Everyday Math Calculators

Which calculator do you need?

Dining out with friends? Open the Tip Calculator: enter the bill, pick a tip percent, set the number of people, and everyone sees their share instantly — no more passing the phone around doing mental math. Shopping a sale, checking a grade, or figuring a raise? The Percentage Calculator is the general-purpose tool: it answers 'what is X% of Y', 'X is what percent of Y', and 'what is the percent change from A to B'.

The underlying formulas are simple. Tip amount equals the bill times the tip percent; each person's share equals the bill plus tip, divided by the number of people. A percent-of calculation is (part divided by whole) times 100, and percent change is (new minus old) divided by old, times 100. Both calculators show the math so you can trust the answer.

Tipping basics

In the United States, 15 to 20 percent of the pre-tax bill is the standard tip for full-service restaurants — 15% for adequate service, 20% or more for great service. Tip on the pre-tax subtotal, since tax is not part of the service you received. For delivery, 10 to 15% is customary; for takeout, a small tip is appreciated but not expected. When a bill is split, decide up front whether everyone pays an equal share or pays for what they ordered — the Tip Calculator handles the equal-share case in one step.

Everyday uses for percentages

Percentages show up far beyond the classroom. Sale prices ('25% off $80'), sales tax added at checkout, grade calculations, pay raises, investment returns, and nutrition labels ('20% of daily value') all run on the same few formulas. Getting comfortable with percent change is especially useful: it is how you compare a raise against inflation, a discount against the original price, or this month's spending against last month's.

Mental math shortcuts worth knowing

Percentages get much easier with a few mental shortcuts. Ten percent of anything is just the decimal point moved one place left: 10% of $64 is $6.40. Fifteen percent — the baseline restaurant tip — is 10% plus half again: $6.40 + $3.20 = $9.60 on that $64 bill. Twenty percent is simply double the 10% figure, and 25% is one quarter of the total. For percent change, anchor on the original number: if your $50 weekly grocery run becomes $65, the $15 difference against the original $50 is 30%. These shortcuts will not replace a calculator for exact splits, but they give you a fast sanity check — if the tool says something wildly different from your mental estimate, recheck your inputs.

Tips for accurate results

Double-check the numbers you type — a misplaced decimal turns 15% into 1.5%. When splitting a bill, agree on the approach before calculating: equal shares are simple, itemized shares are fairer when orders differ wildly. And remember that the tip calculator's suggestions are customs, not rules — adjust for your country and the service you received.