Interest Calculator
Calculate interest earned on a starting balance over any time period, with optional regular contributions and a choice of simple or compound interest.
How to use the Interest Calculator
- Enter your inputs into the Interest Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The interest formulas
Simple: I = P · r · t · · · Compound: A = P (1 + r/n)^(n·t)
P is principal, r is annual rate (decimal), t is time in years, and n is the number of compounding periods per year. Simple interest is paid on the principal only; compound interest is paid on principal plus accumulated interest.
Worked example
$5,000 at 4.5% APY compounded monthly for 10 years: A = 5,000 × (1 + 0.045/12)^120 ≈ $7,829. The same amount at simple interest would be $5,000 + (5,000 × 0.045 × 10) = $7,250 — compounding adds $579.
Frequently asked questions
What is the difference between APR and APY?
APR is the simple annual rate. APY (annual percentage yield) bakes in compounding frequency. For a 5% APR compounded monthly, the APY is about 5.12%.
How do regular contributions change the result?
Each contribution starts compounding from the day it is added, so consistent monthly contributions produce dramatically higher end balances than a single lump sum at the start.
Should I expect simple or compound interest?
Savings accounts, CDs and most investments compound. Some loans (especially short-term and certain car loans) use simple interest only.