How the math works

Compound interest treats the rate as nominal and monthly. A 7% annual rate is 7% ÷ 12 each month. A lump sum compounds for every month. A deposit added at the end of the month does not earn interest in the month it arrives. If the rate is zero, the balance is the starting amount plus the deposits.

SIP uses the same monthly rate, but each deposit arrives at the start of the month, so it compounds for that month too. The same inputs therefore finish a little higher than compound interest. A step-up SIP uses that same start-of-month deposit, then raises the amount once a year. Inflation on that page does not change the balance. It divides the ending balance by the price rise, so the second figure is today’s purchasing power. SWP takes a withdrawal at the start of the month before that month’s growth. If the balance cannot cover a withdrawal, the page stops and says the corpus ran out.

The SIP page can also take the deposit yearly, weekly, or daily. Those use the same beginning-of-period formula, with the annual rate divided by 1, 52, or 365. A lump sum and a fixed deposit are one amount compounded yearly, quarterly, monthly, or daily. A recurring deposit is a monthly installment; the quarterly bank rate is converted to the monthly rate that compounds back to that quarter.

A loan payment is the reducing-balance EMI: interest each month is the remaining balance times the monthly rate, and the rest of the payment reduces the principal. The last payment clears whatever is left. The savings goal calculator runs the end-of-month compound formula backward, either for the number of months or for the deposit.

A tip on one amount is the percentage calculator plus an equal split. Split the bill is the longer form: tax is a percent of the food, rounded to the cent first, and the tip is then a percent of the food or of food plus tax. When the total does not divide evenly, the page also shows the share if every person rounds up to the next cent.

These are civil dates on the Gregorian calendar. The time of day, the time zone, and daylight saving never change a result.

“Days from today” treats the start date as day zero and adds a whole number of calendar days. A negative number counts backward. Leap days are included when the span crosses 29 February.

“Days between” reports the difference (end minus start) and, separately, the count that includes both dates. Weekdays are Monday to Friday. Working days use that weekday count and then subtract a holiday number you type. There is no holiday calendar.

Age is whole years, then whole months, then the remaining days. A birthday on 29 February is observed on 28 February in a non-leap year. That is a convention this site states, not a legal rule.

Each calculator links to the two or three tools that share a formula or a decision — SIP with compound interest, a loan payment with a savings goal, a tip with split the bill. The full list lives on the calculators page, and the same groups are in the footer.

A new variant does not get its own URL when it is a mode of a tool that already exists. Percent increase and percent decrease share one page. “How long” and “how much per month” share the savings goal. Split the bill is its own page because tax and the tip base are not a mode of the tip calculator.