Compound interest calculator with monthly contributions
Most compound interest calculators only handle a single lump sum. This one adds a recurring monthly contribution, which is how most people actually save.
Final balance
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Total contributed
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Total interest earned
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Last updated: September 2026 · Verified by Hugo Cardozo.
How this calculator works
The starting amount grows on its own using standard compound interest: Final = Principal × (1 + r/n)^(n×t), where r is the annual rate, n is how many times per year interest compounds, and t is the number of years. On top of that, each monthly contribution is treated as its own smaller deposit that starts compounding from the month it's added, so a contribution made in year 1 has far longer to grow than one made in year 19.
Worked example
Starting with $5,000, contributing $200 every month, at a 7% annual rate compounded monthly for 20 years: the starting amount alone grows to roughly $20,240, and the monthly contributions add up to $48,000 in deposits but grow to roughly $104,000 thanks to compounding — for a total balance of about $124,000, of which only $53,000 was money you actually put in.
Why monthly contributions change everything
A single lump sum compounds for the full time period, but recurring contributions are what most people actually do — a 401(k) deduction, an automatic transfer to a brokerage account, a round-up savings app. Ignoring contributions dramatically understates how much a real savings habit can grow.
Why compounding frequency matters less than you'd think
Switching from annual to monthly compounding makes a noticeable difference. Switching from monthly to daily makes almost none. The single biggest lever is always the contribution amount and the number of years invested, not the compounding frequency.
Frequently asked questions
Does this account for investment fees or taxes?
No — this is a pure math projection at a fixed rate. Real investment accounts have fees (expense ratios, advisory fees) and, outside of tax-advantaged accounts, may owe capital gains tax, both of which reduce the real return.
What interest rate should I use?
For a savings account or CD, use the actual APY. For long-term stock market investing, 7% (after inflation) is a commonly used conservative historical average, though actual returns vary year to year and aren't guaranteed.
Does the order of contributions matter?
This calculator assumes contributions happen at the end of each month (an "ordinary annuity"). Contributing at the start of the month instead would produce a very slightly higher final balance, since each deposit gets a few extra weeks to grow.
📘 Related guide
Read the full guide: "Compound Interest Explained: Why Time Matters More Than the Rate".
Compound Interest Explained