Calculate compound interest with recurring contributions. Free compound interest calculator with the formula shown - runs entirely in your browser, no signup.
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Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for the formulas used.
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Calculate compound interest with recurring contributions. Free compound interest calculator with the formula shown - runs entirely in your browser, no signup.
Effective yield climbs from 5% to 5.127% as compounding frequency increases.
Transparent methodology
How this calculator works
Formula
FV = principal x (1 + r/12)^(12n) + contribution x (((1 + r/12)^(12n) - 1) / (r/12)) for monthly contributions.
Worked example
$10,000 principal with $200 monthly contributions at 7% for 20 years: $58,000 contributed grows to about $144,572.72, of which $86,572.72 is interest.
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20 years at 7% on $10,000
What this means for your numbers: In the worked example, compounding added $14,697 over simple interest, more than doubling the principal in growth. The final decade contributes more growth than the first two combined.
Illustrative breakdown using the worked example above. The calculator computes your own numbers with the same formula.
A worked example, month by month
Set principal to $5,000, monthly contribution to $200, rate to 5%, compounding to monthly, contributions at end of month, term to 10 years. The monthly rate is (1 + 0.05/12)^(12/12) - 1 = 0.0041667, about 0.41667% per month. Month 1: interest is $5,000 x 0.0041667 = $20.83, then the $200 contribution lands, so the balance closes at $5,220.83.
Rather than stepping through 120 months, use the closed form. The principal grows to $5,000 x (1.0041667)^120 = $8,235.05. The contributions form an annuity: $200 x [(1.0041667)^120 - 1] / 0.0041667 = $200 x 155.2822 = $31,056.44. Total future value: $39,291.50.
Check it against the totals the calculator reports: you contributed $5,000 + $200 x 120 = $29,000, so interest earned is $39,291.50 - $29,000 = $10,291.50. The effective annual yield is (1 + 0.05/12)^12 - 1 = 5.116%, above the 5% nominal rate because intra-year interest itself earns interest. Note the sequencing effect: because contributions occur at month end, each one misses exactly one month of growth; switching to beginning-of-month raises the result to about $39,579.
Where nominal rates mislead
The rate field is a nominal annual rate divided by the compounding frequency, not an APR in the credit-card sense and not necessarily what a bank advertises. A 5% nominal rate compounded monthly yields 5.116% effective; compounded daily it yields 5.127%; compounded annually it yields exactly 5%. If a savings product quotes '5% APY', that is already the effective figure, and entering it as nominal with monthly compounding would overstate growth by roughly 0.1 percentage points per year. On $39,291.50 of balance over 10 years, that mislabel costs about $350.
Two further distortions worth checking against reality. First, the model assumes the rate never changes: 10 years at a fixed 5% is a fiction for most accounts, and a single 1-point drop in year 6 on a $30,000 balance removes roughly $1,500 of the projected endpoint. Second, contributions are assumed exact and constant; skipping three $200 deposits mid-term costs not $600 but about $730 once lost compounding is counted. Treat the output as a scenario under fixed assumptions, not a forecast of any specific account.
