Everyday calculators
The small calculations that come up constantly — split a bill, count the days, convert the units.
- Tip CalculatorWork out the tip, the total, and what each person owes. Enter the bill, a tip percentage, and party size, and optionally round every share up.
- Age CalculatorFind your exact age in years, months and days from any birth date, plus total days, weeks and months lived and a countdown to your next birthday.
- Date Difference CalculatorCount the days, weeks, business days, months and years between two dates, with an option to include the end date in the total.
- Unit ConverterConvert length, weight, and temperature units — metres to miles, kilograms to pounds, Celsius to Fahrenheit — with the exact factors shown.
- Fuel Cost CalculatorWork out what fuel a trip will cost from distance, efficiency and pump price. Handles km or miles and L/100km, km/L or mpg, plus the round trip.
- Discount CalculatorWork out the sale price after a percentage discount, how much you save, and what sales tax adds. Handles tax charged before or after the discount.
- Electricity Cost CalculatorWork out what any appliance costs to run. Enter its wattage, hours used per day, and your rate per kWh to see the daily, monthly, and yearly cost.
- Cooking Measurement ConverterConvert cups, tablespoons, teaspoons, fluid ounces, millilitres, litres, pints and quarts. Enter one amount and read every other kitchen measure.
- GPA CalculatorWork out your term and cumulative GPA from a list of courses and credit hours, on either the standard 4.0 scale or a weighted 5.0 AP and honours scale.
- Paint CalculatorWork out how much paint a room needs from its size, coats and coverage. Deducts doors and windows, rounds tins up, and totals what you will pay.
- Business Days CalculatorCount working days between two dates, or add business days to a date. Skips weekends and US federal holidays, including their observed days.
- Concrete CalculatorWork out concrete for a slab, footing, column or post holes in cubic yards and cubic metres, then how many 40, 60 or 80 lb bags to buy.
- Tile CalculatorWork out how many tiles a floor or wall needs by laying them out row by row, not by dividing area — plus boxes to buy, waste allowance and total cost.
- Grade CalculatorFind the score you need on your final exam to hit a target grade, plus your current weighted grade and the best and worst grades still possible.
- Time Zone ConverterConvert a date and time between two time zones. Shows the local time, whether it lands on the next or previous day, both UTC offsets and the gap.
- Ohm's Law CalculatorEnter any two of voltage, current, resistance and power and get the other two, with the formulas used shown. Handles short and open circuits properly.
- Time Card CalculatorAdd up a week of clock-in and clock-out times, subtract unpaid breaks, split regular from overtime hours, and multiply by your hourly rate.
Small sums that are surprisingly easy to get wrong
The calculations here are individually trivial and collectively the ones people most often botch, usually because a detail is hiding in the definition rather than the arithmetic. Whether a tip goes on the pre-tax or post-tax total changes the answer. Whether a date range counts both endpoints changes the answer. Whether a discount stacks multiplicatively or additively changes the answer, and stacking two 20 percent discounts gives 36 percent off rather than 40.
Each page states which convention it uses rather than leaving you to infer it from the output. Where more than one convention is genuinely in use, the choice is yours to make rather than one made silently on your behalf.
Dates and units, which hide most of the traps
Date arithmetic looks simple and is not. Leap years are not every four years — a century is a leap year only when divisible by 400, so 1900 was not and 2000 was. Months have no fixed length, so adding one month to 31 January has no single correct answer. Time zones shift by political decision, and daylight saving means some local days contain 23 or 25 hours.
Unit conversion carries its own traps. A US gallon is not an imperial gallon, a US pint is 16 fluid ounces while an imperial pint is 20, and a cup differs between US customary, US legal and metric definitions. Where a unit is ambiguous, which one is meant is stated rather than assumed.
Frequently asked questions
Should a tip be calculated before or after tax?
Convention in the United States is to tip on the pre-tax subtotal, since tax is not part of the service being tipped for. In practice many people tip on the total because it is the larger, more visible number, and on a typical bill the difference is small — around a dollar on a 60 dollar meal at 8 percent tax and a 20 percent tip. There is no rule, only a convention, so the calculator lets you choose which base to use.
Do two stacked discounts of 20 percent make 40 percent off?
No. They apply in sequence, each to the reduced price, so 100 becomes 80 and then 64 — a total of 36 percent off, not 40. Discounts multiply rather than add: 0.8 times 0.8 is 0.64. The gap grows with the size of the discounts, so two 50 percent reductions leave 25 percent of the original price rather than nothing at all.
How many business days are between two dates?
Business days exclude weekends, and usually public holidays, so the count depends on which holiday calendar applies. A span of 14 calendar days contains 10 business days when it starts on a Monday, and the answer shifts by a day or two depending on the starting weekday. Whether both endpoints are counted matters too, which is why the convention used is stated on the page rather than left implicit.
Do the date calculators handle leap years correctly?
Yes, including the century rule that catches most implementations out. A year divisible by 4 is a leap year, except that a year divisible by 100 is not, unless it is also divisible by 400. So 1900 had no 29 February and 2000 did. Ages and date spans computed here account for this, which is why an age across a leap day resolves correctly rather than drifting by a day.