Explore compound interest growth

Compound interest in one paragraph

You earn interest on your principal, then the next year you earn interest on both the principal and the interest from the year before. That's it — there's no other secret. The maths is simple; what catches people out is the curve. For the first few years nothing dramatic happens, and then somewhere around year fifteen the line starts bending upward in a way that looks nothing like what you'd sketch from a straight ruler. Most people's mental model of growth is linear, which is why compounding keeps surprising them.

The formula, in plain

The shape below the calculator is A = P(1 + r/n)^nt. Strip the notation and you're saying: take the rate per compounding period, add 1, raise it to the power of how many periods there are in total, and multiply by what you started with. Monthly compounding at 7% annually isn't 7% per month — it's 0.5833% per month, applied 120 times over 10 years. That multiplication is where the acceleration comes from.

Try these scenarios to feel the curve

Using the defaults (1,000 at 7% for 10 years, monthly compounding), you land at 2,010 — roughly double. Stretch the term to 20 years and you're at 4,038 — four times. At 30 years: 8,116 — eight times. Each extra decade doesn't add a fixed amount; it multiplies what's already there. Under the Rule of 72, money at 7% doubles every 10.3 years, so 30 years gets you roughly 2³ = 8× the starting figure. The calculator gives you the precise number; the rule gives you the mental shortcut.

Why starting early beats starting big

This is the most famous compound-interest result and it's genuinely counter-intuitive. A 25-year-old who invests 100 a month for 10 years and then stops — 12,000 total — ends up with more at 65 than a 35-year-old who invests 100 a month for 30 years, putting in 36,000. Same rate, triple the money, smaller final figure. The reason is simply that the early money gets 40 years to compound versus 30. Running both scenarios in this calculator is a useful exercise: it turns a saying into a number you can argue with.

The fee, inflation, and tax trio

Nominal compound growth is the flattering version of the story. What you actually keep is smaller, for three reasons:

Fees. A 1% annual fee on an investment returning 7% doesn't cost you 1% — it costs you roughly 15–20% of your final wealth over a 30-year horizon. The fee compounds too, in the wrong direction. Fund-level, platform, and advice fees stack. Checking what you're actually paying across all three is the single highest-return hour of admin most people can do.

Inflation. The 8,116 from the 30-year example above is in today's money only if inflation is zero. At 2.5% average inflation, the real purchasing power is closer to 3,870. Still a good return — but not eight times your spending power, which is what the nominal figure implies. The inflation input exists precisely to strip that flattering layer off.

Tax. Outside of tax-advantaged savings accounts, pensions, or equivalent wrappers, you pay tax on interest or gains along the way. Effective returns after tax can be 20–40% lower than headline returns depending on your marginal rate and the investment type. The calculator assumes a tax-neutral wrapper; in the real world that assumption only holds if you're actually using one.

Does compounding frequency matter?

Less than the marketing suggests. Going from annual to monthly compounding at 7% over 30 years moves 1,000 from about 7,612 to 8,116 — a 6.6% lift on the final figure. Moving from monthly to daily adds about another 9. The lessonwhether you compound matters enormously; how often matters at the margins. When a provider makes a big deal of "daily compounding", the meaningful comparison is the quoted annual rate, not the frequency.

Common mistakes when running scenarios

A few traps. First, entering nominal historical returns as a forward expectation — equities have averaged roughly 7% real long-term, but a 10-year period can easily deliver half of that. Second, forgetting that the input doesn't include ongoing contributions; if you plan to add to the pot each month, this calculator understates the result. Third, ignoring sequence-of-returns risk — the output assumes smooth growth, but a market that drops 30% in year one and recovers later ends up at the same place mathematically, not emotionally or behaviourally.

Compound interest works against you too

The same mechanic that grows a savings account is what makes a credit card balance at 22% APR so dangerous. 3,000 at 22% compounding monthly doubles roughly every 3.2 years if you make no payments — the same maths, pointed the other way. The debt tools in this library use the same formula; seeing your savings scenario next to your debt scenario is often the most clarifying exercise someone can run. Someone with 20,000 in an tax-advantaged savings account compounding at 5% and 8,000 on a credit card compounding at 22% is losing, not winning.

What the calculator can't model

Every projection tool simplifies. This one assumes a constant rate, a single lump sum, no contributions after day one, and an inflation figure you enter yourself. Real investment journeys include all of those plus withdrawals, market volatility, changing tax situations, and behavioural responses to drawdowns. Treat the output as the cleanest possible version of one scenario, not a forecast. The point of running the numbers is to recalibrate intuition before making a decision — not to predict the future.