Compound interest calculator with monthly contributions
Free compound interest calculator that handles both your starting amount and recurring monthly deposits. See exactly how regular contributions + compounding build wealth.
How does compound interest with monthly contributions work? In this model, growth is applied to the existing balance first and the deposit is added at month-end; that deposit starts earning in the next modeled period. For monthly compounding, FV = P·(1+r/12)12t + PMT·((1+r/12)12t−1)/(r/12), where P is the starting balance, PMT is the monthly deposit, r is a fixed nominal annual rate as a decimal, and t is years. At a fixed 8% nominal rate, $500 deposited at each month-end from a $0 starting balance models to about $91,473 after 10 years, $294,510 after 20, $475,513 after 25, and $745,180 after 30. These are fixed-rate illustrations, not return forecasts.
- $100/mo @ 8% for 30 yrs → $149,000
- $300/mo @ 8% for 30 yrs → $447,000
- $500/mo @ 8% for 30 yrs → $745,000
- $1,000/mo @ 8% for 30 yrs → $1,490,000
Key terms (used throughout this page)
- Future Value of an Annuity
- The closed-form formula for a stream of equal periodic contributions: FV = PMT × [((1+r/n)^(nt) − 1)/(r/n)]. Used in tandem with principal compounding.
- Dollar-Cost Averaging (DCA)
- Investing a fixed dollar amount on a regular schedule. It describes a contribution pattern; it does not eliminate market risk or establish whether this schedule will outperform a lump-sum investment.
- PMT (periodic contribution)
- The recurring deposit amount. This calculator adds each monthly PMT at month-end, so it begins earning growth in the next modeled period.
- Ordinary annuity vs annuity-due
- End-of-period versus start-of-period contributions. A start-of-month deposit receives one additional monthly growth period; match deposit timing when comparing results.
How a starting balance and monthly deposits combine
These three illustrations use the same fixed 8% nominal annual rate, monthly compounding and 30-year term. Deposits are added at each month-end; there are no fees, taxes, withdrawals or inflation adjustment.
- $50,000 initial, $0/month: modeled balance $546,786; total deposits $50,000.
- $0 initial, $500/month: modeled balance $745,180; total deposits $180,000.
- $50,000 initial, $500/month: modeled balance $1,291,966; total deposits $230,000.
The examples use different total deposits, so they illustrate how the inputs combine rather than prove that one contribution schedule is better. Actual returns vary; use the calculator to test assumptions that fit the account and time period you want to model.
The compound interest + contribution formula
With monthly compounding and month-end deposits, the model combines principal growth with the future value of a monthly annuity:
Principal growth: A = P × (1 + r/12)^(12t)
Contribution growth: FV = PMT × [((1 + r/12)^(12t) − 1) / (r/12)]
Total final value: sum of both. Our calculator above does this math automatically — drag the sliders and watch.
How to use this calculator for real scenarios
401(k) with employer match
Enter the current balance and a deposit amount based on your plan documents, including any match that applies. Choose a rate only as an assumption and compare lower and higher scenarios; this calculator does not forecast a plan balance or model vesting, fees, taxes or withdrawals.
Roth IRA contributions
Enter the current balance and a contribution amount after checking current IRS limits and eligibility. For 2026, the general $7,500 IRA contribution limit applies across traditional and Roth IRAs combined; an additional $1,100 catch-up may apply for someone age 50 or older by year-end. Roth IRA eligibility can also depend on income. See the IRS 2026 limits and IRA eligibility and contribution rules. The calculator does not check eligibility or account tax treatment.
Taxable brokerage account
Use the balance and planned monthly deposit you want to model. This calculator does not estimate taxes or after-tax returns; tax treatment depends on the account and your circumstances, so do not subtract a universal percentage from the rate.
High-yield savings account
Use the current account terms and test more than one rate because APYs can change. This calculator expects a nominal annual rate: if a provider gives only an APY, use annual compounding with that effective annual rate or convert it before selecting more frequent compounding, so you do not compound the APY twice.
Compounding frequency: monthly vs quarterly vs annual
With a $0 starting balance, $500 deposited at each month-end for 30 years, and a fixed 8% nominal annual rate, the shared calculator model gives:
- Annual compounding: $704,275 final
- Quarterly compounding: $737,248 final
- Monthly compounding: $745,180 final
- Daily compounding: $749,101 final
The model converts the selected nominal rate and compounding frequency into an equivalent monthly growth factor, applies it to the existing balance, then adds the month-end deposit. Under these inputs, the annual-to-daily difference is about $44,826; that is a scenario result, not a universal product rule. Actual account crediting conventions vary.
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Methodology & sources
Calculations and account-rule references reviewed 2026-10-11. The growth examples are fixed-rate illustrations, not forecasts; verify current account terms, tax rules and contribution limits before using them.
- FV of an annuity formula: standard time-value-of-money mathematics
- Growth values: the shared Snowballr calculation engine using the assumptions stated beside each example
- 2026 IRA contribution limits: IRS cost-of-living adjustments