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Compound Interest Calculator
See how a lump sum and regular contributions grow with compound interest over time. Change the rate, the term and how often interest is added, and watch the curve steepen.
An estimate to help you plan, not financial advice. Investment returns are not guaranteed and can fall as well as rise.
- Final balance
- £68,401
- What you put in
- £34,000
- Interest earned
- £34,401
- Interest as a share
- 50%
How compounding works
Each time interest is added, it joins your balance. The next lot of interest is then worked out on that bigger balance, so your money grows a little faster each period. Leave it long enough and the interest you earn on past interest can outweigh what you originally put in. Time is the biggest lever, which is why the curve above gets steeper the further out you go.
Common questions
What is compound interest?
Compound interest is interest paid on the interest you have already earned, as well as on the original amount. Each time interest is added it joins the balance, so the next lot of interest is worked out on a bigger figure. Over years this snowballs, which is why it is sometimes called the eighth wonder of the world.
What is the difference between compound and simple interest?
Simple interest is always worked out on the original amount, so it grows in a straight line. Compound interest is worked out on the growing balance, so it curves upward and pulls ahead of simple interest the longer you leave it.
How often should interest compound?
The more often interest is added, the slightly higher the end balance, because interest starts earning sooner. Use the frequency above to see the difference between monthly, quarterly and yearly compounding on your figures.
How can I work out compound interest myself?
For a lump sum with no contributions, multiply the amount by (1 plus the rate per period) raised to the number of periods. For example £1,000 at 5% for 3 years is £1,000 × 1.05³, which is about £1,158. The calculator also handles regular contributions, which the simple formula does not.
Does this work for investments as well as savings?
The maths is the same, but investment returns are not guaranteed and can fall as well as rise, so a fixed rate is only ever a rough guide for investing. For cash savings, use the rate your account actually pays.
About this calculator
Uses standard compounding arithmetic, adding your contribution each month and interest at the frequency you choose. It assumes a steady rate, which real savings and investments rarely deliver, and shows the gross figure before any tax or charges. Treat it as a guide to help you plan, not financial advice. Last updated June 2026.