How do I add regular contributions on top of the starting amount?

This calculator models a single lump-sum deposit. For monthly contributions on top of the principal, use the Investment Calculator or Retirement Calculator, both linked below.

Compound Interest Calculator

Principal Amount

Annual Interest Rate

Compounds Per Year

Time Period

years

Final Balance

—

Interest Earned

—

Effective Annual Rate (APY)

—

Updating live using the standard compound interest formula: A = P(1 + r/n)^nt.

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Enter a principal, rate, and time to see the formula with your numbers.

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What your result means

Final Balance is computed with A = P(1 + r/n)^nt — the standard formula published by the U.S. Securities and Exchange Commission on Investor.gov. Every number on this page comes from that one equation.

Interest Earned is everything above your original deposit — the part that came from the rate working on itself. Over long horizons this usually dwarfs the principal, which is the whole reason compounding matters for retirement planning.

Effective Annual Rate (APY) shows what the stated rate actually works out to once compounding frequency is baked in. Use this number, not the nominal rate, when comparing savings accounts or CDs side by side.

Try adjusting the rate by half a percent and watch the final balance move. Over 30 years, a 0.5% difference is rarely a rounding error — it's often tens of thousands of dollars, which is what the "time + rate" combination does to money left alone.

How this calculator works

You enter four things: starting principal, the annual rate the account pays, how often interest is credited (annually, semi-annually, quarterly, monthly, or daily), and how long the money stays invested. The calculator plugs these into A = P(1 + r/n)^nt on every keystroke. It assumes the rate is fixed for the entire period, no additional contributions, no withdrawals, and no taxes or fees taken out while the money compounds. To model regular contributions or tax-advantaged accounts, use the Investment or 401(k) calculators linked below. Full methodology →

What affects your result

Time is the heaviest lever. Doubling the years does far more than doubling the rate, because compounding is exponential in time.

Rate is the second lever. A 7% return versus 5% over 30 years is not a 40% difference — it's roughly an 80% difference in final balance.

Compounding frequency is a minor lever. Daily versus annual at the same stated rate usually changes the outcome by 1–3% over decades. Don't chase it.

Taxes and fees are invisible here. A 1% expense ratio or a 15% capital gains drag quietly eats a meaningful share of the growth shown above. Model those separately.

Inflation shrinks the final number in real terms. A $500,000 nominal balance in 30 years buys roughly what $206,000 does today at 3% inflation.

A worked example

You deposit $10,000 in an account paying 7% annually, compounded monthly, and leave it alone for 30 years. Plugging into A = P(1 + r/n)^nt: A = 10,000 × (1 + 0.07/12)^12×30 = 10,000 × (1.00583)³⁶⁰ ≈ $81,165. The interest portion — $71,165 — is more than seven times the original deposit. That is the entire case for starting early.

Quick questions

Does compounding frequency really change my final balance?

Yes, but less than most people expect. At 7% over 30 years, switching from annual to daily compounding on $10,000 raises the ending balance by roughly 2%. Rate and time dominate — frequency is a tiebreaker, not the main lever.

Should I account for inflation when projecting decades out?

The calculator shows a nominal balance. To see real purchasing power, subtract your inflation assumption from the rate before running the numbers. At 7% nominal and 3% inflation, enter 4% to see the real return. The Inflation Calculator can convert a future balance back to today's dollars directly.

How are compound earnings taxed while the money grows?

In a taxable brokerage account, interest and dividends are generally taxed each year, which shrinks the base that compounds. In a 401(k) or traditional IRA, growth is tax-deferred; in a Roth IRA, qualified withdrawals are tax-free. See IRS Topic 409 for the capital gains rules that apply when you eventually sell.

Why is my effective annual rate higher than the rate I entered?

The effective annual rate — APY — captures the boost from compounding more than once per year. At a 6% nominal rate compounded monthly, the effective rate is about 6.17%. When you're comparing savings accounts, always compare APY to APY, not stated rate to stated rate.

How do I model regular contributions on top of the starting amount?

This calculator is for a single lump-sum deposit, which keeps the formula clean and honest. For ongoing monthly or annual contributions, use the Investment Calculator or the Retirement Calculator.

Compound Interest Calculator

What your result means

How this calculator works

What affects your result

A worked example

Quick questions

Sources

Method & review

Compound Interest Calculator

What your result means

How this calculator works

What affects your result

A worked example

Quick questions

Sources

Related Calculators

Method & review