Constant-rate assumption. The math assumes the rate doesn't change. Real markets don't behave that way — they zig-zag. A 30-year "expected 7%" can deliver anywhere from 4% to 10% averaged over realistic periods. Treat the number as a ballpark. Inflation isn't subtracted. A balance of $1,000,000 in 30 years buys roughly what $400,000 buys today (at 3% inflation). Use the retirement-projection tool for inflation-adjusted figures. Taxes aren't subtracted. Tax-sheltered accounts (ISA, Roth IRA, 401k, pension wrappers) compound gross; taxable brokerage accounts compound net of dividend tax. Difference compounds. Contribution timing matters slightly. This tool assumes end-of-period contributions; beginning-of-period contributions earn one extra period of interest each, ~1× nominal-rate more across the life. Fees are silent killers. A 1% expense ratio over 30 years drains roughly 20% of your final balance. Subtract fund/platform fees from your expected return input.

The Rule of 72 is a useful sanity check. Years to double = 72 ÷ rate. At 8% real return, money doubles every 9 years. At 4%, every 18. The rule is accurate within ~1 year for rates between 2% and 12%. Memorise it and you can verify any compound interest calculator's output mentally — if the doubling pattern doesn't fit, the inputs are wrong. Real returns are what actually matter. Most calculators (including this one) compound nominal returns. To project actual buying power, subtract your assumed inflation rate from the return. A 7% nominal return at 2.5% inflation is a 4.5% real return — the "real" curve is what your future self will spend. Showing your spouse "you'll have €1M at 65" is honest in nominal terms, but it buys what €470K buys today after 30 years of typical inflation. Sequence-of-returns risk applies in withdrawal phase. Compound math assumes a smooth average return, but real markets bounce. A 30-year accumulator who happens to start retirement just before a 30% market drop can end up with materially less than the calculator suggests — because they're now withdrawing from a depressed balance. The defence is bond/cash buffer in the first 5-10 years of retirement, not better return assumptions during accumulation. The "tax wrapper" multiplier is huge over decades. Same monthly contribution, same return, taxable brokerage vs Roth IRA / ISA / PEA: the wrapper saves the dividend/capital-gains tax on every compounded year. Over 30 years, that's typically 15-25% of the final balance back in your pocket. If your country has tax-advantaged retirement accounts with unused contribution room, that's the highest-leverage move in personal finance — see our compound interest article for the math. The 4% rule is a starting point, not a destination. "Withdraw 4% of your starting nest egg, adjusted annually for inflation, and you have a high probability of not running out over 30 years" — the Trinity Study finding. It's a heuristic from US data, not gospel. Real planning factors in: variable spending (most retirees naturally adjust), longer time horizons (40+ years for early retirees), Social Security / state pension floors, and home equity. Use 4% as a rough sanity-check on your nest-egg target, not as a withdrawal recipe.

retirement-projection — inflation-adjusted view + withdrawal phase. mortgage-refi-comparison — compare investing the savings vs. paying down a mortgage. currency-converter — if you're tracking a multi-currency portfolio.

Compound Interest Calculator

Calculate compound interest growth for savings or investments. Set principal, rate, period, contribution frequency. See final balance, total interest, year-by-year breakdown, and growth chart.

Compound-interest models assume a constant rate. Real returns vary year-to-year — these are projections, not guarantees.

Principal (starting balance)

Annual rate (%)

Term (years)

Compounding frequency

Contribution amount

Contribution frequency

Currency symbol

Sync with URL

Result

What is this for?

Compound interest is the engine behind every long-term savings or investment plan: each period's gains start earning their own gains. This tool answers "if I start with X, add Y per month for N years at R% return, what do I end up with?" — and shows the year-by-year split between contributions and interest so you can see exactly where the growth comes from.

The formula

For a starting principal P at annual rate r compounded n times per year over t years, with periodic end-of-period payment PMT:

A = P(1 + r/n)^(nt) + PMT × [(1 + r/n)^(nt) − 1] / (r/n)

The first term is your principal compounding by itself; the second is the future value of the contribution stream. Internally this tool walks year by year (12 sub-steps per year) so it stays accurate even when contribution frequency differs from compounding frequency.

Common gotchas

Constant-rate assumption. The math assumes the rate doesn't change. Real markets don't behave that way — they zig-zag. A 30-year "expected 7%" can deliver anywhere from 4% to 10% averaged over realistic periods. Treat the number as a ballpark.
Inflation isn't subtracted. A balance of $1,000,000 in 30 years buys roughly what $400,000 buys today (at 3% inflation). Use the retirement-projection tool for inflation-adjusted figures.
Taxes aren't subtracted. Tax-sheltered accounts (ISA, Roth IRA, 401k, pension wrappers) compound gross; taxable brokerage accounts compound net of dividend tax. Difference compounds.
Contribution timing matters slightly. This tool assumes end-of-period contributions; beginning-of-period contributions earn one extra period of interest each, ~1× nominal-rate more across the life.
Fees are silent killers. A 1% expense ratio over 30 years drains roughly 20% of your final balance. Subtract fund/platform fees from your expected return input.

Expert notes

The Rule of 72 is a useful sanity check. Years to double = 72 ÷ rate. At 8% real return, money doubles every 9 years. At 4%, every 18. The rule is accurate within ~1 year for rates between 2% and 12%. Memorise it and you can verify any compound interest calculator's output mentally — if the doubling pattern doesn't fit, the inputs are wrong.
Real returns are what actually matter. Most calculators (including this one) compound nominal returns. To project actual buying power, subtract your assumed inflation rate from the return. A 7% nominal return at 2.5% inflation is a 4.5% real return — the "real" curve is what your future self will spend. Showing your spouse "you'll have €1M at 65" is honest in nominal terms, but it buys what €470K buys today after 30 years of typical inflation.
Sequence-of-returns risk applies in withdrawal phase. Compound math assumes a smooth average return, but real markets bounce. A 30-year accumulator who happens to start retirement just before a 30% market drop can end up with materially less than the calculator suggests — because they're now withdrawing from a depressed balance. The defence is bond/cash buffer in the first 5-10 years of retirement, not better return assumptions during accumulation.
The "tax wrapper" multiplier is huge over decades. Same monthly contribution, same return, taxable brokerage vs Roth IRA / ISA / PEA: the wrapper saves the dividend/capital-gains tax on every compounded year. Over 30 years, that's typically 15-25% of the final balance back in your pocket. If your country has tax-advantaged retirement accounts with unused contribution room, that's the highest-leverage move in personal finance — see our compound interest article for the math.
The 4% rule is a starting point, not a destination. "Withdraw 4% of your starting nest egg, adjusted annually for inflation, and you have a high probability of not running out over 30 years" — the Trinity Study finding. It's a heuristic from US data, not gospel. Real planning factors in: variable spending (most retirees naturally adjust), longer time horizons (40+ years for early retirees), Social Security / state pension floors, and home equity. Use 4% as a rough sanity-check on your nest-egg target, not as a withdrawal recipe.

Pairs with

retirement-projection — inflation-adjusted view + withdrawal phase.
mortgage-refi-comparison — compare investing the savings vs. paying down a mortgage.
currency-converter — if you're tracking a multi-currency portfolio.