Interest Calculator
Estimate how much interest your savings will earn over time with compounding and optional periodic contributions.
Example
Start with $10,000 at 5% annual interest, adding $200/month for 10 years with monthly compounding (contributions at end of period).
After 120 months the balance grows to $47,526.55. You deposited $10,000 + $24,000 in contributions = $34,000, so you earned $13,526.55 in interest.
How it works
Enter your starting principal, annual interest rate, time horizon, contribution amount, and how often interest compounds. The calculator grows the principal and each contribution with compound interest and updates live.
Good to know
This Interest Calculator projects how a lump sum plus a steady stream of deposits grows when compound interest is applied. You enter a starting principal, an annual rate, a number of years, a recurring contribution, and how often interest compounds, and it returns the end balance along with a breakdown of total interest, total contributions, and total deposited. It is built for savers, CD and high-yield savings shoppers, and anyone wanting to see the long-run effect of "set it and forget it" deposits without building a spreadsheet.
Reach for it when you want a quick what-if: comparing a one-time deposit against a smaller deposit topped up monthly, testing whether monthly versus annual compounding meaningfully changes the outcome, or sanity-checking a bank's quoted growth figure. Because the math runs live in your browser as you type, it is fast to nudge a single input and watch the end balance move.
Read the result by separating what you put in from what the rate earned. "Total deposited" is principal plus every contribution (your own money), and "Total interest" is everything above that. The three bars show the proportion of the final balance coming from principal, contributions, and interest, which makes it easy to see when compounding starts doing the heavy lifting over long horizons.
One caveat to keep in mind: this tool assumes contributions happen at the same frequency you select for compounding, and that the rate and contribution stay fixed for the whole term. Real accounts often have variable rates, fees, or taxes on interest, so treat the figure as a clean baseline rather than a guaranteed outcome. Switching contribution timing to "Start of period" gives each deposit one extra compounding cycle, which nudges interest slightly higher.
Frequently asked questions
Does this account for the money I add over time?
Yes. Each periodic contribution is treated as a deposit that then earns compound interest for the remaining time. You can also choose whether contributions land at the start or end of each period, which slightly changes the interest earned.
What does the compounding frequency setting do?
It sets how often interest is added to your balance and how often you contribute. More frequent compounding (e.g. monthly vs. annually) earns slightly more interest at the same annual rate because earned interest starts compounding sooner.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is this financial advice?
No. These are educational estimates — consult a qualified financial professional before making decisions.
People also ask
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it grows in a straight line. Compound interest is calculated on the principal plus previously earned interest, so the balance accelerates over time. This tool uses compound interest.
Does contributing at the start of the period really earn more than the end?
Yes, slightly. A start-of-period deposit sits in the account one extra compounding cycle before the next interest is applied, so it earns a little more over the full term. The longer the horizon and the higher the rate, the larger that gap becomes.
What annual interest rate should I enter?
Use the rate your account or investment actually quotes, such as a savings APY or an assumed average return. For neutral planning, many people test a range of conservative and optimistic rates to see how sensitive the outcome is.
How do I figure out how long it takes to double my money?
A common shortcut is the Rule of 72: divide 72 by the annual interest rate to estimate the years to double. For example, at 6% it takes roughly 12 years. It is an approximation, not an exact figure.
Is the interest this calculator shows taxable?
In many jurisdictions interest earned in a standard account is treated as taxable income, while tax-advantaged accounts may defer or exempt it. This calculator does not deduct taxes, so the interest figure is pre-tax. Check your local rules for specifics.
Why does monthly compounding give a higher balance than annual at the same rate?
With more frequent compounding, earned interest is added to the balance sooner and begins earning its own interest earlier. At the same stated annual rate, that produces a higher effective yield and a larger end balance.
Can I use this to plan regular savings deposits like a monthly transfer?
Yes. Enter your recurring amount in the contribution field and choose the matching frequency, and the tool grows each deposit with compound interest for its remaining time. It assumes the contribution stays constant for the entire term.
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