All calculators
43 calculators, grouped by what they are for. Every one of them prints the formula it used, states its assumptions where you can change them, and names the mistake people actually make with that calculation.
Home & construction
Work out what a job needs before you buy it: concrete volume and bags, tiles, flooring, roofing, fencing and mulch — each with the waste allowance shown separately from the net quantity.
- Concrete CalculatorSlab volume, bags to buy, waste allowance.
- Square Footage CalculatorEnter the length and width to get the area in square feet, square yards and square metres.
- Tile CalculatorEnter the area and your tile size to see the tiles needed, the boxes to buy and the spares left over.
- Flooring CalculatorEnter the room area and the coverage printed on the box to see how many boxes to buy.
- Roofing CalculatorEnter the footprint and the pitch to get the true sloped area, in square feet and in squares.
- Fence CalculatorEnter the run and your spacing to see the posts, sections, rails and pickets.
- Mulch CalculatorEnter the bed area and the depth you want to see the volume and the bags to buy.
- Brick CalculatorEnter the wall, the brick and the joint. Openings come off the area, and the order is shown apart from the net count.
- Gravel CalculatorEnter the area and the depth you want when it is finished and rolled. The answer is what to order, not what fits in the hole.
- Paint CalculatorEnter the room and the coverage printed on the tin. Openings come off, coats multiply, and whole tins are what you buy.
Math
Percentages, ratios, fractions, probability and statistics. Every page prints the formula it used and names the mistake people actually make with it.
- Percentage CalculatorEnter a percentage and a number to see the amount, plus the totals with it added and removed.
- Percentage Change CalculatorEnter the starting value and the new value to see how much it changed, in percent.
- Percentage Difference CalculatorEnter two values to compare them without either one being treated as the starting point.
- Fraction to Decimal CalculatorEnter a numerator and a denominator to see the decimal, the percentage and the simplest form.
- Ratio CalculatorEnter two quantities to see the ratio reduced, split as percentages and expressed as 1 to n.
- Rule of Three CalculatorIf A corresponds to B, what corresponds to C? Both proportions are solved at once.
- Reverse Percentage CalculatorYou have the number after the change. This finds the one before it.
- Circle CalculatorEnter a radius. Everything else about the circle follows from it.
- Triangle Area CalculatorTwo ways to the same answer, depending on which measurements you actually have.
- Probability CalculatorEnter the probability of each event to see how they combine.
- Bayes Theorem CalculatorEnter how common the condition is and how the test performs to see what a positive result really implies.
- Combination CalculatorEnter how many items there are and how many you take to see the combinations and permutations.
- Z-Score CalculatorEnter a value with the mean and standard deviation of its distribution.
- Confidence Interval CalculatorEnter your sample mean, standard deviation and size to see the interval around it.
- Sample Size CalculatorEnter the margin of error you want and the size of the group you are studying.
- Average CalculatorPaste or type your numbers, separated by spaces, commas or line breaks.
Money
Margin, markup, discount, tips and loan payments. Three of these get confused with each other constantly, and one of them costs more than people expect — each page shows which is which.
- Discount CalculatorEnter the original price and the discount to see the sale price and the saving.
- Margin CalculatorEnter what an item costs you and what you sell it for.
- Markup CalculatorEnter your cost and the markup you want to apply to get the price to charge.
- Tip CalculatorEnter the bill and the percentage. Tax is kept separate, because tipping on tax is a choice.
- Loan Payment CalculatorEnter the amount, the annual rate and the term. The interest total is the part a monthly figure hides.
- Sales Tax & VAT CalculatorEnter a price and a rate. The tax-exclusive and tax-inclusive figures are both shown.
- Compound Interest CalculatorEnter a starting amount, a rate and a term. What compounding adds is shown as its own figure.
Health
Calculations about the body, published only where the formula is transparent and its limits can be stated plainly. Nothing here is a diagnosis, and nothing here replaces a clinician.
Conversions
Length, area and volume, converted through exact factors with no intermediate rounding. Area and volume are derived from the linear factor rather than carrying their own constant.
- Length ConverterEnter a length in any unit to see it in all the others at once.
- Area ConverterEnter an area in any unit to see it in all the others at once.
- Volume ConverterEnter a volume in any unit to see it in all the others at once.
- Weight ConverterEnter a weight in any unit to see it in all the others at once.
- Speed ConverterEnter a speed in any unit to see it in all the others at once.
- Temperature ConverterEnter a temperature in any scale to see it in the other two, and what a change of that size means.
Time & dates
Days between two dates, age in years, months and days, and averages — calendar arithmetic that leap years and month lengths make easy to get wrong by hand.