Compound interest calculator — user guide
How to open the calculator
The calculator is one of the iBro tools. There are two ways in: find it in the list of all tools, or — if you have used it before — open it from your favourites.
The list of tools
On a phone or tablet, tap the "Apps" icon at the top of the page: it opens the list of every tool, sorted into categories. On a computer the "All tools" item in the top menu does the same. The category you want is "Calculators", and the tool itself is called "Compound interest".
The favourites star
There is a star next to the tool's name. Tap it and the tool goes into your favourites, where it is shown first in the list. If you come here often, that turns three clicks into one.
What is where on the screen
On the left are the boxes where you type your numbers. On the right is a card with the answer. It recalculates by itself the moment you change something — there is no "calculate" button to press. On a wide screen the answer stays in view even while you scroll the boxes down. The choice between "Amount" and "Time" sits as two buttons above the boxes on a phone, and in the menu on the left on a computer.
Amount
This mode answers "how much money will I have after a while". Below is every box in order, from the top down. The boxes in the examples are real ones — you can play with them. They do not touch your own calculation; they are only a sample.
Starting amount
How much money you put in at the very beginning. This is what the interest starts growing on. You can leave it at zero — then everything comes from your top-ups and the interest on them.
Term
Two boxes side by side: a number, and what that number means — days, months or years. That is the stretch of time the calculation covers. The longer it is, the harder compound interest works: near the end, interest is paid on the interest that piled up earlier, not just on what you put in.
Start date
The day the count begins. Today by default. This date does not change the final amount — what matters is how long the term is, not when it starts. It only changes the date under the answer: the day you actually get the money.
Rate, % per year
The very percentage the bank writes in the contract: what it pays over a year. The calculator divides it by the number of payouts in a year (see the next box) and adds it on in pieces.
How often interest is added
How often the interest joins your money: every day, once a month, once every three months, or once a year. The more often, the more you end up with at the same rate — because the interest starts earning its own interest sooner.
Here it is in numbers. A rate of 20% a year, added monthly. That means every month you get 20 ÷ 12 ≈ 1.667% — twelve times in a row, each time on a slightly bigger sum. Over the year that comes to about 21.9%, not 20%. Those extra 1.9% are compound interest. In their adverts, banks call that number the "effective rate".
Regular top-ups
The checkbox turns on adding money as you go: you put in the same amount at even intervals. While it is off, the extra boxes are hidden and only the starting amount counts.
The top-up amount is how much you add each time. Zero here is the same as leaving the box unticked: there will be no top-ups in the calculation.
How often to top up is set by the slider. It has three ready-made stops: every day, once a week, once a month. But you can also set any number of days of your own — drag the slider or type the number. "Once a month" has a subtlety: it does not mean "every 30 days" but the same date of the next month. That is how a real payment behaves, and over a year it does not creep forward by several days.
The answer card
To the right of the boxes is the "Final amount" — how much money there will be at the end of the term. The calculator works it out the way a piggy bank actually grows: step by step, period after period.
The line under the sum — "You will earn … in interest" — is the clean gain: everything you put in yourself (the starting amount and all top-ups) has been subtracted from the total. What is left is exactly what the interest brought.
Lower still, under the thin line, is a date: the day the term ends, counting from the start date you chose.
The "Save calculation" button
It remembers all your numbers together with the answer and puts them on the "Saved data" page. You can come back later without typing anything again. The button has three looks: normal, saying "Save calculation"; a spinning ring while it saves; and then a tick with the word "Saved", which turns back into the normal label after a couple of seconds.
Saving works only when you are signed in: the calculations live on our server, not in your browser. If you are not signed in, the button is replaced by an invitation to sign in or register.
The "See it on a chart" link
It sits under the save button and opens the same calculation on the "Visualisation" page: the same numbers, but instead of a single figure you get a growth line and a table. That page is described below.
Time
Here the question is turned around: you know how much you want to save, and you are asking how long it will take. Almost every box is the same as in "Amount" (explained above). There are only two differences.
Starting amount
Target amount
How much money you want to end up with. This box stands in place of "Term": the time is what the calculator works out for you, from the starting amount, the rate and the top-ups.
