Compound Interest Calculator (with Monthly Contributions)

See how an initial deposit + steady monthly contributions grow over time with compound interest.

Inputs

Starting balance.

Amount you add at the end of each month.

Expected annual return — long-run S&P 500 average is ~7-10% after inflation.

How many years you let the money compound.

How often interest is calculated and added to the balance.

Result

Loading calculator…
—

How to use this calculator

  • Enter your initial deposit (existing savings going in).
  • Set your monthly contribution (what you can realistically add each month).
  • Pick an annual rate — 7% is a reasonable long-run S&P 500 estimate after inflation; high-yield savings sit around 4-5%.
  • Pick the number of years and your compound frequency (monthly is the most common in practice).

About this calculator

Compound interest is interest earned on both your original deposit and on the interest already accrued — money making money on its own money. The longer you let it run, the more dramatic the curve. Adding monthly contributions on top accelerates this further: every dollar you add gets its own runway to compound, so a 30-year-old depositing $500/month for 35 years can end up with more than someone depositing $1,000/month for 15 years starting at 50, even though the lifetime total contributed is the same. The Rule of 72 estimates how long it takes money to double: divide 72 by your annual rate. At 7% annual return, your balance roughly doubles every 10 years. Compound frequency matters less than people think — the difference between annual and daily compounding at 7% over 20 years is under 1%.

Frequently asked

What is compound interest?+
Interest earned on both your original principal and on the interest you have already accumulated. Each compounding period grows the base that next period's interest is calculated against — that's why the curve gets steeper the longer it runs.
How does monthly vs. annual compounding actually differ?+
At 7% over 20 years, monthly compounding produces about 0.7% more than annual compounding. Daily vs. monthly is another fraction of a percent. The real driver of growth is your contribution rate and time, not compounding frequency.
What is a realistic annual return rate?+
High-yield savings: 4-5%. Bonds: 4-6%. US stock index funds (long-run after inflation): 6-7% real, ~10% nominal. Don't plug in 12-15% — that's a sales pitch, not a sustainable return.
What if I miss a monthly contribution?+
The calculator assumes every month is funded. In real life, missing one $500 contribution at year 5 of a 20-year, 7% plan costs you about $1,500 in final balance. Not catastrophic, but visible — the lost interest on the missed contribution compounds for the remaining 15 years.
Are there tax implications?+
In a taxable brokerage account, yes — dividends and realized gains are taxed yearly, which slows compounding. In a 401(k), Roth IRA, or HSA, the calculator's output is closer to reality because growth is tax-deferred or tax-free. Always run your specific situation past an accountant for a real plan.
When should I use this vs. a simple savings calculator?+
Use this when (a) you're adding consistently each month, AND (b) you expect interest/returns above ~2%. For a one-time deposit with no monthly contribution, a basic compound interest calculator is enough. For a savings goal where you need to back-solve for the contribution amount, look for a "savings goal" calculator.

Related calculators

Percentage Math Calculator
Calculate what X% of a value is, find what percentage one number is of another, or compute the percentage change between two numbers.
Tip & Bill Split Calculator
Calculate tip, split the bill among any number of people with per-person subtotal/tip/total, and optionally round each share up to the next dollar.
Present Value Calculator
PV = FV / (1 + r)^n. The current worth of a future cash flow.
Future Value Calculator
FV = PV × (1 + r)^n. What today's amount grows to over time.
Annuity Payment Calculator
PMT = P · r / (1 − (1+r)^(−n)). Payment that fully amortizes a principal over n periods at rate r.
Perpetuity Value Calculator
V = C / r — present value of an infinite stream of equal cash flows.

More tools you might like

Hand-picked tools that pair well with this one — same audience, same intent.

Tip & Bill Split Calculator
Calc

Calculate tip, split the bill among any number of people with per-person subtotal/tip/total, and optionally round each share up to the next dollar.

Percentage Math Calculator
Calc

Calculate what X% of a value is, find what percentage one number is of another, or compute the percentage change between two numbers.

Tip Calculator
Finance

Tip amount and per-person split for any bill.

Percentage Calculator
Finance

Six-mode percentage tool — X% of Y, X is what % of Y, % change, % difference (symmetric), add %, and subtract %.

Loan Payoff Calculator with Extra Payments
Calc

See how extra monthly payments shorten a loan and cut total interest — compare payoff time and interest with and without overpayment.

Real Interest Rate Calculator (Fisher Equation)
Calc

Convert a nominal interest rate and inflation rate into the real interest rate using the exact Fisher equation.