I built this because I got fed up doing compound interest sums on the back of an envelope — badly, usually. Pop your numbers in below and it'll show you exactly how a lump sum, or regular monthly deposits, actually grows over time. No sign-up, no email address wanted, just the maths.
Compound Interest Calculator
Investment growth with regular deposits and compounding
Your results will appear here
Fill in the values on the left and click Calculate
How compound interest actually works
Here's the bit school never really explained properly: compound interest is just interest earning interest. You put money in, it earns interest, and then next year you earn interest on the original amount and on the interest you already made. Do that for long enough and the growth stops being a straight line and starts curving upwards — sometimes dramatically.
Say you put away £10,000 at 5% a year and never touch it. After year one you've got £10,500. Doesn't sound like much. But leave it 20 years and you're sat on over £26,500 — more than double, without adding another penny. That's the whole trick. Time does most of the heavy lifting, not the amount you start with.
Does compounding frequency actually matter?
A bit, yes, though less than people expect. Daily compounding versus annual compounding on the same rate makes a small but real difference — a few pounds here and there on smaller pots, more noticeable once you're into five figures. If your provider compounds monthly rather than annually, that's a genuine (if modest) advantage. Use the compounding frequency dropdown above to see the difference for yourself with your own numbers.
Why regular deposits change the picture completely
Here's where it gets properly interesting. A lump sum growing on its own is one thing, but most of us aren't sitting on a spare £10k — we're chipping in £100 or £200 a month when we can. Add regular deposits into the calculator and you'll see the growth curve change shape entirely, because now you're compounding on a constantly topped-up pot rather than a fixed one. Even modest monthly amounts, left alone for 15–20 years, tend to surprise people. I know it surprised me.
A word of caution
This calculator shows you the maths of compounding — it doesn't and can't predict what any real investment will actually return. Markets go up and down, and a 5% or 7% annual return typed into a box is an assumption, not a promise. Use it to understand how growth works, not as a forecast of what you'll definitely end up with.
Frequently asked questions
Is monthly or annual compounding better for me?+
More frequent compounding is always slightly better for your money, all else being equal — but the difference is usually small compared to the impact of the interest rate itself or how much you're paying in. Don't lose sleep over it.
What annual interest rate should I use for a stocks and shares ISA?+
There's no guaranteed rate — that's the nature of investing, not saving. Many people use long-run historical stock market averages (often cited around 5-7% after inflation for diversified global equities) as a rough planning assumption, not a promise. Worth reading around this properly rather than taking any single figure as gospel.
Does this calculator account for tax?+
No — it shows pure compound growth. If you're investing outside a tax wrapper like an ISA or SIPP, remember your actual returns could be reduced by tax depending on your circumstances.
This calculator is for illustration purposes only and doesn't constitute financial advice.