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Compound interest calculator
Set a starting amount, a monthly deposit and a rate of return; the chart redraws as you drag. Each bar is split into the money you put in and the money the interest made, so the year growth starts out-earning your own deposits is something you can see rather than work out.
How to use it
- Enter the starting amount, what you plan to add each month, the annual return you expect and how many years you will leave it.
- Drag any slider and watch the bars redraw — the point is comparing scenarios, not getting one number.
- Hover, tap or arrow-key across the chart to read any single year; the table underneath has all of them.
- Open “More assumptions” for the compounding basis, contributions that grow with your pay, and an inflation figure — set one and the chart gains a “Today’s money” view that discounts every bar.
The crossover year is the whole story
Compounding is usually explained with a final number, which is the least interesting part of it. The bar chart here is split instead: a grey block for every unit you have deposited, and a violet one for the growth stacked on top. In the early years the violet is a sliver. Then it thickens, and at some point — the page names the year — it becomes taller than everything you have contributed put together. From that moment the account is earning more than you are paying in, and that is the moment the word compounding actually refers to.
Where that crossover falls is far more sensitive to time than to the rate. Adding five years to a twenty-year plan usually moves it more than adding a percentage point to the return, which is worth knowing because one of those two things is under your control and the other is not.
Compounding frequency is not the lever it is sold as
A rate quoted as "6% compounded quarterly" is not 6% a year. Each quarter adds 1.5%, and four of those multiply out to 6.14% — the effective annual rate. Daily compounding on the same nominal 6% gets you 6.18%. That is the entire range: the gap between the worst and best compounding basis at a typical rate is about a fifth of a percentage point, and a headline rate a tenth of a point higher beats all of it.
This tool takes the frequency seriously anyway, because the difference is real and the control lets you see how small it is. The balance is grown by the twelfth root of the effective annual rate each month, so switching the setting changes the answer by exactly the amount the compounding basis is worth and not a unit more.
What a steady rate quietly assumes
Every projection on this page grows the balance by the same percentage every single year. No investment does that. A fund that averages 6% over two decades gets there through years of +21% and years of −14%, and when the bad years arrive changes the outcome even if the average does not — a crash in the year before you stop is a very different event from the same crash in year two. Statisticians call it sequence-of-returns risk, and no single-rate calculator can show it to you.
So read the output as an order of magnitude, not a prediction. It answers "is this plan roughly the right shape?" honestly, and "how much will I have in 2046?" not at all. The inflation field exists for the same reason: a number thirty years out in cash terms flatters itself, and the "in today’s money" column is the one worth reading.
Questions
What formula does this use?
A month-by-month simulation rather than a single closed-form expression, because contributions, a contribution that rises each year, and a compounding frequency that is not monthly do not fit in one tidy formula. Each month the balance is multiplied by the twelfth root of the effective annual rate, and the deposit is added at the start or end of the month depending on the setting you choose.
Does the compounding frequency really matter?
Barely. On a 6% nominal rate, yearly compounding gives 6.00% effective and daily gives 6.18%. It is worth setting correctly for accuracy, but if you are choosing between two accounts, the headline rate and the fees will decide it long before the compounding basis does.
Should I enter the return before or after fees?
After. A fund charging 0.9% a year against one charging 0.1% is an eight-tenths-of-a-point difference compounded for the whole term, which over thirty years is a substantial share of the final pot. Subtract the charge from your expected return before typing it in.
Is the inflation figure deducted from my returns?
No. It drives the "in today’s money" tile, the extra table column and the chart’s “Today’s money” view, which show what the balance would buy at present-day prices. The headline balance stays in future cash terms, because that is the number that will appear on the statement.
Why is my balance in today’s money smaller than what I paid in?
Because only one of the two figures is discounted. "In today’s money" divides the final balance by inflation compounded over the whole term, while "you put in" is the plain sum of deposits made across thirty years, each in the money of its own year. Comparing them is comparing 2059 hryvnia with a mixture of 2026 to 2059 hryvnia, and it makes a growing pot look like a shrinking one. Switch the chart to "Today’s money" and both halves are discounted: what you paid in shrinks too, and the real gain shows. The real return is (1 + return) ÷ (1 + inflation) − 1, and mind which "return" goes in: a 14% rate compounded monthly is 14.93% effective, so against 8% inflation the pot gains 6.4% a year, not the 5.6% the quoted figures suggest. Smaller than the headline either way, but still growth.
Is anything I type stored or sent anywhere?
No. The arithmetic runs in this browser tab, there is no account, and no figure you enter is transmitted or saved. Reload the page and it is gone.
Updated 2026-08-19. Runs fully in your browser — nothing is uploaded.