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Finance & Money

Free Compound Interest Calculator

See how savings grow when interest earns interest - with or without regular contributions.

Free to useNo sign-up requiredNo watermarkRuns in your browser

Last reviewed: 30 September 2026 by Vishal Senthilkumar

This free compound interest calculator shows what a starting balance grows into once interest starts earning interest of its own. Set the principal, the annual rate, the compounding frequency and the time period, and you get the final balance, the total interest earned and a year-by-year view of the growth.

You can also add a regular contribution. That usually matters more than the interest rate over realistic time horizons - a modest monthly deposit compounds into far more than most people expect.

How this tool works

  1. Enter your starting balance

    The principal you begin with. It can be zero if you are starting from nothing and relying entirely on contributions.

  2. Set the annual interest rate

    The nominal annual rate. For a realistic view of savings, consider using a rate net of tax and inflation.

  3. Choose the compounding frequency

    Monthly, quarterly or yearly. More frequent compounding produces slightly more growth for the same nominal rate.

  4. Add a regular contribution if you make one

    A recurring deposit is compounded along with the principal. Leave it empty to see growth on the lump sum alone.

How it works

Simple interest is paid only on the original principal. Compound interest is paid on the principal plus all the interest already added, so the balance grows faster and faster the longer it is left alone.

Compounding frequency is how often interest is added to the balance. Monthly compounding adds one twelfth of the annual rate twelve times a year, and because each addition earns interest for the remaining months, the yearly total slightly exceeds the nominal rate.

Time is the dominant variable. Because growth is exponential rather than linear, the final years of a long investment add far more in absolute terms than the first years - which is why starting earlier beats contributing more later.

Regular contributions are added at each compounding step and then compound themselves. Over decades the total contributed is often smaller than the interest earned on it.

Why time matters more than rate

Compound growth is exponential, so the curve is nearly flat at first and then steepens sharply. Most of the final balance is generated in the last third of the period.

A useful shortcut is the rule of 72: divide 72 by the annual interest rate to get roughly how many years it takes for money to double. At 6% that is about twelve years; at 9%, about eight.

1,000 growing at 7% a year, compounded yearly
After 5 years1,403
After 10 years1,967
After 20 years3,870
After 30 years7,612
After 40 years14,974

How compounding frequency changes the result

For the same nominal annual rate, compounding more often gives a slightly higher balance. The effect is real but modest, and much smaller than the effect of the rate itself or of time.

10,000 at 6% for 10 years, by compounding frequency
Yearly17,908
Quarterly18,140
Monthly18,194

Things this calculator does not know

  • Tax. Interest and investment gains are usually taxable, which reduces the real return.
  • Inflation. A balance that grows 5% a year while prices rise 3% has gained about 2% in purchasing power.
  • Fees. Platform and fund charges come off the return every year and compound against you.
  • Variable returns. Investments do not deliver a steady percentage; a fixed rate is a simplification for planning, not a forecast.

Formula

Compound interest

A = P(1 + r ÷ n)^(n × t)

P is the principal, r the annual rate as a decimal, n the compounds per year, t the years.

Interest earned

interest = A − P

With regular contributions

A = P(1 + i)^N + C × (((1 + i)^N − 1) ÷ i)

i is the rate per period, N the number of periods, C the contribution each period.

Rule of 72

years to double ≈ 72 ÷ annual rate %

Worked examples

10,000 at 6% for 10 years, compounded monthly

The balance reaches about 18,194, meaning roughly 8,194 of interest on a 10,000 stake.

The same money at simple interest

6% simple interest on 10,000 for 10 years is 6,000. Compounding adds about 2,194 more, purely from interest earning interest.

Adding 200 a month

Starting from 10,000 at 6% for 10 years with 200 added monthly, the balance passes 51,000 - of which about 24,000 is contributions and the rest is growth.

Starting ten years earlier

1,000 at 7% for 40 years becomes about 14,974. The same 1,000 for 30 years reaches about 7,612 - the extra decade roughly doubles the outcome.

Frequently asked questions

Is this compound interest calculator free?

Yes, free with no sign-up. Every figure is calculated in your browser and nothing is sent to a server.

What is compound interest?

Interest calculated on the principal plus all previously added interest, rather than on the principal alone. Because each period starts from a larger balance, growth accelerates over time.

What is the compound interest formula?

A = P(1 + r ÷ n)^(n × t), where P is the principal, r the annual rate as a decimal, n the number of compounds per year and t the number of years.

Does monthly compounding beat yearly compounding?

Slightly, for the same nominal rate. 10,000 at 6% over 10 years gives about 17,908 compounded yearly and about 18,194 compounded monthly.

Can I include regular monthly deposits?

Yes. Add a recurring contribution and it is compounded alongside the principal. Over long periods, contributions often account for less of the final balance than the growth on them.

Does this account for tax and inflation?

No. It shows nominal growth at a fixed rate. Real returns are lower once tax, fees and inflation are taken into account.