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Compound interest calculator

There is no shortage of compound interest calculators online. Nearly all of them are the same thing: a web form with three boxes, an answer, and nothing to take away. Change your mind about the rate and you start again. Come back next year and you enter everything from scratch.

These are spreadsheets. You download them, put your numbers in the purple cells, and keep them. Next year you open the same file, update what's changed, and see how far the picture has moved.

That matters more than it sounds for compounding, because the whole point is what happens over decades, and a plan you revisit each year is worth considerably more than a number you saw once on a screen.


Start here — the quick one

If you just want to know what a sum will grow to, this is the one. Enter what you're saving, for how long, at what rate.

Compound interest calculator


The one that does what web calculators can't

The comprehensive version is the reason this page exists. It handles the things a three-box form can't:

  • Compare contributing against not contributing, side by side

  • Add contributions or withdrawals in any year you like - not a fixed monthly figure, but the messy reality of a bonus one year and a car replacement the next

  • See how much came from your money and how much came from returns. This is the number that surprises people, and it's the one most calculators never show

  • A second tab comparing a lump sum across different rates and compounding frequencies, so you can see what monthly versus annual compounding is actually worth

Comprehensive compound interest calculator

That contribution-versus-growth split is worth dwelling on. Early on, almost everything in the balance is money you put there. The crossover, the year where growth has contributed more than you have, usually arrives later than people expect and then changes everything after it. Watching where that lands in your own numbers is more persuasive than any general statement about compounding I could write here.

How long until my money doubles?

The rule of 72 is a shortcut: divide 72 by your rate of return and you get roughly the number of years to double. At 7.2%, about ten years.

It works for anything expressed as a percentage. Inflation, pay rises, rent increases. It isn't exact to the decimal, but it's close enough to be useful and simple enough to do in your head.

The rule of 72 calculator works both directions. Enter a rate and get the doubling time, or enter a timeframe and get the rate you'd need.

Rule of 72 calculator

The rule of 72 grid needs nothing from you at all. It lays out a range of rates and doubling times in one view, which is more useful when you're comparing several scenarios rather than checking one.

Rule of 72 grid


A word on the rate you enter

Compound interest calculators flatter optimism. Nudge the rate from 5% to 8% and the thirty-year number changes dramatically, which makes it tempting to keep nudging.

Two things worth holding onto. Returns aren't smooth. The calculator draws a clean curve, and no real portfolio has ever done that. And the rate should reflect what you're actually invested in, after fees and after tax, not what you hope for.

If the answer only works at 9%, the plan needs changing rather than the assumption.

The number is the easy part

Working out what compounding does is arithmetic. The harder questions are the ones the spreadsheet can't answer. Whether the rate you've assumed matches what you're actually invested in, whether the money is in the right place for when you'll need it, and how much of that return the tax and fees quietly take.

If you'd like to talk that through, get in touch for a free 30 minute chat.

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