Compound Interest and Investment Growth Calculator

What this calculator does

Compound growth is simple in principle and slippery in practice, because real plans have moving parts: money going in monthly while interest compounds quarterly, a fee skimming the balance every year, a withdrawal phase after the saving phase, and inflation quietly repricing the finish line. This calculator iterates the whole plan period by period, exactly as a statement would, and reports the pieces separately: what you put in, what you took out, what growth added, what fees took, and what the ending balance is worth in today’s money. The year-by-year table carries a bar for each year so the acceleration that compounding is famous for is visible, not just asserted, and the whole table downloads as a text file for your own records.

How to use it

1. The rate is a scenario, and the page will keep reminding you. For a savings account, use its actual rate. For investments, the honest use of this page is to run several rates (a cautious one, a middle one, an optimistic one) and read the spread of outcomes as the answer. A single projection at a single assumed return is a story; three projections are a plan.
2. Contribution timing is real money. “Start of each period” and “End of each period” differ by one period’s growth on every contribution, forever. Monthly savers paid at month-end are end-of-period; a deposit made today is start-of-period. Over the one-year test in the examples the difference is $12.68 on $1,200 contributed; over 30 years it is not small, and tools that hide the choice are hiding a real term of the deal.
3. Match contribution frequency to your life, not to the compounding. Contributions can be monthly while compounding is annual, quarterly, daily or continuous: the calculator converts them into each compounding period correctly. What you must not do is enter a monthly amount and select yearly contributions, or vice versa; the labels are plain and the totals line shows what was actually put in, so a mismatch is easy to spot.
4. Fees are charged on the balance, annually, in the open. A 1% annual fee sounds like a rounding error and behaves like a tax on the whole pile every year: on the worked example it consumes $620.09 over five years and leaves the balance $720.12 lower than the same plan without it. If a product quotes an expense ratio or an annual management charge, that is the number for this field.
5. Withdrawals have a start year because plans have phases. Set “withdrawals start after year” to model accumulation first and drawdown later. If withdrawals outrun growth, the page says the year the balance emptied rather than projecting a fiction below zero.
6. The inflation line answers the only question that matters. The nominal ending balance is a future number of dollars; the real value is that balance translated back into today’s purchasing power. At 2.5% inflation over ten years, the flagship example’s $18,193.97 is $14,213.10 of today’s money. Plans that skip this line feel richer than they are.

How it is calculated

The balance is iterated period by period: each period, contributions land (at the start or end of the period, as chosen), interest accrues at the annual rate divided by the periods per year (annually 1, semiannually 2, quarterly 4, monthly 12, weekly 52, daily 365; continuous compounding grows each monthly step by e to the power of rate/12), withdrawals come out at the end of the period once their start year has passed, and at each year end the annual fee percentage is deducted from the balance. Contributions scale between frequencies by the ratio of contributions per year to periods per year, and grow by the annual increase percentage once per year. Effective annual rate = (1 + rate/periods) to the power of periods, minus 1 (for continuous compounding, e to the rate, minus 1). Real value = ending balance / (1 + inflation) to the power of years. Growth reported is the sum of the interest actually credited each period, so the identity always holds: end = start + contributions – withdrawals + growth – fees.

Worked examples

The flagship: $10,000 at 6%, compounded monthly, ten years, nothing added: $18,193.97, of which $8,193.97 is growth. The effective annual rate is 6.17%, the monthly compounding having quietly added a sixth of a point. In today’s money at 2.5% inflation: $14,213.10.

Contributions against annual compounding: $5,000 starting, $200 a month going in at period end, 8% compounded annually, five years: contributions total $12,000, growth adds $4,426.48, ending balance $21,426.48. Note the contributions outgrew the growth in five years; compounding needs time before it takes the lead, and the table shows the year the lead changes if you run it longer.

Continuous compounding: the same $10,000 at 6% for ten years, compounded continuously: $18,221.19. The gap over monthly compounding is $27.22 across a decade, which is the honest size of compounding-frequency bragging: frequency matters, and it matters much less than rate, time and contributions.

