Compound Interest Calculator with Monthly Contributions
Compare beginning or end-of-month deposits, compounding frequency, total contributions and interest. View annual balances and download your projection.
Your inputs
Your result
Enter your values, then choose Calculate.
Estimate based on your inputs. Fees, taxes and market changes are included only where explicitly entered.
Calculation breakdown
Method, assumptions and worked example
Monthly growth rate = (1 + R ÷ m)m/12 − 1, where R is the annual nominal rate as a decimal and m is the number of compounding periods per year. End-of-month balance = previous balance × (1 + monthly rate) + contribution. Beginning-of-month deposits are added before growth.
Why timing matters
A December deposit should not receive a full year of growth. The calculator credits each monthly deposit only from its modeled deposit date. For annual or quarterly compounding, it uses equivalent fractional-period growth; an actual account may accrue or post interest differently.
Worked example
Start with 1,000, deposit 100 per month, use 12% nominal interest compounded monthly, and save for one year. End-of-month deposits produce 2,395.08, comprising 2,200 contributed and 195.08 growth. Beginning-of-month deposits produce 2,407.76.
Compare the two timings with the example button. This is a constant-rate illustration, not a market forecast. Taxes, account fees, changing returns and inflation are excluded. Enter a time horizon in whole months expressed as years; for six months use 0.5.
Frequently asked questions
Does the rate mean APY?
No. It is a nominal annual rate. For an effective annual yield, choose annual compounding; the model then converts that yield to equivalent monthly growth.
Is this the exact interest my bank will pay?
No. Banks may use daily balances, different day counts, posting dates or rounding rules. This model is a planning comparison.
Sources and review
Formula and content reviewed on 9 October 2026. Examples are hypothetical. Tax years and model limits are stated above; this page does not receive live rates.