Compound Interest Calculator

Compound interest in one paragraph

You earn interest on your principal, then the next period you earn interest on both the principal and the interest from the period before. That is the whole mechanism. The maths is simple; what catches people out is the curve. For the first few years nothing dramatic happens, then somewhere around year fifteen the line bends 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 English

The shape below the calculator is FV = P(1 + r/n)^nt + PMT × [((1 + r/n)^nt − 1) / (r/n)]. 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, and multiply by what you started with. Then add the future value of every regular contribution made along the way. 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.

How to use the calculator step by step

Enter your initial amount, the annual interest rate, and how long you plan to leave the money invested. Pick a compounding frequency — annually, semi-annually, quarterly, monthly, weekly, or daily — to match how your account or investment actually credits interest. If you make regular deposits, enter the contribution amount and how often. Add an annual increase percentage if you plan to step up contributions over time, which mirrors how most pension and salary-driven savings actually work. The result, the breakdown table and the comparison chart all recompute as you type. Use the currency selector at the top of the input panel to render every figure on the page in your preferred unit.

What each result means

The headline figure is the future value: principal plus all contributions plus all compounded interest. Below it the calculator surfaces total interest earned (the part that came from the money working rather than from your deposits), total deposited (principal plus the sum of every contribution), and the effective annual yield — the true once-a-year-equivalent rate after compounding. The Time-to-Double figure uses the closed-form result t = ln(2) / (n × ln(1 + r/n)), which is the precise version of the Rule of 72 mental shortcut. When contributions are present the calculator also reports a Money-on-Money Return: the cumulative ratio of interest earned to total deposited, which describes how much extra came back on every unit you put in.

After tax and inflation, what you keep in real terms

Many compound calculators stop at the gross future value. The gross figure is the number on a statement; the net real value is the number that translates into purchasing power. The Tax Rate input applies to the interest portion of each compounding period, not as a one-off tax at the end — which mirrors how a taxable brokerage or savings account behaves outside of a tax-advantaged retirement wrapper. The drag compounds in reverse: a 25% tax on returns does not reduce the final balance by 25%, it reduces it by considerably more over a long horizon because every year's interest is taxed before it can compound. At 7% nominal over 30 years, a 25% tax typically shrinks the final figure by roughly 35-40% rather than 25% (verifiable by setting principal=10,000, rate=7%, years=30, tax_rate=25%, contribution=0 in the calculator and comparing the After-Tax Future Value row to the Future Value row). Layer the inflation input on top to convert that net figure into today's purchasing power, and the resulting Real After-Tax Value row reflects what the projection is worth in goods and services for any goal denominated in real-world spending.

Try these scenarios to feel the curve

Using 1,000 of any currency at 7% for 10 years with monthly compounding, you land at about 2,010 — roughly double. Stretch the term to 20 years and you're at about 4,038 — four times. At 30 years: about 8,116 — eight times. Each extra decade doesn't add a fixed amount; it multiplies what's already there. Now turn on a 200/month contribution at the same rate over 30 years and the figure rises to about 252,000 — the contribution stream becomes the main driver after about year 15. Add a 3% annual contribution increase (matching typical wage growth) and the projection rises to about 342,000. Apply a 25% tax rate on returns and the same scenario lands at about 252,000 instead of 342,000 — about 26% lower because the tax compounds in reverse against every year of interest.

How starting early compares with 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 that the early money gets 40 years to compound versus 30. Running both scenarios side by side here turns a saying into a number that can be checked.

The fee, inflation and tax trio

Nominal compound growth describes the gross outcome only. The figure that gets kept after costs is smaller, for three reasons.

Fees. A 1% annual fee on an investment returning 7% doesn't cost 1% — over a 20-year horizon it reduces the final balance by about 17%, and over 30 years by about 25% (derived by comparing 1.07ⁿ with 1.06ⁿ, which the calculator reproduces by running the same scenario once at rate=7% and once at rate=6%). The fee compounds too, in the wrong direction, and fund-level, platform and advice fees stack.

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. The figure on a future statement is the same; the spending power it represents is not. The inflation input surfaces both numbers.

Tax. The Tax Rate input handles this directly. Outside of ISAs, pensions or equivalent wrappers, tax is paid on interest or gains along the way. The calculator applies the tax rate to the interest portion of every compounding period, so the after-tax balance compounds going forward — matching how the drag works in a taxable account rather than as a single end-of-period deduction.

Does compounding frequency matter?

Less than the marketing often suggests. Going from annual to monthly compounding at 7% over 30 years moves 1,000 from about 7,612 to about 8,117 — a 6.6% lift on the final figure. Moving from monthly to daily adds about another 48 (a further 0.6% lift). The lesson: whether compounding happens matters enormously; how often matters at the margins. When a provider emphasises "daily compounding", the meaningful comparison is the quoted annual rate, not the frequency. The calculator shows the effective annual yield directly so two providers can be compared fairly even if their stated rate and frequency differ.

What the comparison chart shows

The chart plots multiple lines side by side: compound balance (the money working at the rate entered), after-tax balance (when a tax rate above zero is set), simple interest (same rate but interest never reinvested), and no-interest principal (just the deposits). The gap between the compound and simple lines is what compounding contributes — for the first few years the gap is invisible, then it widens. The gap between the gross compound line and the after-tax line is the tax drag, which also widens with time. Looking at all the lines together makes it concrete how much of long-term growth is the rate, how much is the compounding effect, and how much is taxed away.

Reading the year-by-year breakdown table

Below the chart, every period of the projection is laid out in a sortable table: opening balance, contributions for the period, interest earned that period, accrued interest to date, and closing balance. Switch the breakdown to monthly to see month-by-month figures (capped at 360 rows for readable scrolling). Click any column header to sort. The period where annual interest first exceeds the annual contribution is the tipping point where invested money does more work than fresh deposits. For most realistic savings scenarios it lands somewhere between year 12 and year 20.

Things to watch for

A few traps. First, entering nominal historical returns as a forward expectation — long-run global equity returns have commonly been cited around 5-8% real over multi-decade periods (Dimson, Marsh & Staunton, Triumph of the Optimists; Credit Suisse Global Investment Returns Yearbook), but any individual 10-year window can deliver well above or well below that range. Second, forgetting to step up contributions in line with wages — a flat 200/month contribution feels right for the next year but not for year 30, when both salaries and prices have roughly doubled. The annual increase input fixes that. Third, ignoring tax — many online compound calculators omit tax entirely, then surprise users when their real-world results come in 30-40% short of the projection. Fourth, 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, even though the lived experience differs.

compound interest accrues to debt-holders too

The same mechanic that grows a savings account is what makes a credit card balance at 22% APR difficult to clear. 3,000 at 22% compounding monthly doubles roughly every 3.2 years if no payments are made — the same maths, pointed the other way. The debt tools in this library use the same formula; running a savings scenario next to a debt scenario is often a clarifying comparison. For someone with 20,000 in a tax-advantaged account compounding at 5% and 8,000 on a credit card compounding at 22%, the credit card balance compounds faster than the savings — so net worth shrinks even when the savings figure on paper grows.

What the calculator can't model

Every projection tool simplifies. This one assumes a constant rate, a fixed compounding frequency, a flat tax rate (real systems have brackets, allowances, and variable rates), and an inflation figure entered manually. Real investment journeys include changing rates, market volatility, varying tax situations, withdrawals, and behavioural responses to drawdowns. The output is best read as the cleanest possible version of one scenario rather than a forecast. A projection like this is typically used for recalibrating intuition about how long horizons compound, not for predicting the future.