Timing, priced: $100 a month for one year at 12% compounded monthly, nothing at the start: end-of-period contributions finish at $1,268.25; start-of-period contributions finish at $1,280.93. The $12.68 is one extra month of growth on each deposit, compounding quietly.

The fee, priced: $10,000 at 8% annually for five years reaches $14,693.28 clean, and $13,973.16 with a 1% annual fee, which took $620.09 in fees and cost a further $100.03 in growth those fees would have earned. Fees are the only negative compounding in most portfolios, and they compound just as reliably.

Running dry: $1,000 with $100 a month withdrawn and no growth empties during the first year, and the page says so in a warning line instead of showing a negative balance with a straight face.

Reading your result

Read the “You put in” line against the ending balance before anything else: the distance between them, minus fees, is what compounding actually did for you, and early in any plan it is small, because compounding’s contributions arrive late and then all at once. Read the year table’s growth column as a story with a known shape: the year growth exceeds annual contributions is the year the plan starts funding itself, and most honest plans cross it somewhere in the second decade. And read the real-value line as the verdict: a plan whose nominal balance doubles while its real value rises by half has done half as well as it feels, and knowing that in advance is the difference between a target and a mirage.

Limitations

• A constant rate is the model’s spine and its fiction. Real returns arrive in an order, and the order matters when money is moving in and out: poor early years with contributions still arriving are survivable; poor early years in withdrawal can be fatal to a plan that the same average return, smoothed, says is safe. This page shows the smoothed path, labelled as a scenario, and the withdrawal warning only catches the crudest version of the risk.
• No tax, no product terms. Interest tax, capital gains treatment, early-withdrawal penalties, contribution limits, employer matches and account rules all change real outcomes and none of them are modelled here. The Savings and Retirement calculators on this site handle their own corners of that world.
• Rates are nominal and user-supplied. Nothing here knows what any account pays today; the rate you type is the rate that compounds. Check product rates at the source before treating an output as a plan.
• Daily compounding uses a 365-day year. Leap years are ignored, as they are in most quoted compounding conventions; across decades the difference is within the noise of the rate assumption itself.

Frequently asked questions

How does compound interest actually work?

Each period, the interest is calculated on everything that is there, including the interest from earlier periods, so the balance grows by a percentage of a growing number. That is the whole mechanism, and its consequence is the shape in the year-table bars: slow, then faster, then startling. On the flagship example, year one adds about $617 of growth; year ten adds about $1,057, from the same 6% rate, because the base it acts on has nearly doubled.

What is the effective annual rate, and why is it higher than my rate?

Compounding more often than yearly means interest starts earning its own interest inside the year, so the true yearly growth exceeds the quoted nominal rate: 6% compounded monthly is 6.17% effective. The gap is small at savings rates and grows with the rate itself. Comparing two products on nominal rates with different compounding is comparing unlike with unlike; the effective rate is the common ruler, which is why it is printed on every result here.

How much do regular contributions change things?

Early on, they are the plan: in the five-year example, contributions supplied $12,000 of the ending balance against growth’s $4,426.48, because compounding had little base and little time. Left running, the same plan’s growth column eventually passes its contribution column and never looks back. The practical reading is that the contribution amount and the years are levers you control entirely, while the rate is the lever you mostly do not, which is an argument for setting the first two generously and the third one honestly.

Is continuous compounding worth chasing?

No, and the worked example prices the chase: $27.22 over ten years on $10,000 at 6%, the difference between daily-and-beyond frequencies and monthly. It exists on this page because the mathematics is classical and some textbooks and products quote it, not because any saver should choose a product for it. Rate, fees and time swamp it by orders of magnitude.

Related calculators

• All Money and Finance Calculators
• Savings and CD Calculator
• Inflation Calculator
• Retirement Calculator
• CAGR Calculator

Method: period-by-period iteration with explicit contribution timing, frequency conversion and annual fee deduction, exactly as defined on this page; growth, contributions, withdrawals and fees reported as separate totals that reconcile to the ending balance. Returns are user-supplied scenarios, not predictions. Last reviewed: 13 October 2026.