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Type a question the way you would say it, or pick a calculator. Every answer shows the formula, your numbers in it, and each step.
- 22calculators, each with its method on the page
- 6subjects, from home projects to school grades
- 66worked examples, each re-run against the calculator
- 0sign-ups. Answers are calculated in your browser
Start from a real question
Each one opens the calculator with these numbers already in, so you can change one and see what moves.
- Percentage CalculatorWhat is 15% of 200?15% of 200 is 30.
- Flooring Calculator12 × 10 room, 10% waste, 22.5 area per boxBuy 6 boxes; estimated surplus after the waste-adjusted requirement is 3 area units.
- Date Difference Calculator1 January to 18 October 2026290 days — 41 weeks and 3 days, or 9 months and 17 days.
- Recipe Scaling CalculatorA recipe for 4, cooked for 6Multiply everything by 1.5: 375 g flour, 3 eggs, 450 ml milk.
- Grade CalculatorHomework, midterm and finalThe weighted grade is 84.3.
- Fuel Cost Calculator300 miles at 30 mpg, fuel at 3.50 a gallonThe trip costs 35.00 in fuel.
- Aspect Ratio CalculatorFind height for a 16:9 frame that is 1920 pixels wide1920 × 1080 preserves a 16:9 aspect ratio.
- Percentage Change CalculatorA price rising from 80 to 100A 25% increase.
- Discount Calculator25% off a price of 80The sale price is 60; you save 20.
- Tip Calculator84.50 with an 18% tip, split three waysTip 15.21, total 99.71; each pays 33.24 (three shares make 99.72).
Calculators by subject
22 calculators, each with worked examples, common mistakes and the method behind it.
Math
7 calculatorsPercentages, percentage change, averages, discounts, tips, unit prices and ratios — solve for the value you are missing and see the formula and every step.
- Percentage CalculatorA percent of a number, a percentage rate, or the original value.
- Aspect Ratio CalculatorThe missing width or height for a ratio, or the simplest form of a size.
- Percentage Change CalculatorPercentage increase or decrease from an old value to a new one.
- Discount CalculatorSale price and saving from a percent-off discount, even when two stack.
- Tip CalculatorThe tip on a bill, the total, and each person’s share.
- Unit Price CalculatorWhich of two package sizes is cheaper per unit, and by how much.
- Average CalculatorThe mean and median of a list of numbers, and how spread out they are.
Home & garden
7 calculatorsMaterial estimates for home and garden projects, with measurements, waste allowance and product coverage kept as inputs you control.
- Flooring CalculatorWhole boxes of flooring for a room, with your waste allowance.
- Paint CalculatorGallons or litres of wall paint for a room, and how many tins to buy.
- Tile CalculatorTiles and boxes for a floor or wall, counting the grout joint and cuts.
- Concrete CalculatorConcrete for a slab in cubic yards or metres, and how many bags.
- Brick CalculatorBricks for a single-layer wall, allowing for mortar joints and openings.
- Mulch CalculatorMulch or bark for a garden bed, in cubic yards or metres and in bags.
- Gravel CalculatorGravel for a path or drive, by volume and by weight in tons or tonnes.
Time & date
2 calculatorsDays between dates and exact ages, counted on the real calendar with leap years and month lengths handled for you.
Cooking
1 calculatorKitchen arithmetic: scale a recipe to the number of people you are cooking for, with every amount worked out from one factor.
Education
2 calculatorsWeighted grades and the score you need on a final, worked out from your marks and the weight of each assessment.
Automotive
3 calculatorsFuel for a trip, how long the drive takes, and your car’s real fuel economy, in miles or kilometres.
What every answer comes with
The same four things on every calculator, so you never have to take a number on trust.
The formula, named and versioned
Every calculator prints the equation it uses, with a version number, so a change to the method is visible.
Your numbers, substituted
The same equation is shown again with your values in it, followed by every step to the answer.
Assumptions you control
Anything that changes a result — a waste allowance, the coverage on a tin — is an input you set, restated beside the answer.
Calculated on your device
What you type is calculated in your browser. It is not sent anywhere or saved, and there is nothing to sign up for.
- Walls of 12, 8, 12 and 8 ft add up to 40 ft; 40 × 8 ft = 320 sq ft.
- Take off one door and one window, 40 sq ft together: 320 − 40 = 280 sq ft.
- Two coats: 280 × 2 = 560 sq ft. At 350 sq ft per gallon, 560 ÷ 350 = 1.6 gallons.
- 1.6 gallons in 1-gallon tins rounds up to 2 tins.
Buy 2 one-gallon tins; about 0.4 gallon is left for touch-ups.
Open this example in the paint calculator →Edited by Kadir Özbek
Each calculator’s formula, worked examples and wording are reviewed before it is published, and reported errors are checked against the formula and its tests. Read the editorial policy.