Interest Calculator
Project interest earned on a balance with regular contributions and any compounding period.
About Interest Calculator
Compound interest accelerates growth because each period's interest itself earns interest in the next period. The effective annual rate (APY) captures the true yearly yield regardless of how often the account compounds. Regular contributions stack on top via the future-value-of-an-annuity formula, which assumes each contribution earns interest for the remaining periods.
Formulas
Future value with contributions
FV = PV × (1+r)^n + PMT × [(1+r)^n − 1] / r
- PV
- Starting balance (principal)
- r
- Periodic interest rate (annual rate ÷ compounding periods)
- n
- Total number of compounding periods
- PMT
- Contribution per period
- FV
- Future value
Effective annual rate
EAR = (1 + r_nominal / n)^n − 1
- r_nom
- Nominal annual interest rate
- n
- Compounding periods per year
Worked example
With the default values below, Interest Calculator returns Future Balance of $31,998.32. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Starting Balanceminimum 0 $
- 10,000 $
- Annual Interest Rateaccepts 0–100
- 5
- Investment Period (years)accepts 1–50
- 10
- Compounding Frequencyone of Daily, Weekly, Monthly, Quarterly, Semi-annually, Annually
- Monthly
- Regular Contributionminimum 0 $
- 100 $
- Contribution Timingone of End of period (ordinary annuity), Start of period (annuity due)
- End of period (ordinary annuity)
Results
- Future Balance
- $31,998.32
- Interest Earned
- $9,998.32
- Total Contributions
- $12,000.00
- Effective Annual Rate (APY)The true annual yield after compounding.
- 5.1162%
Step by step
- 1Starting balance= $10,000.00
- 2Compounding frequency= 12× per year (120 periods)
- 3Periodic rate= 0.4167%
- 4Growth on principal= $16,470.09
- 5Contributions + their growth= $15,528.23
- 6Future value= $31,998.32Effective annual rate: 5.1162%
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This is an estimate for general information and not professional financial, tax or legal advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What is the difference between APR and APY?
APR (Annual Percentage Rate) is the nominal rate without accounting for compounding. APY (Annual Percentage Yield, also called EAR) reflects the actual return after compounding within the year. An account with 5% APR compounded monthly has an APY of about 5.116%. For savings accounts, always compare APY.
Does compounding frequency matter much?
Yes, but the difference shrinks as frequency increases. Going from annual to monthly compounding on a 5% rate raises APY from 5.000% to 5.116% — meaningful over decades. Going from monthly to daily only adds another ~0.006%. The biggest jump is always from annual to more-frequent compounding.
Why add contributions at the start vs. end of the period?
An annuity-due (start-of-period contributions) earns one extra period of interest on every payment compared to an ordinary annuity (end-of-period). Over 10 years of $100/month at 5%, that difference is about $500 — not huge, but worth knowing when your bank processes deposits.
What if my savings rate changes over time?
This calculator assumes a constant rate and contribution. For variable scenarios — laddering CDs, stepping up contributions — run the calculator in segments and treat each segment's ending balance as the next segment's starting balance.