Annual rate, %
Regular contribution
The rest of the boxes — starting amount, rate, how often interest is added, regular top-ups — work exactly as they do in "Amount". See their descriptions in the section above.
The answer card
Instead of a sum, here you get time: "It will take …" in words, say "2 years 3 months". The line below repeats the sum you wanted to reach. And the date under the line shows roughly the day you get there.
If the starting amount is already the target or more, it says "Target already reached" instead of a time. There is nothing to work out — the money is there.
If the rate is zero and there are no top-ups, the money will never grow on its own. In that case the calculator honestly says "This rate will never get there" rather than inventing a number.
Saved data
Every calculation you saved sits here as its own card. The card shows the name, the main numbers (starting amount, rate, how often interest is added, and either the term or the target) and buttons for what you can do with it.
- Open
Puts the saved numbers back into the calculator. Every box fills itself in with the values from the moment you saved — and you can change anything from there.
- Open on the chart
The same thing, but straight onto the "Visualisation" page: the calculation is restored and drawn as a line right away.
- Rename
The little pencil by the name lets you type your own name right on the card. Until you do, the date you saved it is shown instead.
- Delete
The bin removes one card. It will not ask "are you sure?" — the calculation goes at once.
- Clear all
The button above the list wipes every saved calculation at once. Useful when you want to start the list from scratch.
Calculations are kept on the server and tied to your account. They will not disappear when you sign out and back in, and they will be there if you open the site on another phone or computer.
Visualisation
Exactly the same sums as in "Amount", shown differently: a line and a table instead of one number. That way you see not only what you end up with, but how it grew along the way.
The four tiles at the top
"Final amount" and "Earned in interest" are the same figures as in "Amount". "Paid in" is how much you put in yourself: the starting amount plus every top-up, without interest. That tile lights up in colour only when top-ups are switched on — without them, "paid in" would nearly always be the same as the starting amount. "Period" is the whole term in words, so the three sums above it have something to be measured against.
The terms of the calculation
This is a card holding all the boxes, right there on the chart page: starting amount, rate, term, how often interest is added, and top-ups. They are the same boxes as in "Amount" (described above). Change a number and the chart and the table underneath redraw at once.
One difference from "Amount": how often to top up is chosen from a list of three — every day, once a week, once a month — rather than with a slider. If you had earlier set your own number of days with the slider, it appears in the list as an extra line the first time. The calculation does not change; you can just see what is in force.
The card's heading is a button with an arrow. Press it and the card folds away; press again and it opens. Your numbers stay where they were. Handy to fold it once the terms are set and you only want to look at the chart.
The "All calculation settings" link under the boxes takes you back to "Amount" with the same numbers. Every box is available there, including the choice of days, months or years, which this page does not have.
The growth chart
The line shows how much money is in the account on each day. Along the bottom is time, from the start of the term to the end; up the left side are the sums. When there are two lines, the gap between them is the interest you have earned: the wider it grows towards the end, the more visibly compound interest is working.
The labels above the chart appear only when there are two lines — that is, when top-ups are switched on. With one line there is nothing to label: it is named in the card's heading.
Hover the mouse over the chart (on a phone, touch it with a finger). A vertical line appears, along with a small box showing the exact date and the sum at that point. If top-ups are on, it also shows how much of your own money you had put in by that day.
The table by period
The same calculation, line by line: the date, the top-up for that period (this column only appears if top-ups are on), the interest added, and how much money there was at the end of the period. The rows are split up by year — the year stands as its own label, so you do not have to read every date in a long table.
The first year has no label: it is first anyway, and you can see which year it is in the very first date. A year label marks the move into the next year; it does not open a list.
The down arrow by the table heading saves the whole schedule as a PDF — all of it, every period to the end of the term, not just what fitted on the screen. If the term is long and interest is added daily, the file takes a moment to build: the button blinks while it works, so you can see it is busy rather than broken.
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This is a forecast, not a promise
The calculator assumes the rate never changes. In real life a bank can change its terms, part of the gain goes to tax, and taking the money out early usually means less interest.