Calculators
Compound Interest Calculator
Estimate final balance, contributions, and interest earned with compounding.
Compound Interest Calculator tool
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About This Tool
Estimate how a balance grows when interest is added back into the principal over time. Enter a starting principal, annual rate, time horizon, compounding frequency, and optional recurring contributions to model a simple savings or investment scenario. The calculator separates growth from the starting principal and growth from later contributions, which makes it easier to see what came from deposits versus interest. It is a mathematical scenario tool, not a forecast, guarantee, investment recommendation, or account quote.
How To Use It
- Enter the starting principal. Use zero when you want to model contributions without an initial balance.
- Enter the assumed annual rate and time in years. Use a negative rate for a conservative loss scenario when appropriate.
- Choose how often interest is compounded: annually, quarterly, monthly, or daily.
- Optionally add a recurring annual, monthly, biweekly, or weekly contribution and choose whether each deposit happens at the beginning or end of its contribution period.
- Review final balance, total contributions, interest earned, and the balance composition shares before using the estimate in planning notes.
Examples
Monthly compounding
10,000 at 7% for 10 years with monthly compounding grows to about 20,097 before any extra contributions, assuming the 7% annual rate stays constant.
Monthly savings plan
Adding 250 per month at the end of each period raises the same 10-year estimate to about 63,368. Beginning-of-period deposits would be slightly higher because each deposit has more time to compound.
Zero-interest contribution plan
At 0%, the result is just principal plus completed contributions. A starting balance of 500 with 100 monthly deposits for one year reaches 1,700.
Negative assumed return
At -12% with monthly compounding, each month is modeled at -1%. The final balance can be lower than total deposits, and interest earned is shown as a negative number.
Partial-year timing
For partial years, end-of-period contributions are counted only after completed contribution periods, while beginning-of-period contributions can be included at the start of a period.
Useful Notes
Principal growth formula
The starting principal uses A = P(1 + r/n)^(nt), where P is principal, r is the annual rate as a decimal, n is compounding periods per year, and t is years. The displayed principal growth is the future value of that starting balance before adding contribution growth.
Contribution timing
Each recurring contribution is grown from its deposit time to the end of the selected horizon. Beginning-of-period contributions grow for one extra contribution interval compared with end-of-period contributions, so they produce a slightly larger result when the rate is positive.
Different schedules
The contribution schedule can differ from the compounding schedule. For example, weekly contributions can be modeled with monthly or daily compounding. The calculator evaluates each contribution against the selected compounding rate rather than assuming deposits and compounding always happen on the same date.
Partial years
The principal uses the exact entered year value as an exponent. Recurring contributions are counted according to completed end-of-period intervals or started beginning-of-period intervals, which helps avoid adding deposits that would not occur within a short partial period.
Assumptions and exclusions
Results assume the annual rate stays constant and that contributions are made on schedule. Negative assumed rates are supported for loss scenarios as long as each compounding period still has a positive growth factor. Taxes, account fees, inflation, market volatility, withdrawals, missed deposits, changing rates, and account-specific rules are not included.
Planning use
Use the result for arithmetic checks and scenario comparison, not as a promise of future performance. Real savings accounts, loans, investments, and retirement accounts can use different compounding, crediting, tax, fee, and withdrawal rules.
FAQ
Does this include regular deposits?
Yes. You can include annual, monthly, biweekly, or weekly contributions and choose whether each contribution is made at the beginning or end of its period.
Why does contribution timing change the result?
A beginning-of-period contribution is invested or saved earlier than an end-of-period contribution, so it earns one additional interval of growth when the rate is positive.
Does this account for taxes or fees?
No. The estimate uses the rate you enter and does not subtract taxes, account fees, inflation, or changing rates.
Can I use a 0% rate?
Yes. At 0%, the final balance equals the starting principal plus the counted contributions, and interest earned is shown as zero.
Can I enter a negative rate?
Yes. A negative assumed annual rate models a balance that loses value over time. The rate must stay above the point where a single compounding period would reduce the balance by 100% or more.
Is the result guaranteed?
No. It is a calculation from the values entered. Actual account or investment results can differ because rates, fees, taxes, deposits, withdrawals, and market conditions can change.