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? Each month, your existing balance earns interest AND a new deposit is added, then the entire new total compounds the following month. The formula: FV = P·(1+r/12)n·12 + PMT·((1+r/12)n·12−1)/(r/12). Realistic example: $500/mo at 8% compounded monthly grows to $91,500 in 10 yrs, $294,500 in 20 yrs, $452,000 in 25 yrs, and $745,000 in 30 yrs.
- $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. Smooths entry price and matches monthly-contribution behavior.
- PMT (periodic contribution)
- The recurring deposit amount. Monthly in this calculator. Each PMT earns interest from the day it lands.
- Ordinary annuity vs annuity-due
- End-of-period vs start-of-period contributions. Difference over 30 years is ~1 month of growth — small but non-zero.
Why monthly contributions matter more than starting amount
Two scenarios at 8% annual return for 30 years:
- $50,000 initial, $0/month: grows to $503,000
- $0 initial, $500/month: grows to $745,000
Starting with zero but contributing $500/month beats a $50,000 lump sum with no contributions — by $242,000. Monthly contributions compound over the full timeline, while the initial amount only compounds once. Consistency beats size.
The compound interest + contribution formula
Most online "compound interest calculators" only handle the principal. Real-world investing needs both formulas combined:
Principal growth: A = P × (1 + r/n)^(nt)
Contribution growth: FV = PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
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
Initial: current 401(k) balance. Monthly: your contribution + employer match. Rate: 7-9% for balanced target-date funds. Years: until retirement.
Roth IRA contributions
Initial: current Roth balance. Monthly: up to $583/month ($7,000/year 2025 limit). Rate: your investment mix — 8-10% for all-stock, 6-7% for balanced.
Taxable brokerage account
Same inputs, but remember returns are taxed. Reduce your rate by 0.5-1% to approximate tax drag on dividends and short-term gains.
High-yield savings account
Initial: current balance. Monthly: automatic transfers from checking. Rate: your APY (currently 4-5% for top HYSAs). Years: time until you need the money.
Compounding frequency: monthly vs quarterly vs annual
At 8% nominal rate over 30 years with $500/month contributions:
- Annual compounding: $679,000 final
- Quarterly compounding: $724,000 final
- Monthly compounding: $745,000 final
- Daily compounding: $753,000 final
The difference between annual and daily is ~11%. Most investment products compound either monthly (mutual funds, index funds) or daily (savings accounts). For simplicity, use monthly as your default.
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Methodology & sources
- FV of an annuity formula: standard time-value-of-money mathematics
- S&P 500 long-run nominal return ~10%: NYU Stern (Damodaran) historical data set
- HYSA rates: FDIC weekly national rate cap and top-bank disclosures
- 401(k) and Roth IRA limits: IRS Publication 590 and contribution limit notices