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Compound interest calculator

Find out what your savings or investments could grow to. Add a starting amount and a monthly contribution, and the chart shows how much comes from you and how much from interest.

Annual interest rate
Years
Interest added
Final balance
—
You pay in Interest earned

Calculated instantly in your browser — nothing you enter is sent or stored.

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How to use the calculator

  1. Enter your starting amount (it can be zero).
  2. Enter how much you will add each month.
  3. Set the expected yearly interest rate or return and the number of years.
  4. Choose how often interest is added: monthly, quarterly or yearly.

In each bar of the chart, the blue part is the money you paid in and the green part above it is interest earned. Over long periods the green part usually becomes the bigger one. That is compounding.

The compound interest formula

For a single amount with no contributions:

A = P × (1 + r ÷ k)^(k × t)

  • A = final amount, P = starting amount
  • r = yearly rate as a decimal (6% = 0.06)
  • k = how many times a year interest is added
  • t = years

With regular monthly contributions, the calculator adds each deposit at the start of the month and credits interest on the whole balance at each interest date.

Worked example

5,000 invested at 6% a year, compounded yearly, for 10 years:

A = 5,000 × 1.06¹⁰ = 5,000 × 1.7908 ≈ 8,954. You earned about 3,954 in interest without adding anything.

Add 200 a month and, with monthly compounding, the balance after 10 years is roughly 42,000, of which you paid in 29,000.

The rule of 72

To estimate how long it takes to double your money, divide 72 by the yearly rate. At 6% that is 72 ÷ 6 ≈ 12 years. At 8% it is about 9 years.

Things to keep in mind

  • Inflation reduces what your money can buy. A 6% return with 3% inflation is roughly a 3% real return.
  • Investment returns are not guaranteed. Shares can fall, and past returns do not predict future ones.
  • Fees and taxes reduce growth. A 1% yearly fee can take a large share of the gains over 30 years.

Frequently asked questions

What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is also paid on interest you have already earned, so growth speeds up over time.
Does compounding more often make a big difference?
A small one. At 6%, yearly compounding gives 6.00% a year, while monthly compounding gives about 6.17%. Time and the rate itself matter far more.
What interest rate should I use?
For a savings account, use the advertised AER. For a diversified long-term investment, many people use a cautious 4–6% to plan. That is an assumption, not a promise.
Is this financial advice?
No. The calculator shows the maths of a constant rate. Real investments go up and down. Speak to a qualified financial adviser for personal advice.

Last reviewed: November 13, 2026

Results are estimates for general information only and are not financial, tax, medical or legal advice. Check important figures with a qualified professional.