Compound Interest Calculator

How compound interest works

Compound interest is the process of earning interest on both your original principal and the interest that has already accumulated. Each compounding period, your interest is added to your balance, and then that larger balance earns interest in the next period.

This creates exponential growth rather than the linear growth you’d see with simple interest. Over short time horizons the difference is minor; over decades, it becomes dramatic.

The compound interest formula

The standard formula is:

A = P × (1 + r/n)^(n×t)

Where:

• A = Final balance
• P = Principal (starting amount)
• r = Annual interest rate expressed as a decimal (e.g. 7% = 0.07)
• n = Number of compounding periods per year
• t = Time in years

If you make regular contributions each period, the total value includes the future value of those contributions:

FV_contributions = PMT × [((1 + r/n)^(n×t) − 1) / (r/n)]

Where PMT is the contribution per compounding period.

Compounding frequencies explained

The difference between monthly and daily compounding at typical savings rates is small (usually under 0.1% of the total), but it becomes meaningful at higher rates or longer terms.

How time affects your returns

Time is the most powerful variable in compound interest. Consider $10,000 invested at 7% annual rate, compounded monthly:

The balance roughly doubles every decade at 7%, which aligns with the Rule of 72 (72 ÷ 7 ≈ 10.3 years to double).

Regular contributions: the power of consistent investing

Adding consistent contributions to a compounding investment is one of the highest-leverage moves available to individual investors. A $500/month contribution at 8% annual rate compounded monthly for 30 years grows to approximately $745,000 — on just $180,000 of total contributions. The remaining $565,000 is pure compound growth.

This is the mathematical foundation behind 401(k) investing, index fund dollar-cost averaging, and any long-term wealth-building strategy. The formula works regardless of whether markets go up or down in any given year, because the average return is what matters over long periods.

What this calculator does not account for

This is an educational estimation tool. Real-world returns differ from these projections because:

• Investment returns are variable. This calculator assumes a fixed annual rate. Stock market returns fluctuate year to year; the 7–10% historical average for the S&P 500 is not guaranteed in any specific period.
• Inflation. A dollar today has more purchasing power than a dollar in 20 years. For inflation-adjusted projections, subtract the expected inflation rate (historically ~3%) from the nominal return.
• Taxes. Interest and investment gains may be taxed as ordinary income (interest) or capital gains (investments) depending on account type. Tax-advantaged accounts (401k, IRA, ISA) defer or eliminate this drag.
• Fees. Investment management fees reduce effective returns. A 1% annual fee compounded over 30 years can consume roughly 25% of your potential gains.

For financial planning decisions involving significant sums, consult a qualified financial adviser who can account for your specific tax situation, time horizon, and risk tolerance.

Using the calculator effectively

1. Enter your starting amount. This is the money you have today to invest or deposit.
2. Set the annual rate. For savings accounts, use the APY. For investment projections, use a conservative estimate of your expected average annual return (many planners use 6–7% for diversified stock portfolios).
3. Choose compounding frequency. Match it to your account’s actual terms. Monthly is correct for most savings accounts.
4. Set your time horizon. The longer the period, the more dramatic the compound effect.
5. Add regular contributions if applicable. This should match what you realistically plan to add per compounding period.