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Compound Interest Calculator

See how your savings grow with compound interest — with or without regular contributions.

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The amount you start with

Yearly interest rate

Advanced options

Deposit per compounding period (optional)

Your result

Fill in the fields above to see your result.

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What the compound interest calculator does

This compound interest calculator shows how much your money grows once interest is added to your balance and starts earning interest of its own. Enter a starting amount, an annual rate and a number of years, and the tool returns the final value along with how much of it is pure interest.

It fits anyone saving or investing over time: a savings account, a certificate of deposit, bonds, a fund, or retirement contributions. If you top up your balance on a regular basis, switch on the regular contribution option and the calculator will add it every compounding period. You also get a stacked chart (money paid in plus interest) and a year-by-year table, so you can watch the balance build over time.

How to use the calculator

  1. Initial amount — the balance you start with. It can be zero if you are building savings purely from regular contributions.
  2. Annual rate (%) — the nominal yearly interest rate. Enter it even when compounding happens more than once a year; the calculator splits it across periods for you.
  3. Period (years) — how many years you want to project.
  4. Compounding (advanced options) — how often interest is added to the balance: yearly, quarterly or monthly. More frequent compounding gives a slightly higher result at the same rate.
  5. Regular contribution (optional) — a fixed deposit made every compounding period. With monthly compounding that is a monthly deposit; with yearly compounding, once a year.

The result and chart update instantly, so you can swap numbers and compare scenarios on the fly.

How compound interest works

Simple interest is charged only on your starting amount. Compound interest works differently: interest is added to the balance, and in the next period it is calculated on that larger amount. Interest earns interest — that is the whole idea.

The more often interest is added (the compounding frequency) and the longer the period, the stronger this snowball effect becomes.

The formula

For the starting amount alone, the calculator uses the classic future value formula:

FV = P × (1 + r/n)^(n×t)

where:

  • P — initial amount,
  • r — annual rate (e.g. 5% = 0.05),
  • n — number of compounding periods per year (1, 4 or 12),
  • t — number of years.

If you add regular contributions, the calculator also adds the future value of that deposit stream (an ordinary annuity — a deposit at the end of each period):

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

where C is the deposit per compounding period. The final value is the sum of both parts. At a 0% rate there is no division by zero — the deposits simply add up.

The interest earned figure is the final value minus everything you actually put in: the initial amount plus every contribution.

A step-by-step example

You put $10,000 aside at 5% a year, compounded monthly, for 10 years, with no extra deposits.

  1. Convert the rate to a period rate: r/n = 0.05 / 12 = 0.0041667 per month.
  2. Count the periods: n×t = 12 × 10 = 120 months.
  3. Raise to the power: (1 + 0.0041667)^120 = 1.647009.
  4. Multiply by the principal: $10,000 × 1.647009 = $16,470.09.

Interest is 16,470.09 − 10,000 = $6,470.09. Your $10,000 grew by almost 65%, even though the rate is only 5% a year — the extra comes from compounding.

For comparison, with yearly compounding instead of monthly, the same $10,000 at 5% over 10 years reaches $16,288.95. The roughly $181 gap is the payoff for adding interest more often.

The power of compound interest

Compound interest really picks up speed with time, because the base it is calculated on grows every year. Stay with $10,000 at 5% compounded monthly and look at the balance across the years:

  • after 5 years: $12,833.59,
  • after 10 years: $16,470.09,
  • after 20 years: $27,126.40,
  • after 30 years: $44,677.44.

Notice the pace. In the first decade the balance grows by about $6,470. In the second (from $16,470 to $27,126) by more than $10,600. In the third (from $27,126 to $44,677) by more than $17,500. You add nothing, yet each decade contributes more than the one before. That is why time matters more than the size of any single deposit — and why the growth chart on this page gets steeper year after year.

Frequently asked questions

What is compound interest?

Compound interest is interest calculated not only on your starting amount but also on interest already added. After each period the interest increases your balance, and the next period is calculated on that larger figure. As a result the balance grows faster and faster — interest earns interest.

The gap over simple interest widens with time: simple interest always applies to the same starting amount, while compound interest applies to a growing balance.

How often should interest compound?

Compounding is the moment interest is added to the balance. The calculator supports yearly, quarterly and monthly. The more frequent it is, the higher the result at the same nominal rate, though the differences are small and shrink with each step up in frequency.

Frequency depends on the product, so check your account or deposit terms. In the example on this page, monthly instead of yearly added about $181 over 10 years.

Does the calculator account for tax and inflation?

No. The calculator shows the nominal, gross value — before any tax on investment gains and without adjusting for inflation. The real purchasing power of the money will be lower after several years, and interest is usually taxable.

Treat the result as a starting point for planning. The exact tax rules and the effect of inflation depend on the product and country, so factor them in separately when you estimate a real return.

What goes in the regular contribution field?

Enter the amount you add every compounding period. With monthly compounding that is a monthly deposit; quarterly, every quarter; yearly, once a year. The calculator assumes a deposit at the end of each period and adds its future value on top of the initial amount.

If you do not plan regular top-ups, leave the field empty or enter zero — only the growth of the initial amount is then calculated.