Use this interest calculator to estimate how your money grows over time with both simple and compound interest. Enter a starting balance, interest rate, time period, and optional regular contributions to see the difference between simple interest and compound interest side by side.
Enter a starting amount, interest rate, and time period above, then select Calculate Interest Growth to see how your money grows with simple and compound interest.
After you calculate results, this section will show a year-by-year view of your starting balance, contributions, interest earned, and ending balance.
This interest calculator is designed to be flexible enough for savings accounts, CDs, investment balances, or long-term cash reserves. Here’s what each input means and how it affects your results:
In this context, principal means your original deposit, interest rate is the annual return your money earns, and term is how long you leave the money invested. Together, these determine how quickly your balance grows.
The calculator uses standard time-value-of-money formulas to estimate both simple and compound interest. Here is a summary of the core equations:
Simple interest assumes interest is calculated only on the original principal:
I = P × r × t A = P + I
Where:
P = principal (starting amount)
r = annual interest rate (decimal, so 5% = 0.05)
t = time in years
I = interest earned
A = total value after interest
When interest is compounded, each period's interest is calculated on the updated balance:
A = P × (1 + r / n)^(n × t)
Where:
n = number of compounding periods per year (for example,
12 for monthly)
Other variables match the definitions above.
If you add the same amount each compounding period, the future value with contributions is:
A = P × (1 + r / n)^(n × t) + PMT ×
[((1 + r / n)^(n × t) - 1) / (r / n)]
Where PMT is the regular contribution per period. The
calculator uses this formula to estimate your final balance and then
backs out total interest by subtracting the original principal and all
contributions.
Simple interest is calculated only on your original principal, while compound interest is calculated on your principal plus any previously earned interest. Over longer periods, compound interest usually leads to significantly higher balances.
The more often interest is compounded (monthly or daily instead of annually), the more frequently your balance is updated and the slightly more interest you earn. The difference between monthly and daily compounding is often small, but it can add up over many years.
Use the annual percentage rate (APR) quoted by your bank, credit union, lender, or investment provider. If your account lists an APY (annual percentage yield), that figure already reflects compounding and may be slightly higher than the APR.
Yes. Enter an amount in the regular contribution field to model recurring deposits each compounding period. This is especially useful for planning automatic transfers into a savings, CD ladder, or investment account.
This tool focuses on savings and investment growth. For loans, it's better to use the dedicated Loan Payment Calculator or Mortgage Calculator on Solverly, which account for amortization and fixed monthly payments.
No. The calculator assumes a constant interest rate, regular contributions, and idealized compounding. Real-world accounts may have fees, minimum balance requirements, changing rates, or different posting schedules, so use these numbers as planning estimates rather than guaranteed returns.
Based on 3 sources
Last updated: 12/12/2025
This interest calculator and accompanying explanations were prepared for Solverly.net by Michael Lighthall. The formulas follow commonly accepted finance references for simple interest, compound interest, and regular contributions, and are informed by guidance from U.S. consumer finance regulators.
Cite this calculator as:
Lighthall, Michael. "Interest Calculator" at Solverly.net,
https://solverly.net/calculators/interest-calculator.