What this does
It projects a balance forward. You set a starting amount, an annual rate, a term in years, how often interest compounds — annually, quarterly, monthly or daily — and an optional deposit made once per compounding period. It returns the future value, splits it into what you put in versus what the interest added, and draws that split as a bar. The arithmetic happens in your browser, so nothing about your savings is transmitted and the page works offline.
What is the compound interest formula?
For a lump sum it is A = P(1 + r/n)nt — P is the
principal, r the annual rate as a decimal, n the compounding periods per year,
t the years. Deposits are added as an ordinary annuity on top:
PMT × ((1 + r/n)nt − 1) ÷ (r/n), i.e. each deposit
lands at the end of its period and compounds for the periods that
remain. At a rate of zero that term would divide by zero, so it collapses to
deposit × number of periods instead. The period count is
round(years × n), which is what lets you type fractional years.
How do I project my savings?
- Enter the starting amount — zero is fine if you are beginning from nothing.
- Enter the annual rate as a percentage, not a decimal: 6, not 0.06.
- Set the number of years and pick a compounding frequency.
- Enter the recurring deposit. Watch the label — it says each month, each quarter, each year or each day to match the frequency you chose.
What does a worked example look like?
The default setup: $1,000 to start, 6%, 20 years, compounded monthly, $200 a month. Future value $95,718.38, of which $49,000 is your own money and $46,718.38 is interest. Strip out the deposits and the effect is clearer: $1,000 at 5% for 10 years compounds to $1,628.89 yearly, or $1,647.01 monthly. Deposits alone, $100 a month at 6% for 10 years, reach $16,387.93 on $12,000 paid in. Every figure is rounded to two decimals for display only; the maths underneath is not rounded between periods.
Does compounding frequency make much difference?
Less than people expect. Same $1,000 at 5% for 10 years:
| Compounding | Periods | Balance |
|---|---|---|
| Annually | 10 | $1,628.89 |
| Quarterly | 40 | $1,643.62 |
| Monthly | 120 | $1,647.01 |
| Daily | 3,650 | $1,648.66 |
Rate and time move the number far more than frequency does. One caveat: since the deposit is per period, switching frequency to daily also means depositing that amount every day — adjust it, or the totals will jump.
What does this calculator leave out?
Tax, fees, inflation, and any change in the rate. It assumes one constant rate applied evenly for the whole term and deposits that never miss or grow. Real accounts and investments do none of those things reliably, so treat the output as a comparison between scenarios — earlier start versus larger deposit, say — rather than a prediction, and not as financial advice.
Where do the percentages come from?
If you need to work a rate out first — a gain as a percentage, or what one number is of another — use the percentage calculator. For splitting a bill, tip calculator. Currency is displayed with a dollar sign, but the maths is unit-free: read it as whatever currency you entered.