The SIP Calculator answers one question: if you contribute a fixed amount every month for a set number of years at an expected annual return, what is the pot worth at the end? All the arithmetic runs in your browser from the code bundled with the page, and the three inputs plus the result never leave your machine.
Three inputs, one button
There are exactly three fields. Monthly Investment takes the amount you put in each month (placeholder 1,000). Expected Annual Return takes a percentage, not a decimal, and its label appends the percent sign (placeholder 12). Years takes the horizon in whole years (placeholder 10). Nothing is computed as you type: the result and the table appear only after you press Calculate. Two guards decide whether anything happens at all. If the monthly amount is empty, invalid, zero, or negative, the calculator shows nothing. If the years field is empty or does not parse, the horizon defaults to 10, and a negative years value is rejected with no result. A fractional years value such as 5.5 is truncated to 5 before any math runs.
How the future value is computed
The annual return is divided by 100 and then by 12 to get a monthly rate i, and the horizon becomes n = years times 12 months. When i is not exactly zero, the future value is the annuity-due formula: monthly times ((1 + i) to the n minus 1) divided by i, then multiplied by (1 + i). The final factor exists because the model assumes each contribution is made at the start of its month, so even the first one compounds for a full month. When i is exactly zero, a flat branch returns monthly times n, which is just the sum of the contributions. A negative return is not special-cased: the same annuity formula runs with a negative i, so the future value can end up below the total invested.
The four result rows and their currency
The result panel lists four rows in a fixed order: Total Invested (monthly times n), Total Interest (future value minus invested), Total Value, and Return %, which is total interest over total invested times 100. Every amount is formatted with two decimals and comma grouping. The currency symbol depends on the locale: the Chinese (zh-CN) and Japanese interfaces prefix amounts with the yen, every other locale with the dollar sign. Total Interest, Total Value, and Return % are colored in the gain color when positive and the loss color when negative, so a losing scenario is visible at a glance. Total Invested stays neutral.
The year-by-year table
Below the result panel, one table row appears per year, labeled Y1 through Yn. Each row is the cumulative position after that year has delivered its 12 contributions: invested is monthly times the month count, value is the same annuity formula evaluated at that month count, and interest is the difference. Table cells are always prefixed with a dollar sign and rounded to whole dollars, even on locales where the result panel uses the yen. The interest column is colored by sign, so rows where the pot shrank show a negative interest figure in the loss color.
What you can copy out
The copy button in the result panel writes four lines to the clipboard in result order, one key and value per line: totalInvested, totalInterest, totalValue, and returnPercent. In the 500 per month, 20 year, 10 percent scenario it copies totalInvested: $120,000.00 and totalValue: $382,848.45.
What is not here, and where to go next
The calculator models one fixed monthly contribution: it has no mid-plan amount changes, no expense ratios or fees, no tax on the gains, and no quarterly or weekly frequency. If your money sits as a single lump sum instead of monthly contributions, the Compound Interest Calculator is the matching tool. If the instrument pays dividends that you reinvest into more shares, the DRIP Calculator compounds those payouts for you. If you already know the start and end values and want the implied rate, the CAGR Calculator works in the reverse direction. And for a step-by-step companion, read the compound interest guide.