About the Compound Interest Calculator
This compound interest calculator projects how a starting balance grows when interest is earned on interest. Enter a principal, an annual rate and a number of years, optionally add a regular contribution for every period, and choose monthly, quarterly or yearly compounding. It returns the future value, the total you contributed, and the interest earned — the part that compounding created for you.
Small inputs produce surprising outputs: 1,000 at 7% compounded monthly for 30 years grows to over 8,100 without adding a cent, and regular deposits multiply that effect dramatically. Because the calculator recomputes the moment you type, it is ideal for exploring savings goals, investment scenarios and retirement projections side by side. Everything runs client-side in your browser — free, instant, with no sign-up and nothing uploaded anywhere.
Features
- Monthly, quarterly or yearly compounding options
- Optional regular contribution every period
- Future value, contributions and interest shown separately
- Ideal for savings, investing and retirement planning
- Recalculates instantly as you change inputs
- Runs fully in your browser, no sign-up
How to calculate compound interest
- Enter your starting amount (the principal).
- Enter the annual interest rate and the number of years.
- Optionally add a contribution made every compounding period.
- Pick the compounding frequency: yearly, quarterly or monthly.
- Read the future value, total contributed and interest earned.
Frequently asked questions
What is the compound interest formula?
Without contributions, the future value is A = P × (1 + r/n)^(n×t), where P is the principal, r the annual rate as a decimal, n the compounding periods per year and t the years. With contributions, this calculator compounds period by period, adding your deposit after each interest step.
When are my contributions added?
Each contribution is applied at the end of every compounding period, after that period's interest is credited. If you compound monthly, the contribution field means a monthly deposit; if quarterly, a quarterly one. Match the frequency to how often you actually plan to save.
Which compounding frequency should I choose?
Use whatever your account or investment actually does: most savings accounts compound monthly, many bonds pay quarterly or yearly. More frequent compounding earns slightly more at the same nominal rate, because interest starts earning interest sooner. Try all three options to see the difference for your numbers.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so growth is linear. Compound interest is calculated on the principal plus all previously earned interest, so growth accelerates over time. Over long horizons the gap becomes dramatic — that acceleration is what this calculator makes visible.
How long does it take to double my money?
A quick estimate is the Rule of 72: divide 72 by the annual rate. At 6% your money doubles in roughly 12 years; at 9%, about 8 years. Enter your own rate and years here to see the exact figure, including the boost from any regular contributions.