How to use it
- Enter what you start with, what you add each month (a negative amount for withdrawals), the number of years and the yearly interest rate.
- Choose how often interest compounds: yearly, twice a year, quarterly, monthly, daily or continuously. Tick the box if you deposit at the start of each month.
- Read the final balance, your deposits and the interest earned, the APY and the doubling time, then the chart and the year-by-year table.
How it calculates
Compound interest is interest earned on earlier interest as well as on what you put in; the Consumer Financial Protection Bureau’s example is $1,000 at 5% compounded once a year, which becomes $1,050 after one year and $1,102.50 after two. For a lump sum the calculator uses the textbook formula A = P(1 + r/n)nt. Monthly deposits are handled by converting the chosen compounding into the equivalent monthly growth, (1 + r/n)n/12, so every deposit compounds from the month it goes in and the balance at each compounding date is exactly what the formula gives. Balances are computed in closed form rather than by adding up hundreds of rounded months, so nothing drifts over long periods. The inputs mirror the SEC’s Investor.gov calculator: starting amount, monthly contribution, years, rate and compounding frequency.
What we tested
On October 14, 2026 we compared the page’s own functions with an independent 60-digit decimal simulation that grows every deposit month by month, on 1,507 random cases (all six compounding choices, deposits and withdrawals, start or end of month, rates from −5% to 15%, up to 50 years): all 30,032 year-end balances agreed to the cent, and the largest difference anywhere was 0.0056 of a cent, on a balance of tens of millions. All 333 lump-sum cases also equal the textbook formula to the cent, and the CFPB example comes out at exactly $1,050.00 and $1,102.50. In Chromium: the default $10,000 plus $200 a month at 5% for 20 years showed $109,333.14 ($58,000.00 deposited, $51,333.14 interest) with 20 table rows and a 20-bar chart; the CFPB example showed $1,050.00 after one year and $1,102.50 after two; daily compounding of $10,000 at 5% for 10 years showed $16,486.65 and an APY of 5.1267%; $200 a month at 6% for 30 years showed $200,903.01 (end of month) and $201,907.52 (start); $1,000 monthly withdrawals from $100,000 at 4% ran out in month 122; a non-number showed a message; and the page fitted a 390 px phone with no errors.
| Example | Result | Note |
|---|---|---|
| CFPB example: $1,000 at 5% compounded yearly, 1 and 2 years | $1,050.00 and $1,102.50 | Exact match |
| $10,000 + $200 a month, 5%, monthly, 20 years | $109,333.14 | Deposits $58,000.00 |
| $10,000, 5%, daily, 10 years | $16,486.65 | APY 5.1267% |
| $10,000, 5%, continuous, 10 years | $16,487.21 | APY 5.1271% |
| $100,000, withdraw $1,000 a month, 4% | runs out in month 122 | |
| $500 at the start of each month, 7% yearly, 40 years | $1,242,758.23 | Deposits $240,000.00 |
Tips
- Time matters most: $200 a month at 6% grows to $92,408.18 over 20 years and $200,903.01 over 30, more than double for 50% more deposits.
- Compare savings accounts by APY, not by the rate and the compounding separately.
- Investment returns are not a fixed rate: use a conservative figure, and remember taxes, fees and inflation are not included.
Sources
Checked on .
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools