Compound Interest Calculator with Monthly Contributions

Project long-term growth when you invest a set amount every month

Quick answer

Investing $300 a month at a 7% annual return compounds to roughly $156,000 after 20 years — against only $72,000 deposited. Run your own numbers in the compound interest calculator to adjust contribution size, rate, and horizon.

The growth table most people don't expect to see

Deposits are linear; growth is not. The last decade of a 30-year plan produces more growth than the first two combined — which is why starting early beats starting big.

Breaking down the future value formula

The calculator multiplies your monthly contribution by the compound growth factor:

FV = P × [((1 + r/n)^(n×t) − 1) / (r/n)]

where r is the annual rate, n is the number of compounding periods per year, and t is years invested. Adding an initial principal simply adds another future value component — PV × (1 + r/n)^(n×t) — that grows independently alongside your contributions.

Worked example: $10,000 initial plus $300/month at 7% for 20 years:

• Initial principal: $10,000 × (1.005833)^240 ≈ $40,400
• Contribution stream: $300 × [((1.005833)^240 − 1) / 0.005833] ≈ $155,800
• Combined: ~$196,200, of which $82,000 is what you actually deposited

Comparing contribution cadences

Monthly contributions keep cash working more consistently than quarterly or annual deposits — the difference is real but modest. Investing the same $3,600 per year monthly instead of as one January lump sum historically adds well under 1% to long-run results, because lump sums spend part of the year in cash.

Weekly or bi-weekly automatic investing changes the outcome by even less. Cadence is a convenience decision, not a returns decision. The schedule output in the calculator lets you visualize how different cadences accumulate over identical horizons.

Adjusting for inflation and risk

• Inflation: at 3% inflation, $156,000 in 20 years buys roughly what $86,000 buys today. Plan in real (inflation-adjusted) terms for goals decades out.
• Rate scenarios: run optimistic, base, and conservative returns (e.g., 5% / 7% / 9% for a stock-heavy mix). The spread between them is your planning margin.
• Taxes: 401(k) and IRA contributions let compounding work pre-tax; a taxable account shaves the effective rate each year.
• Fees: a 1% annual fee over 30 years consumes roughly a quarter of your final balance — check expense ratios.

Consistency tips for monthly investors

• Automate transfers right after payday so the money never sits in checking.
• Raise contributions with every raise — even 1% per year compounds meaningfully.
• Rebalance annually so new contributions keep your target allocation.
• Ignore the news between reviews: contributions during downturns buy more shares.

Plan other parts of your financial picture

Project single deposits or irregular cash flows in the future value calculator, or translate your savings trajectory into retirement income with the retirement calculator.

Turn monthly habits into long-term wealth

Small monthly contributions compound into significant balances when you stay disciplined. Test scenarios in the calculator, document your plan, and revisit it every year to keep growth on track.