Why these five?

They are the calculations people search for every single day: how much to tip, what their grades average out to, what a percentage really means in dollars, how old someone is exactly, and what compound growth does to savings. Each tool here is built to answer in one screen, with the inputs pre-filled with a worked example so you can see how it behaves before typing anything.

Private by design

Every calculation happens locally in your browser with JavaScript. Your numbers never leave your device — there is no server storing them, no account, and no tracking of the values you enter.

How it works

The tip is bill × tip%, the total is bill + tip, and each person's share is the total divided by the number of people. In the US, 15–20% is the customary range for table service at a restaurant; 18% is pre-filled because it is the most common default on card readers.

Tip is normally calculated on the pre-tax subtotal. If your bill total includes tax and you want to tip on the subtotal only, subtract the tax first, then enter that amount.

The grading scale

A+ and A = 4.0, A− = 3.7, B+ = 3.3, B = 3.0, B− = 2.7, C+ = 2.3, C = 2.0, C− = 1.7, D+ = 1.3, D = 1.0, F = 0.0. GPA = Σ (grade points × credits) ÷ Σ credits.

Schools vary: some cap A+ at 4.0, some award 4.3, and weighted high-school GPAs add points for honors and AP classes. This calculator uses the standard unweighted 4.0 college scale — check your institution's handbook if your transcript uses a different one.

The formulas

X% of Y = (X ÷ 100) × Y · X as % of Y = (X ÷ Y) × 100 · % change = ((B − A) ÷ A) × 100. A positive change is an increase, a negative one a decrease. Handy checks: a discount is "X% of Y" subtracted from the price; a grade is "X as % of Y"; investment returns are "% change".

How exact age is counted

The calculator subtracts the birth date from today the way people count birthdays: full years first, then full months, then the remaining days. "Total days lived" counts every calendar day since birth, leap days included — most people are surprised how quickly it passes 10,000.

The formula

Future value = P(1 + r/n)nt + PMT × [((1 + r/n)nt − 1) ÷ (r/n)], where P is the starting amount, r the annual rate, n how often interest compounds per year, t the years, and PMT the deposit per compounding period. The second half is why starting early beats depositing more: time is the exponent.