How These Calculators Work
The math behind every tool on the site, spelled out the way you'd explain it over the fence. Each answer links to the calculator that does it for you.
How do I calculate a percentage?
To find X percent of a number, multiply the number by X divided by 100. So 15 percent of 80 is 80 × 0.15, which is 12. Going the other way, to find what percent X is of Y, divide X by Y and multiply by 100.
Skip the arithmetic: the percentage calculator shows the working too.
How do I calculate percent increase or decrease?
Subtract the old value from the new one, divide that difference by the old value, then multiply by 100. From 50 up to 75, the difference is 25, and 25 ÷ 50 is 0.5, so a 50 percent increase. A negative answer means the value fell.
The third box on the percentage calculator handles change in either direction.
How many square feet is a 12x12 room?
A 12 by 12 foot room is 144 square feet, because area is length times width. In other units that's 16 square yards (divide by 9) or about 13.4 square meters (multiply by 0.0929). Odd-shaped room? Split it into rectangles and add them.
The square footage calculator takes feet and inches and totals multiple rooms.
How do you solve a proportion?
Cross multiplication. In a:b = c:d the cross products match, meaning a × d = b × c. Multiply the two values that sit diagonally from each other, divide by the remaining known value, and the missing number falls out. To check any answer, confirm the cross products agree.
Leave one box empty on the ratio calculator and it solves for that one.
How do you add fractions with different denominators?
Rewrite both fractions over their least common denominator, then add the tops. For 3/4 + 1/6 the least common denominator is 12, so it becomes 9/12 + 2/12 = 11/12. If the result's top and bottom share a factor, divide it out to simplify.
The fraction calculator writes out the LCD step every time.
What is the difference between mean and median?
The mean adds every value and divides by the count; the median is the middle value once the list is sorted. They often land close together, but a few extreme values drag the mean around while the median stays put, which is why home prices get reported as medians.
The average calculator reports both, plus mode, range, sum and count.