a higher nominal target to compensate.
If you would rather invest a single lump sum and never top it up, the investment calculator is built for that; for a steady recurring deposit into the markets, the SIP calculator models that scenario.
Frequently asked questions
What interest rate should I enter?
Match it to your actual account or investment. Savings accounts: 3–5%. CDs: 4–6%. Bond funds: 4–6%. Broad stock index funds: 8–10% historically. Use a conservative figure for goal planning — returns are not guaranteed, and an optimistic rate can leave you short.
How much difference does compounding frequency make?
More frequent compounding earns slightly more. On $100,000 at 8% for 10 years, monthly compounding produces about $6,000 more than annual. The gap widens with higher rates and longer terms. Set it to match your actual account: most savings compound daily, CDs monthly.
Should I use the step-up feature?
Yes, if your contributions will realistically grow over time. A 5% annual step-up (matching typical salary growth) can add 30–50% to the final balance over 20 years. It's the highest-leverage setting in this calculator. Set it to 0 only if you're certain contributions will stay flat.
How does inflation affect my result?
The inflation adjustment shows your future balance in today's purchasing power. A $500,000 balance in 20 years buys far less than $500,000 today. At 3% inflation, you'd need roughly $900,000 nominal to match today's $500,000. Always check the inflation-adjusted figure when planning goals.
Can I model irregular deposits like annual bonuses?
Yes. Use the One-time deposits & withdrawals section to add events at specific months and years. Each event is applied at that exact point in the simulation and shows as a dot on the growth chart. Use negative amounts for withdrawals.
What is the difference between this and the SIP calculator?
Both model recurring investments with compound growth. The SIP calculator uses terminology and defaults common in mutual fund investing (SIP amount, maturity value). This calculator is more general: it handles any savings account, CD, or investment with a starting balance, regular additions, and a known compounding frequency.