This page explains exactly how our calculators work, so you can check any number yourself.
Compound interest
The calculator starts with your initial deposit and adds your regular deposit at the end of each deposit period (every week, every 2 weeks, every month, every 3 months or every year). Interest is added as often as you choose: daily, weekly, monthly, quarterly or annually.
For a single deposit with no extra money added, the result follows the standard formula:
A = P × (1 + r/n)^(n × t)
Where A is the final balance, P the starting amount, r the yearly rate as a decimal, n how many times per year interest is added and t the number of years.
When you add regular deposits, we turn the yearly rate into the equivalent rate for each deposit period and add one period at a time:
rate per period = (1 + r/n)^(n / deposits per year) − 1 new balance = old balance × (1 + rate per period) + deposit
Simple interest
With simple interest you only earn interest on the money you deposited, never on interest you already earned. Each period we add: money deposited so far × yearly rate ÷ deposits per year.
What our results leave out
- Taxes on interest or investment gains.
- Inflation (our results are in today’s money).
- Fees, account limits or changes in the interest rate over time.
- Rounding: results are shown rounded to the nearest whole unit.
How we check them
Before a calculator goes live we compare its results with known examples and with public tools, such as the compound interest calculator on Investor.gov from the U.S. Securities and Exchange Commission. For example, $10,000 at 7% compounded monthly for 20 years gives $40,387 in both.
Spotted something that looks wrong? Please tell us at monetaryreports@gmail.com.
Last updated: November 2026.